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Message-ID: <20210428150952.mdnvl7i4kimgwswh@example.org>
Date: Wed, 28 Apr 2021 17:09:52 +0200
From: Alexey Gladkov <legion@...nel.org>
To: Oliver Sang <oliver.sang@...el.com>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
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>,
Andrew Morton <akpm@...ux-foundation.org>,
Christian Brauner <christian.brauner@...ntu.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 Wed, Apr 28, 2021 at 10:36:40PM +0800, Oliver Sang wrote:
> hi, Alexey Gladkov,
>
> On Fri, Apr 23, 2021 at 09:44:31AM +0200, Alexey Gladkov wrote:
> > On Fri, Apr 23, 2021 at 10:47:22AM +0800, Oliver Sang wrote:
> > > hi, Eric,
> > >
> > > On Thu, Apr 08, 2021 at 01:44:43PM -0500, Eric W. Biederman wrote:
> > > > Linus Torvalds <torvalds@...ux-foundation.org> writes:
> > > >
> > > > > 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.
> > > >
> > > > 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.
> > >
> > > this report is upon v10. do you have newer version which hope bot test?
> >
> > Yes. I posted a new version of this patch set. I would be very grateful if
> > you could test it.
> >
> > https://lore.kernel.org/lkml/cover.1619094428.git.legion@kernel.org/
> >
>
> we tested this v11 version, and found the regression reduced to about 1.6%.
> please be noted, according to our previous experience, the stress-ng is
> kind of sensitive testsuite, so we normally wouldn't report <3% regression.
Thank you very much for testing and good news for me !!!
Do you have a place where its possible to see if the patch has been tested?
I mean test passed or not.
> =========================================================================================
> class/compiler/cpufreq_governor/disk/kconfig/nr_threads/rootfs/tbox_group/test/testcase/testtime/ucode:
> interrupt/gcc-9/performance/1HDD/x86_64-rhel-8.3/100%/debian-10.4-x86_64-20200603.cgz/lkp-ivb-2ep1/sigsegv/stress-ng/60s/0x42e
>
> commit:
> 00a58a6af1c4 ("Reimplement RLIMIT_MSGQUEUE on top of ucounts")
> 8932738fc10c ("Reimplement RLIMIT_SIGPENDING on top of ucounts")
>
> 00a58a6af1c473c5 8932738fc10c4398521892adfe6
> ---------------- ---------------------------
> %stddev %change %stddev
> \ | \
> 4.745e+08 -1.6% 4.669e+08 stress-ng.sigsegv.ops
> 7908964 -1.6% 7781343 stress-ng.sigsegv.ops_per_sec
>
> Below is some data of results from your new branch and base.
> b3ad8e1fa3fd8 ucounts: Set ucount_max to the largest positive value the type can hold 7783421.61 7794441.59 7775793.52 7773683.6 7760744.1 7757720.33
> 8932738fc10c4 Reimplement RLIMIT_SIGPENDING on top of ucounts 7755985.06 7780646.72 7783944.12 7809090.98 7798193.32 7760202.59
> 00a58a6af1c47 Reimplement RLIMIT_MSGQUEUE on top of ucounts 7940474.72 7912442.26 7879195.61 7869803.63 7912693.69 7939175.48
> e75074781f173 selftests/resctrl: Change a few printed messages 7660254.5 7676124.45 7745330.79 7736754.88 7716834.93 7660143.13
> 87f1c20e2effd Documentation: kselftest: fix path to test module files 7729609.16 7726906.92 7760819.26
> 06bd03a57f8c2 selftests/resctrl: Fix MBA/MBM results reporting format 7692866.06 7730606.11 7681414.48
> a38fd87484648 Linux 5.12-rc2 7724932.06
>
>
> > > please be noted, sorry to say, due to various reasons, it will be a
> > > big challenge for us to capture each version of a patch set.
> > >
> > > e.g. we didn't make out a similar performance regression for
> > > v8/v9 version of this one..
> > >
> > > >
> > > > 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.
> > > >
> > > > Eric
> > >
> >
> > --
> > Rgrds, legion
> >
>
--
Rgrds, legion
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