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Message-ID: <20200224003301.GA5061@shbuild999.sh.intel.com>
Date: Mon, 24 Feb 2020 08:33:01 +0800
From: Feng Tang <feng.tang@...el.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Jiri Olsa <jolsa@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
kernel test robot <rong.a.chen@...el.com>,
Ingo Molnar <mingo@...nel.org>,
Vince Weaver <vincent.weaver@...ne.edu>,
Jiri Olsa <jolsa@...nel.org>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Arnaldo Carvalho de Melo <acme@...hat.com>,
"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
Ravi Bangoria <ravi.bangoria@...ux.ibm.com>,
Stephane Eranian <eranian@...gle.com>,
Thomas Gleixner <tglx@...utronix.de>,
LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
andi.kleen@...el.com, "Huang, Ying" <ying.huang@...el.com>
Subject: Re: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops
-5.5% regression
Hi Linus,
On Sun, Feb 23, 2020 at 09:37:06AM -0800, Linus Torvalds wrote:
> On Sun, Feb 23, 2020 at 6:11 AM Feng Tang <feng.tang@...el.com> wrote:
> >
> > I tried to use perf-c2c on one platform (not the one that show
> > the 5.5% regression), and found the main "hitm" points to the
> > "root_user" global data, as there is a task for each CPU doing
> > the signal stress test, and both __sigqueue_alloc() and
> > __sigqueue_free() will call get_user() and free_uid() to inc/dec
> > this root_user's refcount.
>
> What's around it for you?
>
> There might be that 'uidhash_lock' spinlock right next to it, and
> maybe that exacerbates the issue?
The system map shows:
ffffffff8225b520 d __syscall_meta__ptrace
ffffffff8225b560 d args__ptrace
ffffffff8225b580 d types__ptrace
ffffffff8225b5c0 D root_user
ffffffff8225b680 D init_user_ns
ffffffff8225b880 d ratelimit_state.56624
ffffffff8225b8c0 d event_exit__sigsuspend
I also searched the uidhack_lock,
ffffffff82b04c80 b uidhash_lock
> > Then I added some alignement inside struct "user_struct" (for
> > "root_user"), then the -5.5% is gone, with a +2.6% instead.
>
> Do you actually need to align things inside the struct, or is it
> sufficient to just align the structure itself?
Initially I justed add the ____cacheline_aligned after the definition
of struct 'user_struct', which only makes the following 'init_user_ns'
aligned, and test result doesn't show much change.
Then I added
struct user_struct {
+ char dummy[0] ____cacheline_aligned;
to make 'root_user' itself aligned.
> IOW, is the cache conflicts _within_ the user_struct itself, or is it
> with some nearby data (like that uidhash_lock or whatever?)
>From the perf c2c data, and the source code checking, the conflicts
only happens for root_user.__count, and root_user.sigpending, as
all running tasks are accessing this global data for get/put and
other operations.
> > One thing I don't understand is, this -5.5% only happens in
> > one 2 sockets, 96C/192T Cascadelake platform, as we've run
> > the same test on several different platforms. In therory,
> > the false sharing may also take effect?
>
> Is that the biggest machine you have access to?
>
> Maybe it just isn't noticeable with smaller core counts. A lot of
> conflict loads tend to have "exponential" behavior - when things get
> overloaded, performance plummets because it just makes things worse as
> everybody gets slower at that contention point and now it gets even
> more contended...
No, it's not the biggest, I tried another machine 'Xeon Phi(TM) CPU 7295',
which has 72C/288T, and the regression is not seen. This is the part
confusing me :)
Also I've tried to limit the concurrent task number from 192 to 96/48/24/6/1,
and the regression number did get smaller following the task decrease.
Thanks,
Feng
> Linus
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