[PATCH v1][RFC] Speed up next/step while debugging multithreaded programs on AIX.

[PATCH v1][RFC] Speed up next/step while debugging multithreaded programs on AIX.

[Aditya Vidyadhar Kamath]

From: Aditya Vidyadhar Kamath <aditya.kamath1@ibm.com>

During go compiler debugging on AIX my collegue and I noticed that
the next command was pretty slow compared to Linux - around 7 seconds
per step when program has atleast 3 or more threads running. A simple
walk through a function took around a minute. In compiler codes this is
slow.

TO measure this properly I wrote a benchmark pasted below. The program spawns
20 worker threads , then stops inside a function with 10 simple
that GDB steps over one by one. The GDB batch script breaks at that
function and issues 10 next commands and then quits. Timing the whole
run gives a fair picture of how much overhead each `next` carries.
Both the .c file and batch file are pasted below.

Before this patch, on AIX 7.3 ppc64 with 20 background threads:

real    1m10.05s
user    0m17.59s
sys     0m20.43s

That is roughly 7 seconds per `next` step.

After this patch:

real    0m20.37s
user    0m5.94s
sys     0m6.53s

About 2 seconds per step.  3.5x faster for the same workload.

Now why was it slow?

Every time the inferior stops -- even for a single-step trap from next
- the AIX thread layer called pd_update(), which did two very expensive
things unconditionally:

1. pthdb_session_update ()

This function walks through all of the pthread library's internal
data structures inside the inferior process, reading them out via
pdc_read_data() callbacks. With 20 threads that is over 300 memory
reads from the inferior on every single stop, even when absolutely
nothing about the threads has changed.

2. sync_threadlists()

This follows the session update with a full rebuild of GDB's thread
list: iterate every thread via pthdb_pthread(), call
pthdb_pthread_tid() for each one, allocate two sorted arrays, diff
them against GDB's internal list, and reconcile the differences.
Again, all of this on every stop, even when the thread population
has not changed at all since the last stop.

On top of that, get_signaled_thread() was calling getthrds() asking for
one kernel thread descriptor at a time, which costs one syscall per
thread in the process.

None of this work is wrong but it is needed when threads actually come
and go.  But during a next or step sequence through a function, the
thread list does not change. Paying for a full rescan on every stop is
something not needed I think.

So what changes this patch does is:

1. Batch getthrds() calls get_signaled_thread ()

The original code passed 1 as the count argument to getthrds(), so
it did one syscall per kernel thread to find the one that stopped.
Changed to fetch 64 descriptors per call and scan the batch in a
loop. This alone cuts the getthrds() cost from O(N) syscalls to
O(N/64).

2. Track the thread count - aix_thread_variables, pd_activate

Added a last_thread_count field to aix_thread_variables.  It records
how many threads libpthdebug reported the last time a full sync was
done. Initialised to -1 in the constructor so the very
first stop always does a full sync. Also reset to -1 in pd_activate()
so that whenever a new debug session is opened the first stop is
always fully synced regardless of the stop kind.

3. Skip the sync on clean step stops

pd_update() now takes a bool step_stop parameter.  When that is true
and last_thread_count is not -1, it skips pthdb_session_update() and
sync_threadlists() entirely.

When a sync is needed (not a step stop, or first stop after
activation), a new helper count_pthdb_threads() does a lightweight
pass through pthdb_pthread() just to count threads. It only calls
sync_threadlists() if that count differs from last_thread_count.
This avoids the expensive pthdb_pthread_tid() calls and array
allocations on stops where the thread count has not changed.

4. Detect step stops in wait() and pass the flag down

wait() already has the stop kind and signal number before it calls
pd_update(). A TARGET_WAITKIND_STOPPED with GDB_SIGNAL_TRAP is what
the kernel reports for a completed single-step. wait() now computes
step_stop from those two fields and passes it to pd_update().

int current_count = count_pthdb_threads (data);
          if (current_count != data->last_thread_count)

This makes sure we never miss any genuinely coming new threads and are
in sync.

So these are the  things I thought and did. Let me know what you think.

