08ed4efad6: stress-ng.sigsegv.ops_per_sec -41.9% regression

[Alexey Gladkov]

On Thu, Apr 08, 2021 at 09:22:40AM -0700, Linus Torvalds wrote:
> On Thu, Apr 8, 2021 at 1:32 AM kernel test robot <oliver.sang at 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.

Yes and right now I'm trying to rewrite this patch.

> 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.

Thanks for decrypting this! I spent some time to understand this report
and still wasn't sure I understood it.

-- 
Rgrds, legion