[PATCH 4/4] gdb: change regcache list to be a map

Simon Marchi
2020-07-20T20:41:22Z

One regcache object is created for each stopped thread and is stored in
the regcache::regcaches linked list. Looking up a regcache for a given
thread is therefore in O(number of threads). Stopping all threads then
becomes O((number of threads) ^ 2). It becomes noticeable when
debugging thousands of threads, which is typical with GPU targets. This
patch replaces the linked list with an std::unordered_multimap, indexed
by (target, ptid).

I originally designed it using an std::unordered_map with (target, ptid,
arch) as the key, because that's how lookups are done (in
get_thread_arch_aspace_regcache). However, the registers_changed_ptid
function, also somewhat on the hot path (it is used when resuming
threads), needs to delete all regcaches associated to a given (target,
ptid) tuple. Using (target, ptid) as a key allows to do this more
efficiently (see exception below). If the key of the map was (target,
ptid, arch), we'd have to walk all items of the map.

The lookup (in get_thread_arch_aspace_regcache), walks over all existing
regcaches belonging to this (target, ptid), looking to find the one with
the right arch. This is ok, as there will be very few regcaches for a
given key (typically one). Lookups become faster when the number of
threads grows, compared to the linked list. With a small number of
threads, it will probably be a bit slower to do a map lookup than to
walk a few linked list nodes, but I don't think it will be noticeable in
practice.

The function registers_changed_ptid deletes all regcaches related to a
given (target, ptid). We must now handle the different cases
separately:

- NULL target and minus_one_ptid: we delete all the entries
- NULL target and non-minus_one_ptid: invalid (checked by assert)
- non-NULL target and non-minus_one_ptid: we delete all the entries
associated to that tuple, this is done efficiently
- a non-NULL target and minus_one_ptid: we delete all the entries
associated to that target, whatever the ptid. This is the slightly
annoying case, as we can't easily look up all items having this target
in their key. I implemented it by walking the list, which is not
ideal.

The function regcache_thread_ptid_changed is called when a thread
changes ptid. It is implemented efficiently using the map, although
that's not very important: it is not called often, mostly when creating
an inferior, on some specific platforms.

Note: In hash_target_ptid, I am combining hash values from std::hash by
summing them. I don't think it's ideal, since std::hash is just the
identity function for base types. But I don't know what would be better
to reduce the change of collisions. If anybody has a better idea, I'd
be interested.

This patch is a tiny bit from ROCm GDB [1] we would like to merge
upstream. Laurent Morichetti gave be these performance numbers:

The benchmark used is:

time ./gdb --data-directory=data-directory /extra/lmoriche/hip/samples/0_Intro/bit_extract/bit_extract -ex "set pagination off" -ex "set breakpoint pending on" -ex "b bit_extract_kernel if \$_thread == 5" -ex run -ex c -batch

It measures the time it takes to continue from a conditional breakpoint with
2048 threads at that breakpoint, one of them reporting the breakpoint.

baseline:
real 0m10.227s
real 0m10.177s
real 0m10.362s

with patch:
real 0m8.356s
real 0m8.424s
real 0m8.494s

[1] https://github.com/ROCm-Developer-Tools/ROCgdb

gdb/ChangeLog:

* regcache.c (struct target_ptid): New struct.
(hash_target_ptid): New struct.
(target_ptid_regcache_map): New type.
(regcaches): Change type to target_ptid_regcache_map.
(get_thread_arch_aspace_regcache): Update to regcaches' new
type.
(regcache_thread_ptid_changed): Likewise.
(registers_changed_ptid): Likewise.
(regcaches_size): Likewise.
(regcaches_test): Update.
(regcache_thread_ptid_changed): Update.
* gdbsupport/ptid.h (hash_ptid): New struct.