==================
Benchmark program (bench_next.c):

    /* Spawns NUM_THREADS worker threads that spin, then steps through
       10 assignments in do_steps() so GDB can time each next.  */
    #include <stdio.h>
    #include <stdlib.h>
    #include <pthread.h>
    #include <unistd.h>

    #ifndef NUM_THREADS
    #define NUM_THREADS 20
    #endif

    static volatile int keep_running = 1;

    static void *
    worker (void *arg)
    {
      while (keep_running)
        sched_yield ();
      return NULL;
    }

    void
    do_steps (void)
    {
      volatile int a = 1;
      volatile int b = 2;
      volatile int c = a + b;
      volatile int d = c * 2;
      volatile int e = d - a;
      volatile int f = e + c;
      volatile int g = f / 2;
      volatile int h = g + 1;
      volatile int i2 = h * h;
      volatile int j = i2 - b;
      (void)j;
    }

    int
    main (void)
    {
      pthread_t threads[NUM_THREADS];
      int i;

      for (i = 0; i < NUM_THREADS; i++)
        pthread_create (&threads[i], NULL, worker, NULL);

      do_steps ();

      keep_running = 0;
      for (i = 0; i < NUM_THREADS; i++)
        pthread_join (threads[i], NULL);

      return 0;
    }

====================
Build done with
gcc -O0 -g -gdwarf -maix64 -DNUM_THREADS=20 -o bench_next_bin bench_next.c -lpthread
=====================
GDB batch script (bench_next.gdb):

    set pagination off
    set confirm off

    break do_steps
    run

    next
    next
    next
    next
    next
    next
    next
    next
    next
    next

    quit
========================
Then run,
time gdb -batch -x bench_next.gdb ./bench_next_bin
---
 gdb/aix-thread.c | 117 +++++++++++++++++++++++++++++++++++++----------
 1 file changed, 94 insertions(+), 23 deletions(-)

diff --git a/gdb/aix-thread.c b/gdb/aix-thread.c
index 5ac71d22237..5c43e1130e4 100644
--- a/gdb/aix-thread.c
+++ b/gdb/aix-thread.c
@@ -173,6 +173,11 @@ static pthdb_callbacks_t pd_callbacks = {
 /* Aix variable structure.  */
 struct aix_thread_variables
 {
+  aix_thread_variables ()
+    : pd_able (0), pd_active (0), pd_session (0), pd_brk_addr (0),
+      arch64 (0), last_thread_count (-1)
+  {}
+
   /* Whether the current application is debuggable by pthdb.  */
   int pd_able;
 
@@ -192,6 +197,11 @@ struct aix_thread_variables
 
   /* Describes the number of thread exit events reported.  */
   std::unordered_set<pthdb_pthread_t> exited_threads;
+
+  /* Last known libpthdebug thread count.  Used to skip sync_threadlists()
+     when the thread population has not changed since the previous stop,
+     which is the common case during stepping.  -1 means unknown/force sync.  */
+  int last_thread_count;
 };
 
 /* Key to our per-inferior data.  */
@@ -733,6 +743,11 @@ state2str (pthdb_state_t state)
     }
 }
 
+/* Number of thread descriptors to fetch per getthrds() call.  The original
+   code fetched one at a time, which costs one syscall per thread.  Fetching
+   in batches reduces that to one syscall per GETTHRDS_BATCH threads.  */
+#define GETTHRDS_BATCH 64
+
 /* Search through the list of all kernel threads for the thread
    that has stopped on a SIGTRAP signal, and return its TID.
    Return 0 if none found.  */
@@ -740,20 +755,18 @@ state2str (pthdb_state_t state)
 static pthdb_tid_t
 get_signaled_thread (int pid)
 {
-  struct thrdsinfo64 thrinf;
+  struct thrdsinfo64 thrinf[GETTHRDS_BATCH];
   tid_t ktid = 0;
+  int count, i;
 
