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[Bug gdb/19828] 7.11 regression: non-stop gdb -p <process from a container>: internal error
- From: "cvs-commit at gcc dot gnu.org" <sourceware-bugzilla at sourceware dot org>
- To: gdb-prs at sourceware dot org
- Date: Tue, 24 May 2016 13:55:42 +0000
- Subject: [Bug gdb/19828] 7.11 regression: non-stop gdb -p <process from a container>: internal error
- Auto-submitted: auto-generated
- References: <bug-19828-4717 at http dot sourceware dot org/bugzilla/>
https://sourceware.org/bugzilla/show_bug.cgi?id=19828
--- Comment #6 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Pedro Alves <palves@sourceware.org>:
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=774113b02f41ded4d9ba4d18571ee5024312ad1b
commit 774113b02f41ded4d9ba4d18571ee5024312ad1b
Author: Pedro Alves <palves@redhat.com>
Date: Tue May 24 14:47:57 2016 +0100
[Linux] Optimize PID -> struct lwp_info lookup
Hacking the gdb.threads/attach-many-short-lived-threads.exp test to
spawn thousands of threads instead of dozens, and running gdb under
perf, I saw that GDB was spending most of the time in find_lwp_pid:
- captured_main
- 93.61% catch_command_errors
- 87.41% attach_command
- 87.40% linux_nat_attach
- 87.40% linux_proc_attach_tgid_threads
- 82.38% attach_proc_task_lwp_callback
- 81.01% find_lwp_pid
5.30% ptid_get_lwp
+ 0.10% ptid_lwp_p
+ 0.64% add_thread
+ 0.26% set_running
+ 0.24% set_executing
0.12% ptid_get_lwp
+ 0.01% ptrace
+ 0.01% add_lwp
attach_proc_task_lwp_callback is called once for each LWP that we
attach to, found by listing the /proc/PID/task/ directory. In turn,
attach_proc_task_lwp_callback calls find_lwp_pid to check whether the
LWP we're about to try to attach to is already known. Since
find_lwp_pid does a linear walk over the whole LWP list, this becomes
quadratic. We do the /proc/PID/task/ listing until we get two
iterations in a row where we found no new threads. So the second and
following times we walk the /proc/PID/task/ dir, we're going to take
an even worse find_lwp_pid hit.
Fix this by adding a hash table keyed by LWP PID, for fast lookup.
The linked list embedded in the LWP structure itself is kept, and made
a double-linked list, so that removals from that list are O(1). An
earlier version of this patch got rid of this list altogether, but
that revealed hidden dependencies / assumptions on how the list is
sorted. For example, killing a process and then waiting for all the
LWPs status using iterate_over_lwps only works as is because the
leader LWP is always last in the list. So I thought it better to take
an incremental approach and make this patch concern itself _only_ with
the PID lookup optimization.
gdb/ChangeLog:
2016-05-24 Pedro Alves <palves@redhat.com>
PR gdb/19828
* linux-nat.c (lwp_lwpid_htab): New htab.
(lwp_info_hash, lwp_lwpid_htab_eq, lwp_lwpid_htab_create)
(lwp_lwpid_htab_add_lwp): New functions.
(lwp_list): Tweak comment.
(lwp_list_add, lwp_list_remove, lwp_lwpid_htab_remove_pid): New
functions.
(purge_lwp_list): Rewrite, using htab_traverse_noresize.
(add_initial_lwp): Add lwp to htab too. Use lwp_list_add.
(delete_lwp): Use lwp_list_remove. Remove htab too.
(find_lwp_pid): Search in htab.
(_initialize_linux_nat): Call lwp_lwpid_htab_create.
* linux-nat.h (struct lwp_info) <prev>: New field.
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