Messages in this thread | | | From | Linus Torvalds <> | Date | Thu, 8 Apr 2021 09:22:40 -0700 | Subject | Re: 08ed4efad6: stress-ng.sigsegv.ops_per_sec -41.9% regression |
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On Thu, Apr 8, 2021 at 1:32 AM kernel test robot <oliver.sang@intel.com> wrote: > > FYI, we noticed a -41.9% regression of stress-ng.sigsegv.ops_per_sec due to commit > 08ed4efad684 ("[PATCH v10 6/9] Reimplement RLIMIT_SIGPENDING on top of ucounts")
Ouch.
I *think* this test may be testing "send so many signals that it triggers the signal queue overflow case".
And I *think* that the performance degradation may be due to lots of unnecessary allocations, because ity looks like that commit changes __sigqueue_alloc() to do
struct sigqueue *q = kmem_cache_alloc(sigqueue_cachep, flags);
*before* checking the signal limit, and then if the signal limit was exceeded, it will just be free'd instead.
The old code would check the signal count against RLIMIT_SIGPENDING *first*, and if there were m ore pending signals then it wouldn't do anything at all (including not incrementing that expensive atomic count).
Also, the old code was very careful to only do the "get_user()" for the *first* signal it added to the queue, and do the "put_user()" for when removing the last signal. Exactly because those atomics are very expensive.
The new code just does a lot of these atomics unconditionally.
I dunno. The profile data in there is a bit hard to read, but there's a lot more cachee misses, and a *lot* of node crossers:
> 5961544 +190.4% 17314361 perf-stat.i.cache-misses > 22107466 +119.2% 48457656 perf-stat.i.cache-references > 163292 ą 3% +4582.0% 7645410 perf-stat.i.node-load-misses > 227388 ą 2% +3708.8% 8660824 perf-stat.i.node-loads
and (probably as a result) average instruction costs have gone up enormously:
> 3.47 +66.8% 5.79 perf-stat.overall.cpi > 22849 -65.6% 7866 perf-stat.overall.cycles-between-cache-misses
and it does seem to be at least partly about "put_ucounts()":
> 0.00 +4.5 4.46 perf-profile.calltrace.cycles-pp.put_ucounts.__sigqueue_free.get_signal.arch_do_signal_or_restart.exit_to_user_mode_prepare
and a lot of "get_ucounts()".
But it may also be that the new "get sigpending" is just *so* much more expensive than it used to be.
Linus
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