-  while (1)
+  while ((count = getthrds (pid, thrinf, sizeof (thrinf[0]),
+			    &ktid, GETTHRDS_BATCH)) > 0)
     {
-      if (getthrds (pid, &thrinf,
-		    sizeof (thrinf), &ktid, 1) != 1)
-	break;
-
       /* We also need to keep in mind Trap and interrupt or any
 	 signal that needs to be handled in pd_update ().  */
-
-      if (thrinf.ti_cursig)
-	return thrinf.ti_tid;
+      for (i = 0; i < count; i++)
+	if (thrinf[i].ti_cursig)
+	  return thrinf[i].ti_tid;
     }
 
   /* Didn't find any thread stopped on a SIGTRAP signal.  */
@@ -861,13 +874,39 @@ sync_threadlists (pid_t pid)
       }
 }
 
+/* Count the number of live pthreads visible to libpthdebug.
+   Used before sync_threadlists() to check whether the thread population
+   has changed since the last stop.  */
+
+static int
+count_pthdb_threads (struct aix_thread_variables *data)
+{
+  pthdb_pthread_t pdtid;
+  int cmd, n = 0;
+
+  for (cmd = PTHDB_LIST_FIRST;; cmd = PTHDB_LIST_NEXT)
+    {
+      int status = pthdb_pthread (data->pd_session, &pdtid, cmd);
+      if (status != PTHDB_SUCCESS || pdtid == PTHDB_INVALID_PTHREAD)
+	break;
+      n++;
+    }
+  return n;
+}
+
 /* Synchronize libpthdebug's state with the inferior and with GDB,
    generate a composite process/thread <pid> for the current thread,
-   Return the ptid of the event thread if one can be found, else
-   return a pid-only ptid with PID.  */
+   return the ptid of the event thread if one can be found, else
+   return a pid-only ptid with PID.
+
+   STEP_STOP should be set when the stop is a single-step SIGTRAP.
+   In that case, if the thread list was already synced at a previous
+   stop, both pthdb_session_update() and sync_threadlists() are skipped.
+   Those two calls dominate the cost of every stop in a multithreaded
+   program, so skipping them during stepping gives a large speedup.  */
 
 static ptid_t
-pd_update (pid_t pid)
+pd_update (pid_t pid, bool step_stop = false)
 {
   int status;
   ptid_t ptid;
@@ -880,15 +919,36 @@ pd_update (pid_t pid)
   if (!data->pd_active)
     return ptid_t (pid);
 
-  status = pthdb_session_update (data->pd_session);
-  if (status != PTHDB_SUCCESS)
-    return ptid_t (pid);
-
-  /* Attempt to sync_threadlists () only when debugging object files
-     and not core files since list of threads never change for core
-     files.  */
-  if (target_has_execution ())
-    sync_threadlists (pid);
+  /* Skip the session update and thread list sync when stepping.
+     pthdb_session_update() reads through all pthread data structures in
+     the inferior on every call, and sync_threadlists() follows it with
+     O(N) kernel calls per thread.  During a next/step sequence the thread
+     list does not change, so both calls can be omitted as long as we have
+     done at least one full sync since activation (last_thread_count >= 0).
+     Any other stop kind (breakpoints, real signals) always does a full
+     sync because threads may have been created or destroyed.  */
+  bool need_sync = !step_stop || (data->last_thread_count < 0);
+
+  if (need_sync)
+    {
+      status = pthdb_session_update (data->pd_session);
+      if (status != PTHDB_SUCCESS)
+	return ptid_t (pid);
+
+      /* Attempt to sync_threadlists() only when debugging object files
+	 and not core files since the list of threads never changes for
+	 core files.  Even for live inferiors, skip the sync when the
+	 thread count matches the previous stop.  */
+      if (target_has_execution ())
+	{
+	  int current_count = count_pthdb_threads (data);
+	  if (current_count != data->last_thread_count)
+	    {
+	      sync_threadlists (pid);
+	      data->last_thread_count = current_count;
+	    }
+	}
+    }
 
   /* Define "current thread" as one that just received a trap signal.  */
 
@@ -921,7 +981,12 @@ pd_activate (pid_t pid)
 			       PTHDB_FLAG_REGS, &pd_callbacks,
 			       &data->pd_session);
   if (status == PTHDB_SUCCESS)
-    data->pd_active = 1;
+    {
+      data->pd_active = 1;
+      /* Reset last_thread_count so that the very first stop after
+	 activation always does a full sync_threadlists() pass.  */
+      data->last_thread_count = -1;
+    }
 }
 
