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

[Alexey Gladkov]

On Thu, Apr 08, 2021 at 01:44:43PM -0500, Eric W. Biederman wrote:
> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:
> 
> > On Thu, Apr 8, 2021 at 1:32 AM kernel test robot <oliver.sang@xxxxxxxxx> 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.
> 
> We were cautiously optimistic when no test problems showed up from
> the last posting that there was nothing to look at here.
> 
> Unfortunately it looks like the bots just missed the last posting. 
> 
> So it seems we are finally pretty much at correct code in need
> of performance tuning.
> 
> > 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).
> 
> This is an interesting test in a lot of ways as it is testing the
> synchronous signal delivery path caused by an exception.  The test
> is either executing *ptr = 0 (where ptr points to a read-only page)
> or it executes an x86 instruction that is excessively long.
> 
> I have found the code but I haven't figured out how it is being
> called yet.  The core loop is just:
> 	for(;;) {
> 		sigaction(SIGSEGV, &action, NULL);
> 		sigaction(SIGILL, &action, NULL);
> 		sigaction(SIGBUS, &action, NULL);
> 
> 		ret = sigsetjmp(jmp_env, 1);
> 		if (done())
>                 	break;
> 		if (ret) {
>                 	/* verify signal */
>                 } else {
>                 	*ptr = 0;
>                 }
> 	}
> 
> Code like that fundamentally can not be multi-threaded.  So the only way
> the sigpending limit is being hit is if there are more processes running
> that code simultaneously than the size of the limit.
> 
> Further it looks like stress-ng pushes RLIMIT_SIGPENDING as high as it
> will go before the test starts.
> 
> 
> > 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. That seems a likely culprit.
> 
> > 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.
> 
> That too is possible.
> 
> That node-load-misses number does look like something is bouncing back
> and forth between the nodes a lot more.  So I suspect stress-ng is
> running multiple copies of the sigsegv test in different processes at
> once.
> 
> 
> 
> That really suggests cache line ping pong from get_ucounts and
> incrementing sigpending.
> 
> It surprises me that obtaining the cache lines exclusively is
> the dominant cost on this code path but obtaining two cache lines
> exclusively instead of one cache cache line exclusively is consistent
> with a causing the exception delivery to take nearly twice as long.
> 
> For the optimization we only care about the leaf count so with a little
> care we can restore the optimization.  So that is probably the thing
> to do here.  The fewer changes to worry about the less likely to find
> surprises.
> 
> 
> 
> That said for this specific case there is a lot of potential room for
> improvement.  As this is a per thread signal the code update sigpending
> in commit_cred and never worry about needing to pin the struct
> user_struct or struct ucounts.  As this is a synchronous signal we could
> skip the sigpending increment, skip the signal queue entirely, and
> deliver the signal to user-space immediately.  The removal of all cache
> ping pongs might make it worth it.
> 
> There is also Thomas Gleixner's recent optimization to cache one
> sigqueue entry per task to give more predictable behavior.  That
> would remove the cost of the allocation.

https://git.kernel.org/pub/scm/linux/kernel/git/legion/linux.git/commit/?h=patchset/per-userspace-rlimit/v11.1&id=08db0c814926c6f16e08de99b2de34c8b5ff68ce

You mean something like this ? I did it on top of Thomas Gleixner's
patches.

-- 
Rgrds, legion