[PATCH 3/4] remote: get rid of all the T packets when synching the thread list

Pedro Alves
2014-10-02T16:21:45Z

This commit avoids the prune_threads call in the remote target's
target_update_thread_list's implemention, eliminating all the "thread
alive" RSP traffic (one packet per thread) whenever we fetch the
thread list.

IOW, this:

Sending packet: $Tp2141.2150#82...Packet received: OK
Sending packet: $Tp2141.214f#b7...Packet received: OK
Sending packet: $Tp2141.2141#82...Packet received: OK
... more T packets; it's one per previously known live thread ...
Sending packet: $qXfer:threads:read::0,fff#03...Packet received: l<threads>\n<thread id="p2141.2141" core="2"/>\n<thread id="p2141.214f" core="1"/>\n<thread id="p2141.2150" core="2"/>\n</threads>\n

Becomes:

Sending packet: $qXfer:threads:read::0,fff#03...Packet received: l<threads>\n<thread id="p2141.2141" core="2"/>\n<thread id="p2141.214f" core="1"/>\n<thread id="p2141.2150" core="2"/>\n</threads>\n

Tested on x86_64 Fedora 20, native gdbserver:
- tests the qXfer:threads:read method.

Tested on x86_64 Fedora 20, native gdbserver with qXfer:threads:read
force-disabled in gdbserver:
- So that GDB falls back to the qfThreadInfo/qsThreadInfo method.

And also manually smoked tested force disabling both
qXfer:threads:read and qfThreadInfo/qsThreadInfo in gdbserver.

gdb/
2014-10-02 Pedro Alves <palves@redhat.com>

* gdbthread.h (ALL_NON_EXITED_THREADS_SAFE): New macro.
* remote.c (remote_update_thread_list): Skip calling prune_threads
if any thread listing method is supported, and instead walk over
the set of remote threads listed, deleting those that are not
found in GDB's thread list.
---
gdb/gdbthread.h | 8 ++++++++
gdb/remote.c | 38 ++++++++++++++++++++++++++++++++++----
2 files changed, 42 insertions(+), 4 deletions(-)

diff --git a/gdb/gdbthread.h b/gdb/gdbthread.h
index 31675a1..b85e9d7 100644
--- a/gdb/gdbthread.h
+++ b/gdb/gdbthread.h
@@ -329,6 +329,14 @@ extern struct thread_info *iterate_over_threads (thread_callback_func, void *);
for (T = thread_list; T; T = T->next) \
if ((T)->state != THREAD_EXITED)

+/* Like ALL_NON_EXITED_THREADS, but allows deleting the currently
+ iterated thread. */
+#define ALL_NON_EXITED_THREADS_SAFE(T, TMP) \
+ for ((T) = thread_list; \
+ (T) != NULL ? ((TMP) = (T)->next, 1): 0; \
+ (T) = (TMP)) \
+ if ((T)->state != THREAD_EXITED)
+
extern int thread_count (void);

/* Switch from one thread to another. */
diff --git a/gdb/remote.c b/gdb/remote.c
index 74f57f8..8913cd2 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -2763,9 +2763,7 @@ remote_update_thread_list (struct target_ops *ops)
struct remote_state *rs = get_remote_state ();
struct threads_listing_context context;
struct cleanup *old_chain;
-
- /* Delete GDB-side threads no longer found on the target. */
- prune_threads ();
+ int got_list = 0;

context.items = NULL;
old_chain = make_cleanup (clear_threads_listing_context, &context);
@@ -2779,8 +2777,31 @@ remote_update_thread_list (struct target_ops *ops)
{
int i;
struct thread_item *item;
+ struct thread_info *tp, *tmp;
+
+ got_list = 1;
+
+ /* CONTEXT now holds the current thread list on the remote
+ target end. Delete GDB-side threads no longer found on the
+ target. */
+ ALL_NON_EXITED_THREADS_SAFE (tp, tmp)
+ {
+ for (i = 0;
+ VEC_iterate (thread_item_t, context.items, i, item);
+ ++i)
+ {
+ if (ptid_equal (item->ptid, tp->ptid))
+ break;
+ }

- /* Now add threads we don't know about yet to our list. */
+ if (i == VEC_length (thread_item_t, context.items))
+ {
+ /* Not found. */
+ delete_thread (tp->ptid);
+ }
+ }
+
+ /* And now add threads we don't know about yet to our list. */
for (i = 0;
VEC_iterate (thread_item_t, context.items, i, item);
++i)
@@ -2804,6 +2825,15 @@ remote_update_thread_list (struct target_ops *ops)
}
}

+ if (!got_list)
+ {
+ /* If no thread listing method is supported, then query whether
+ each known thread is alive, one by one, with the T packet.
+ If the target doesn't support threads at all, then this is a
+ no-op. See remote_thread_alive. */
+ prune_threads ();
+ }
+
do_cleanups (old_chain);
}

--
1.9.3