 /* AIX implementation of update_thread_list.  */
@@ -1125,7 +1190,13 @@ aix_thread_target::wait (ptid_t ptid, struct target_waitstatus *status,
 	pd_activate (ptid.pid ());
     }
 
-  return pd_update (ptid.pid ());
+  /* Tell pd_update() whether this is a single-step SIGTRAP so it can
+     skip the expensive pthdb_session_update() and sync_threadlists()
+     calls.  Breakpoints, real signals and other stop kinds always get
+     the full sync.  */
+  bool step_stop = (status->kind () == TARGET_WAITKIND_STOPPED
+		    && status->sig () == GDB_SIGNAL_TRAP);
+  return pd_update (ptid.pid (), step_stop);
 }
 
 /* Supply AIX altivec registers, both 64 and 32 bit.  */
-- 
2.51.2

Re: [PATCH v1][RFC] Speed up next/step while debugging multithreaded programs on AIX.

[Abhay Kandpal]

[-- Attachment #1: Type: text/plain, Size: 15289 bytes --]

Hi Aditya,

On 09/09/26 11:13, Aditya Vidyadhar Kamath wrote:
> From: Aditya Vidyadhar Kamath<aditya.kamath1@ibm.com>
>
> During go compiler debugging on AIX my collegue and I noticed that
> the next command was pretty slow compared to Linux - around 7 seconds
> per step when program has atleast 3 or more threads running. A simple
> walk through a function took around a minute. In compiler codes this is
> slow.
>
> TO measure this properly I wrote a benchmark pasted below. The program spawns
> 20 worker threads , then stops inside a function with 10 simple
> that GDB steps over one by one. The GDB batch script breaks at that
> function and issues 10 next commands and then quits. Timing the whole
> run gives a fair picture of how much overhead each `next` carries.
> Both the .c file and batch file are pasted below.
>
> Before this patch, on AIX 7.3 ppc64 with 20 background threads:
>
> real    1m10.05s
> user    0m17.59s
> sys     0m20.43s
>
> That is roughly 7 seconds per `next` step.
>
> After this patch:
>
> real    0m20.37s
> user    0m5.94s
> sys     0m6.53s
>
> About 2 seconds per step.  3.5x faster for the same workload.
>
> Now why was it slow?
>
> Every time the inferior stops -- even for a single-step trap from next
> - the AIX thread layer called pd_update(), which did two very expensive
> things unconditionally:
>
> 1. pthdb_session_update ()
>
> This function walks through all of the pthread library's internal
> data structures inside the inferior process, reading them out via
> pdc_read_data() callbacks. With 20 threads that is over 300 memory
> reads from the inferior on every single stop, even when absolutely
> nothing about the threads has changed.
>
> 2. sync_threadlists()
>
> This follows the session update with a full rebuild of GDB's thread
> list: iterate every thread via pthdb_pthread(), call
> pthdb_pthread_tid() for each one, allocate two sorted arrays, diff
> them against GDB's internal list, and reconcile the differences.
> Again, all of this on every stop, even when the thread population
> has not changed at all since the last stop.
>
> On top of that, get_signaled_thread() was calling getthrds() asking for
> one kernel thread descriptor at a time, which costs one syscall per
> thread in the process.
>
> None of this work is wrong but it is needed when threads actually come
> and go.  But during a next or step sequence through a function, the
> thread list does not change. Paying for a full rescan on every stop is
> something not needed I think.
>
> So what changes this patch does is:
>
> 1. Batch getthrds() calls get_signaled_thread ()
>
> The original code passed 1 as the count argument to getthrds(), so
