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Message-ID: <CAHk-=wigPx+MMQMQ-7EA0pq5_5+kMCNV4qFsOss-WwdCSQmb-w@mail.gmail.com>
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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