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| author | Pedro Alves <palves@redhat.com> | 2016-05-24 14:47:57 +0100 |
|---|---|---|
| committer | Pedro Alves <palves@redhat.com> | 2016-05-24 14:48:57 +0100 |
| commit | 1ad3de988d2f41c72de66613c68ed78507a3abbd (patch) | |
| tree | 15bbafba25939c164b8f05e4407dd57df129d187 | |
| parent | 95e94c3f18aaf34fadcd9a2a882ffe6147b9acc3 (diff) | |
| download | binutils-gdb-1ad3de988d2f41c72de66613c68ed78507a3abbd.tar.gz | |
[Linux] Avoid refetching core-of-thread if thread hasn't run
Hacking the gdb.threads/attach-many-short-lived-threads.exp test to
spawn thousands of threads instead of dozens, I saw GDB having trouble
keeping up with threads being spawned too fast, when it tried to stop
them all. This was because while gdb is doing that, it updates the
thread list to make sure no new thread has sneaked in that might need
to be paused. It does this a few times until it sees no-new-threads
twice in a row. The thread listing update itself is not that
expensive, however, in the Linux backend, updating the threads list
calls linux_common_core_of_thread for each LWP to record on which core
each LWP was last seen running, which opens/reads/closes a /proc file
for each LWP which becomes expensive when you need to do it for
thousands of LWPs.
perf shows gdb in linux_common_core_of_thread 44% of the time, in the
stop_all_threads -> update_thread_list path in this use case.
This patch simply makes linux_common_core_of_thread avoid updating the
core the thread is bound to if the thread hasn't run since the last
time we updated that info. This makes linux_common_core_of_thread
disappear into the noise in the perf report.
gdb/ChangeLog:
2016-05-24 Pedro Alves <palves@redhat.com>
PR gdb/19828
* linux-nat.c (linux_resume_one_lwp_throw): Clear the LWP's core
field.
(linux_nat_update_thread_list): Don't fetch the core if already
known.
| -rw-r--r-- | gdb/ChangeLog | 8 | ||||
| -rw-r--r-- | gdb/linux-nat.c | 9 |
2 files changed, 16 insertions, 1 deletions
diff --git a/gdb/ChangeLog b/gdb/ChangeLog index dd6282404c0..ba4fc8f2ee7 100644 --- a/gdb/ChangeLog +++ b/gdb/ChangeLog @@ -1,6 +1,14 @@ 2016-05-24 Pedro Alves <palves@redhat.com> PR gdb/19828 + * linux-nat.c (linux_resume_one_lwp_throw): Clear the LWP's core + field. + (linux_nat_update_thread_list): Don't fetch the core if already + known. + +2016-05-24 Pedro Alves <palves@redhat.com> + + PR gdb/19828 * linux-tdep.c (find_mapping_size): Delete. (linux_vsyscall_range_raw): Rewrite reading from /proc/PID/task/PID/maps directly instead of using diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c index 19fe12f80a3..dbadf2900e0 100644 --- a/gdb/linux-nat.c +++ b/gdb/linux-nat.c @@ -1432,6 +1432,7 @@ linux_resume_one_lwp_throw (struct lwp_info *lp, int step, status. Note that we must not throw after this is cleared, otherwise handle_zombie_lwp_error would get confused. */ lp->stopped = 0; + lp->core = -1; lp->stop_reason = TARGET_STOPPED_BY_NO_REASON; registers_changed_ptid (lp->ptid); } @@ -3785,7 +3786,13 @@ linux_nat_update_thread_list (struct target_ops *ops) /* Update the processor core that each lwp/thread was last seen running on. */ ALL_LWPS (lwp) - lwp->core = linux_common_core_of_thread (lwp->ptid); + { + /* Avoid accessing /proc if the thread hasn't run since we last + time we fetched the thread's core. Accessing /proc becomes + noticeably expensive when we have thousands of LWPs. */ + if (lwp->core == -1) + lwp->core = linux_common_core_of_thread (lwp->ptid); + } } static char * |