> it did one syscall per kernel thread to find the one that stopped.
> Changed to fetch 64 descriptors per call and scan the batch in a
> loop. This alone cuts the getthrds() cost from O(N) syscalls to
> O(N/64).
>
> 2. Track the thread count - aix_thread_variables, pd_activate
>
> Added a last_thread_count field to aix_thread_variables.  It records
> how many threads libpthdebug reported the last time a full sync was
> done. Initialised to -1 in the constructor so the very
> first stop always does a full sync. Also reset to -1 in pd_activate()
> so that whenever a new debug session is opened the first stop is
> always fully synced regardless of the stop kind.
>
> 3. Skip the sync on clean step stops
>
> pd_update() now takes a bool step_stop parameter.  When that is true
> and last_thread_count is not -1, it skips pthdb_session_update() and
> sync_threadlists() entirely.
>
> When a sync is needed (not a step stop, or first stop after
> activation), a new helper count_pthdb_threads() does a lightweight
> pass through pthdb_pthread() just to count threads. It only calls
> sync_threadlists() if that count differs from last_thread_count.
> This avoids the expensive pthdb_pthread_tid() calls and array
> allocations on stops where the thread count has not changed.
>
> 4. Detect step stops in wait() and pass the flag down
>
> wait() already has the stop kind and signal number before it calls
> pd_update(). A TARGET_WAITKIND_STOPPED with GDB_SIGNAL_TRAP is what
> the kernel reports for a completed single-step. wait() now computes
> step_stop from those two fields and passes it to pd_update().
>
> int current_count = count_pthdb_threads (data);
>            if (current_count != data->last_thread_count)
>
> This makes sure we never miss any genuinely coming new threads and are
> in sync.
>
> So these are the  things I thought and did. Let me know what you think.
>
> ==================
> Benchmark program (bench_next.c):
>
>      /* Spawns NUM_THREADS worker threads that spin, then steps through
>         10 assignments in do_steps() so GDB can time each next.  */
>      #include <stdio.h>
>      #include <stdlib.h>
>      #include <pthread.h>
>      #include <unistd.h>
>
>      #ifndef NUM_THREADS
>      #define NUM_THREADS 20
>      #endif
>
>      static volatile int keep_running = 1;
>
>      static void *
>      worker (void *arg)
>      {
>        while (keep_running)
>          sched_yield ();
>        return NULL;
>      }
>
>      void
>      do_steps (void)
>      {
>        volatile int a = 1;
>        volatile int b = 2;
>        volatile int c = a + b;
>        volatile int d = c * 2;
>        volatile int e = d - a;
>        volatile int f = e + c;
>        volatile int g = f / 2;
>        volatile int h = g + 1;
>        volatile int i2 = h * h;
>        volatile int j = i2 - b;
>        (void)j;
>      }
>
>      int
>      main (void)
>      {
>        pthread_t threads[NUM_THREADS];
>        int i;
>
>        for (i = 0; i < NUM_THREADS; i++)
>          pthread_create (&threads[i], NULL, worker, NULL);
>
>        do_steps ();
>
>        keep_running = 0;
>        for (i = 0; i < NUM_THREADS; i++)
>          pthread_join (threads[i], NULL);
>
>        return 0;
>      }
>
> ====================
> Build done with
> gcc -O0 -g -gdwarf -maix64 -DNUM_THREADS=20 -o bench_next_bin bench_next.c -lpthread
> =====================
> GDB batch script (bench_next.gdb):
>
>      set pagination off
>      set confirm off
>
>      break do_steps
>      run
>
>      next
>      next
>      next
>      next
>      next