Change-Id: Iabb0a1111707936ca111ddb13f3b09efa83d3402
---
gdb/regcache.c | 192 ++++++++++++++++++++++++++++++++--------------
gdbsupport/ptid.h | 16 ++++
2 files changed, 152 insertions(+), 56 deletions(-)

diff --git a/gdb/regcache.c b/gdb/regcache.c
index fb20250d20f0..eed8a8bde6e5 100644
--- a/gdb/regcache.c
+++ b/gdb/regcache.c
@@ -29,7 +29,7 @@
#include "reggroups.h"
#include "observable.h"
#include "regset.h"
-#include <forward_list>
+#include <unordered_map>

/*
* DATA STRUCTURE
@@ -313,32 +313,73 @@ reg_buffer::assert_regnum (int regnum) const
gdb_assert (regnum < gdbarch_num_regs (arch ()));
}

-/* Global structure containing the current regcache. */
+/* Key type for target_ptid_regcache_map. */
+
+struct target_ptid
+{
+ target_ptid (process_stratum_target *target, ptid_t ptid)
+ : target (target), ptid (ptid)
+ {}
+
+ process_stratum_target *target;
+ ptid_t ptid;
+
+ bool operator== (const target_ptid &other) const
+ {
+ return (this->target == other.target
+ && this->ptid == other.ptid);
+ }
+};
+
+/* Hash function for target_ptid. */
+
+struct hash_target_ptid
+{
+ size_t operator() (const target_ptid &val) const
+ {
+ std::hash<long> h_long;
+ hash_ptid h_ptid;
+
+ return h_long ((long) val.target) + h_ptid (val.ptid);
+ }
+};
+
+/* Type to map a (target, ptid) tuple to a regcache. */
+
+using target_ptid_regcache_map
+ = std::unordered_multimap<target_ptid, regcache *, hash_target_ptid>;
+
+/* Global structure containing the existing regcaches. */

/* NOTE: this is a write-through cache. There is no "dirty" bit for
recording if the register values have been changed (eg. by the
user). Therefore all registers must be written back to the
target when appropriate. */
-static std::forward_list<regcache *> regcaches;
+static target_ptid_regcache_map regcaches;

struct regcache *
get_thread_arch_aspace_regcache (process_stratum_target *target,
- ptid_t ptid, struct gdbarch *gdbarch,
+ ptid_t ptid, gdbarch *arch,
struct address_space *aspace)
{
gdb_assert (target != nullptr);

- for (const auto &regcache : regcaches)
- if (regcache->target () == target
- && regcache->ptid () == ptid
- && regcache->arch () == gdbarch)
- return regcache;
-
- regcache *new_regcache = new regcache (target, gdbarch, aspace);
+ /* Look up the regcache for this (target, ptid, arch). */
+ target_ptid key (target, ptid);
+ auto range = regcaches.equal_range (key);
+ for (auto it = range.first; it != range.second; it++)
+ {
+ if (it->second->arch () == arch)
+ return it->second;
+ }

- regcaches.push_front (new_regcache);
+ /* It does not exist, create it. */
+ regcache *new_regcache = new regcache (target, arch, aspace);
new_regcache->set_ptid (ptid);

+ /* Insert it in the map. */
+ regcaches.insert (std::make_pair (key, new_regcache));
+
return new_regcache;
}

@@ -417,10 +458,25 @@ static void
regcache_thread_ptid_changed (process_stratum_target *target,
ptid_t old_ptid, ptid_t new_ptid)
{
- for (auto &regcache : regcaches)
+ /* Find all the regcaches to updates. */
+ std::vector<regcache *> regcaches_to_update;
+ target_ptid old_key (target, old_ptid);
+ auto range = regcaches.equal_range (old_key);
+ for (auto it = range.first; it != range.second; it++)
+ regcaches_to_update.push_back (it->second);
+
+ /* Remove all entries associated to OLD_KEY. We could have erased the items
+ in the previous `for`, but it is only safe to erase items while iterating
+ starting with C++14. */
+ regcaches.erase (old_key);
+
+ /* Update the regcaches' ptid, insert them back in the map with an updated
+ key. */
+ target_ptid new_key (target, new_ptid);
+ for (regcache *rc : regcaches_to_update)
{
- if (regcache->ptid () == old_ptid && regcache->target () == target)
- regcache->set_ptid (new_ptid);
+ rc->set_ptid (new_ptid);
+ regcaches.insert (std::make_pair (new_key, rc));
}
}

