linux-kernel - Re: 08ed4efad6: stress-ng.sigsegv.ops_per

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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