Date: Thu, 8 Apr 2021 09:22:40 -0700 From: Linus Torvalds <torvalds@...ux-foundation.org> To: kernel test robot <oliver.sang@...el.com> Cc: Alexey Gladkov <gladkov.alexey@...il.com>, 0day robot <lkp@...el.com>, LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org, "Huang, Ying" <ying.huang@...el.com>, Feng Tang <feng.tang@...el.com>, zhengjun.xing@...el.com, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Linux Containers <containers@...ts.linux-foundation.org>, Linux-MM <linux-mm@...ck.org>, Alexey Gladkov <legion@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, Christian Brauner <christian.brauner@...ntu.com>, "Eric W . Biederman" <ebiederm@...ssion.com>, Jann Horn <jannh@...gle.com>, Jens Axboe <axboe@...nel.dk>, Kees Cook <keescook@...omium.org>, Oleg Nesterov <oleg@...hat.com> Subject: Re: 08ed4efad6: stress-ng.sigsegv.ops_per_sec -41.9% regression On Thu, Apr 8, 2021 at 1:32 AM kernel test robot <oliver.sang@...el.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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