>      next
>      next
>      next
>      next
>      next
>
>      quit
> ========================
> Then run,
> time gdb -batch -x bench_next.gdb ./bench_next_bin
> ---
>   gdb/aix-thread.c | 117 +++++++++++++++++++++++++++++++++++++----------
>   1 file changed, 94 insertions(+), 23 deletions(-)
>
> diff --git a/gdb/aix-thread.c b/gdb/aix-thread.c
> index 5ac71d22237..5c43e1130e4 100644
> --- a/gdb/aix-thread.c
> +++ b/gdb/aix-thread.c
> @@ -173,6 +173,11 @@ static pthdb_callbacks_t pd_callbacks = {
>   /* Aix variable structure.  */
>   struct aix_thread_variables
>   {
> +  aix_thread_variables ()
> +    : pd_able (0), pd_active (0), pd_session (0), pd_brk_addr (0),
> +      arch64 (0), last_thread_count (-1)
> +  {}
> +
>     /* Whether the current application is debuggable by pthdb.  */
>     int pd_able;
>   
> @@ -192,6 +197,11 @@ struct aix_thread_variables
>   
>     /* Describes the number of thread exit events reported.  */
>     std::unordered_set<pthdb_pthread_t> exited_threads;
> +
> +  /* Last known libpthdebug thread count.  Used to skip sync_threadlists()
> +     when the thread population has not changed since the previous stop,
> +     which is the common case during stepping.  -1 means unknown/force sync.  */
> +  int last_thread_count;
>   };
>   
>   /* Key to our per-inferior data.  */
> @@ -733,6 +743,11 @@ state2str (pthdb_state_t state)
>       }
>   }
>   
> +/* Number of thread descriptors to fetch per getthrds() call.  The original
> +   code fetched one at a time, which costs one syscall per thread.  Fetching
> +   in batches reduces that to one syscall per GETTHRDS_BATCH threads.  */
> +#define GETTHRDS_BATCH 64
> +
>   /* Search through the list of all kernel threads for the thread
>      that has stopped on a SIGTRAP signal, and return its TID.
>      Return 0 if none found.  */
> @@ -740,20 +755,18 @@ state2str (pthdb_state_t state)
>   static pthdb_tid_t
>   get_signaled_thread (int pid)
>   {
> -  struct thrdsinfo64 thrinf;
> +  struct thrdsinfo64 thrinf[GETTHRDS_BATCH];
>     tid_t ktid = 0;
> +  int count, i;
>   
> -  while (1)
> +  while ((count = getthrds (pid, thrinf, sizeof (thrinf[0]),
> +			    &ktid, GETTHRDS_BATCH)) > 0)
>       {
> -      if (getthrds (pid, &thrinf,
> -		    sizeof (thrinf), &ktid, 1) != 1)
> -	break;
> -
>         /* We also need to keep in mind Trap and interrupt or any
>   	 signal that needs to be handled in pd_update ().  */
> -
> -      if (thrinf.ti_cursig)
> -	return thrinf.ti_tid;
> +      for (i = 0; i< count; i++) + if (thrinf[i].ti_cursig) + return thrinf[i].ti_tid; } 
> /* Didn't find any thread stopped on a SIGTRAP signal. */ @@ -861,13 
> +874,39 @@ sync_threadlists (pid_t pid) } } +/* Count the number of 
> live pthreads visible to libpthdebug. + Used before sync_threadlists() 
> to check whether the thread population + has changed since the last 
> stop. */ + +static int +count_pthdb_threads (struct 
> aix_thread_variables *data) +{ + pthdb_pthread_t pdtid; + int cmd, n = 
> 0; + + for (cmd = PTHDB_LIST_FIRST;; cmd = PTHDB_LIST_NEXT) + { + int 
> status = pthdb_pthread (data->pd_session, &pdtid, cmd);
> +      if (status != PTHDB_SUCCESS || pdtid == PTHDB_INVALID_PTHREAD)
> +	break;
> +      n++;
> +    }
> +  return n;
> +}
> +
>   /* Synchronize libpthdebug's state with the inferior and with GDB,
>      generate a composite process/thread <pid> for the current thread,
> -   Return the ptid of the event thread if one can be found, else
> -   return a pid-only ptid with PID.  */
