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
