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

Linus Torvalds
2021-04-08T16:23:07Z

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