> +   return the ptid of the event thread if one can be found, else
> +   return a pid-only ptid with PID.
> +
> +   STEP_STOP should be set when the stop is a single-step SIGTRAP.
> +   In that case, if the thread list was already synced at a previous
> +   stop, both pthdb_session_update() and sync_threadlists() are skipped.
> +   Those two calls dominate the cost of every stop in a multithreaded
> +   program, so skipping them during stepping gives a large speedup.  */
>   
>   static ptid_t
> -pd_update (pid_t pid)
> +pd_update (pid_t pid, bool step_stop = false)
>   {
>     int status;
>     ptid_t ptid;
> @@ -880,15 +919,36 @@ pd_update (pid_t pid)
>     if (!data->pd_active)
>       return ptid_t (pid);
>   
> -  status = pthdb_session_update (data->pd_session);
> -  if (status != PTHDB_SUCCESS)
> -    return ptid_t (pid);
> -
> -  /* Attempt to sync_threadlists () only when debugging object files
> -     and not core files since list of threads never change for core
> -     files.  */
> -  if (target_has_execution ())
> -    sync_threadlists (pid);
> +  /* Skip the session update and thread list sync when stepping.
> +     pthdb_session_update() reads through all pthread data structures in
> +     the inferior on every call, and sync_threadlists() follows it with
> +     O(N) kernel calls per thread.  During a next/step sequence the thread
> +     list does not change, so both calls can be omitted as long as we have
> +     done at least one full sync since activation (last_thread_count >= 0).
> +     Any other stop kind (breakpoints, real signals) always does a full
> +     sync because threads may have been created or destroyed.  */
> +  bool need_sync = !step_stop || (data->last_thread_count < 0);
> +
> +  if (need_sync)
> +    {
> +      status = pthdb_session_update (data->pd_session);
> +      if (status != PTHDB_SUCCESS)
> +	return ptid_t (pid);
> +
> +      /* Attempt to sync_threadlists() only when debugging object files
> +	 and not core files since the list of threads never changes for
> +	 core files.  Even for live inferiors, skip the sync when the
> +	 thread count matches the previous stop.  */
> +      if (target_has_execution ())
> +	{
> +	  int current_count = count_pthdb_threads (data);
> +	  if (current_count != data->last_thread_count)
> +	    {
> +	      sync_threadlists (pid);
> +	      data->last_thread_count = current_count;
> +	    }
> +	}
> +    }
>   
>     /* Define "current thread" as one that just received a trap signal.  */
>   
> @@ -921,7 +981,12 @@ pd_activate (pid_t pid)
>   			       PTHDB_FLAG_REGS, &pd_callbacks,
>   			       &data->pd_session);
>     if (status == PTHDB_SUCCESS)
> -    data->pd_active = 1;
> +    {
> +      data->pd_active = 1;
> +      /* Reset last_thread_count so that the very first stop after
> +	 activation always does a full sync_threadlists() pass.  */
> +      data->last_thread_count = -1;
> +    }
>   }
>   
>   /* AIX implementation of update_thread_list.  */
> @@ -1125,7 +1190,13 @@ aix_thread_target::wait (ptid_t ptid, struct target_waitstatus *status,
>   	pd_activate (ptid.pid ());
>       }
>   
> -  return pd_update (ptid.pid ());
> +  /* Tell pd_update() whether this is a single-step SIGTRAP so it can
> +     skip the expensive pthdb_session_update() and sync_threadlists()
> +     calls.  Breakpoints, real signals and other stop kinds always get
> +     the full sync.  */
> +  bool step_stop = (status->kind () == TARGET_WAITKIND_STOPPED
> +		    && status->sig () == GDB_SIGNAL_TRAP);
> +  return pd_update (ptid.pid (), step_stop);