@@ -438,19 +494,53 @@ regcache_thread_ptid_changed (process_stratum_target *target,
void
registers_changed_ptid (process_stratum_target *target, ptid_t ptid)
{
- for (auto oit = regcaches.before_begin (), it = std::next (oit);
- it != regcaches.end (); )
+ if (target == nullptr)
{
- struct regcache *regcache = *it;
- if ((target == nullptr || regcache->target () == target)
- && regcache->ptid ().matches (ptid))
- {
- delete regcache;
- it = regcaches.erase_after (oit);
- }
- else
- oit = it++;
+ /* Since there can be ptid clashes between targets, it's not valid to
+ pass a ptid without saying to which target it belongs. */
+ gdb_assert (ptid == minus_one_ptid);
+
+ /* Delete all the regcaches. */
+ for (auto pair : regcaches)
+ delete pair.second;
+
+ regcaches.clear ();
}
+ else if (ptid != minus_one_ptid)
+ {
+ /* Non-NULL target and non-minus_one_ptid, delete all regcaches belonging
+ to this (TARGET, PTID). */
+ target_ptid key (target, ptid);
+ auto range = regcaches.equal_range (key);
+ for (auto it = range.first; it != range.second; it++)
+ delete it->second;
+
+ regcaches.erase (key);
+ }
+ else
+ {
+ /* Non-NULL target and minus_one_ptid, delete all regcaches associated
+ to this target.
+
+ We unfortunately don't have an efficient way to do this. Fall back
+ to iterating all items to find all those belonging to TARGET.
+
+ Note that in C++11, it's not safe to erase map entries while
+ iterating, so we keep track of them and delete them at the end. */
+ std::vector<target_ptid> keys;
+
+ for (auto pair : regcaches)
+ {
+ if (pair.second->target () == target)
+ {
+ keys.push_back (pair.first);
+ delete pair.second;
+ }
+ }
+
+ for (auto key : keys)
+ regcaches.erase (key);
+ }

if ((target == nullptr || current_thread_target == target)
&& current_thread_ptid.matches (ptid))
@@ -1431,13 +1521,6 @@ register_dump::dump (ui_file *file)

namespace selftests {

-static size_t
-regcaches_size ()
-{
- return std::distance (regcaches.begin (),
- regcaches.end ());
-}
-
/* Wrapper around get_thread_arch_aspace_regcache that does some self checks. */

static void
@@ -1457,7 +1540,7 @@ static void
regcaches_test (void)
{
/* It is empty at the start. */
- SELF_CHECK (regcaches_size () == 0);
+ SELF_CHECK (regcaches.size () == 0);

ptid_t ptid1 (1), ptid2 (2), ptid3 (3);

@@ -1469,52 +1552,52 @@ regcaches_test (void)
test_get_thread_arch_aspace_regcache (&test_target1, ptid1,
target_gdbarch (),
NULL);
- SELF_CHECK (regcaches_size () == 1);
+ SELF_CHECK (regcaches.size () == 1);

/* Get regcache from (target1,ptid2), a new regcache is added to
REGCACHES. */
test_get_thread_arch_aspace_regcache (&test_target1, ptid2,
target_gdbarch (),
NULL);
- SELF_CHECK (regcaches_size () == 2);
+ SELF_CHECK (regcaches.size () == 2);

/* Get regcache from (target1,ptid3), a new regcache is added to
REGCACHES. */
test_get_thread_arch_aspace_regcache (&test_target1, ptid3,
target_gdbarch (),
NULL);
- SELF_CHECK (regcaches_size () == 3);
+ SELF_CHECK (regcaches.size () == 3);

/* Get regcache from (target1,ptid2) again, nothing is added to
REGCACHES. */
test_get_thread_arch_aspace_regcache (&test_target1, ptid2,
target_gdbarch (),
NULL);
- SELF_CHECK (regcaches_size () == 3);
+ SELF_CHECK (regcaches.size () == 3);