Doesn't a breakpoint stop also arrive as TARGET_WAITKIND_STOPPED with GDB_SIGNAL_TRAP?
get_signaled_thread() already looks for the thread stopped on SIGTRAP to find the
current thread, and that has to work at breakpoint stops too, so I would expect
a breakpoint hit to look the same as a single-step here.

If so, breakpoint stops also get step_stop = true and skip the sync, which is the
opposite of what the commit message says:

"Any other stop kind (breakpoints, real signals) always does a full sync
because threads may have been created or destroyed."

The last_thread_count < 0 check only covers the first stop after pd_activate().
After that first sync, last_thread_count is >= 0 for the rest of the session
and need_sync becomes just !step_stop.  So from the second stop onwards
a breakpoint hit skips the sync, and any threads created since the previous
stop are missed.

The benchmark won't show this: all 20 threads are created before do_steps()
and there is only one breakpoint, so the thread list never changes after the
first stop.

resume() is already told whether GDB asked for a single step, and it already
has the aix_thread_variables pointer in hand.  Could you save the flag there
and use it here instead of inferring it from the signal?

/* aix_thread_target::resume (), after the existing
data = get_thread_data_helper_for_ptid (ptid); */
data->last_resume_step = step;

/* in wait () */
bool step_stop = (data->last_resume_step
&& status->kind () == TARGET_WAITKIND_STOPPED
&& status->sig () == GDB_SIGNAL_TRAP);

That would also handle "next" over a function call, where GDB puts a temporary
breakpoint at the return address and continues rather than single-stepping
through the callee.  resume() is called with step = 0 in that case, so the
stop would correctly get a full sync even though the user typed "next".

Regards,
Abhay

>   }
>   
>   /* Supply AIX altivec registers, both 64 and 32 bit.  */

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Re: [PATCH v1][RFC] Speed up next/step while debugging multithreaded programs on AIX.

[Aditya Kamath]

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Hi Abhay and community members,


Thank you very much for the feedback. Please see my comments below to your concerns and I am sending v2 of this patch after this soon.

>Doesn't a breakpoint stop also arrive as >TARGET_WAITKIND_STOPPED with GDB_SIGNAL_TRAP?
>get_signaled_thread() already looks for the thread stopped >on SIGTRAP to find the
>current thread, and that has to work at breakpoint stops >too, so I would expect
>a breakpoint hit to look the same as a single-step here.

>If so, breakpoint stops also get step_stop = true and skip >the sync

>The last_thread_count < 0 check only covers the first stop >after pd_activate().
After that first sync, >last_thread_count is >= 0 for the rest of the session
and >need_sync becomes just !step_stop.  So from the second stop >onwards
a breakpoint hit skips the sync, and any threads >created since the previous
stop are missed.

You are right here. A user breakpoint hit, a software single-step completion, and the thread-creation stub breakpoint all arrive with exactly the same WAITKIND_STOPPED + GDB_SIGNAL_TRAP. The v1 version of this patch couldn't tell them apart, so step_stop was true for breakpoint hits too, and the sync was wrongly skipped. I checked this. I did not think in this angle and just thought about speeding up. Thanks for pointing it out. Yeah the benchmark will not point this out.

>resume() is already told whether GDB asked for a single >step, and it already
>has the aix_thread_variables pointer in hand.  Could you >save the flag there
>and use it here instead of inferring it from the signal?

>/* aix_thread_target::resume (), after the existing
>data = get_thread_data_helper_for_ptid (ptid); */
>data->last_resume_step = step;

>/* in wait () */
>bool step_stop = (data->last_resume_step
>&& status->kind () == TARGET_WAITKIND_STOPPED
>&& status->sig () == GDB_SIGNAL_TRAP);

>That would also handle "next" over a function call, where GDB puts a temporary
>breakpoint at the return address and continues rather than single-stepping
>through the callee.  resume() is called with step = 0 in that case, so the
>stop would correctly get a full sync even though the user typed "next”.


I like this idea but while implementing came across something else. On AIX rs6000_software_single_step() is registered as the architecture's next-PC provider. infrun.c's maybe_software_singlestep() calls it, which inserts breakpoints at the next instruction(s) and returns hw_step = false, so do_target_resume() always passes step=0. Saving that would have made last_resume_step always 0, killing the optimisation entirely.

In the debug log I saw every do_target_resume call showed step=0, even for next.

The right signal is whether software single-step breakpoints were inserted for the current thread at the time of the resume. That is what thread_has_single_step_breakpoints_set() reports, and it is set when and only when GDB is software-single-stepping through source lines.

So in resume() in v2 version of this patch you will see:

struct thread_info *tp = inferior_thread ();
data->last_resume_step = thread_has_single_step_breakpoints_set (tp);


This is 1 when GDB inserted single-step breakpoints which is a next/step resume, and 0 for any free continue or temporary-breakpoint resume. The wait() side then checks both last_resume_step and GDB_SIGNAL_TRAP — both must be true for the sync skip to apply.

Let me know your thoughts in the next version of this patch.

Have a nice day ahead.

Thank you and regards,

Aditya.



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