/* Get regcache from (target2,ptid2), a new regcache is added to
REGCACHES, since this time we're using a different target. */
test_get_thread_arch_aspace_regcache (&test_target2, ptid2,
target_gdbarch (),
NULL);
- SELF_CHECK (regcaches_size () == 4);
+ SELF_CHECK (regcaches.size () == 4);

/* Mark that (target1,ptid2) changed. The regcache of (target1,
ptid2) should be removed from REGCACHES. */
registers_changed_ptid (&test_target1, ptid2);
- SELF_CHECK (regcaches_size () == 3);
+ SELF_CHECK (regcaches.size () == 3);

/* Get the regcache from (target2,ptid2) again, confirming the
registers_changed_ptid call above did not delete it. */
test_get_thread_arch_aspace_regcache (&test_target2, ptid2,
target_gdbarch (),
NULL);
- SELF_CHECK (regcaches_size () == 3);
+ SELF_CHECK (regcaches.size () == 3);

/* Confirm that marking all regcaches of all targets as changed
clears REGCACHES. */
registers_changed_ptid (nullptr, minus_one_ptid);
- SELF_CHECK (regcaches_size () == 0);
+ SELF_CHECK (regcaches.size () == 0);
}

class target_ops_no_register : public test_target_ops
@@ -1847,27 +1930,24 @@ regcache_thread_ptid_changed ()
get_thread_arch_aspace_regcache (&target1.mock_target, old_ptid, arch, NULL);
get_thread_arch_aspace_regcache (&target2.mock_target, old_ptid, arch, NULL);

- /* Return whether a regcache for (TARGET, PTID) exists in REGCACHES. */
- auto regcache_exists = [] (process_stratum_target *target, ptid_t ptid)
+ /* Return the count of regcaches for (TARGET, PTID) in REGCACHES. */
+ auto regcache_count = [] (process_stratum_target *target, ptid_t ptid)
{
- for (regcache *rc : regcaches)
- {
- if (rc->target () == target && rc->ptid () == ptid)
- return true;
- }
+ target_ptid key (target, ptid);

- return false;
+ auto range = regcaches.equal_range (key);
+ return std::distance (range.first, range.second);
};

- gdb_assert (regcaches_size () == 2);
- gdb_assert (regcache_exists (&target1.mock_target, old_ptid));
- gdb_assert (regcache_exists (&target2.mock_target, old_ptid));
+ gdb_assert (regcaches.size () == 2);
+ gdb_assert (regcache_count (&target1.mock_target, old_ptid) == 1);
+ gdb_assert (regcache_count (&target2.mock_target, old_ptid) == 1);

thread_change_ptid (&target1.mock_target, old_ptid, new_ptid);

- gdb_assert (regcaches_size () == 2);
- gdb_assert (regcache_exists (&target1.mock_target, new_ptid));
- gdb_assert (regcache_exists (&target2.mock_target, old_ptid));
+ gdb_assert (regcaches.size () == 2);
+ gdb_assert (regcache_count (&target1.mock_target, new_ptid) == 1);
+ gdb_assert (regcache_count (&target2.mock_target, old_ptid) == 1);
}

} // namespace selftests
diff --git a/gdbsupport/ptid.h b/gdbsupport/ptid.h
index ef52d5551260..a528312bad5e 100644
--- a/gdbsupport/ptid.h
+++ b/gdbsupport/ptid.h
@@ -32,6 +32,8 @@
thread_stratum target that might want to sit on top.
*/

+#include <functional>
+
class ptid_t
{
public:
@@ -143,6 +145,20 @@ class ptid_t
long m_tid;
};

+/* Functor to hash a ptid. */
+
+struct hash_ptid
+{
+ size_t operator() (const ptid_t &ptid) const
+ {
+ std::hash<long> long_hash;
+
+ return (long_hash (ptid.pid ())
+ + long_hash (ptid.lwp ())
+ + long_hash (ptid.tid ()));
+ }
+};
+
/* The null or zero ptid, often used to indicate no process. */

extern const ptid_t null_ptid;
--
2.26.2