From: "Huang\, Ying" <ying.huang@intel.com>
To: Feng Tang <feng.tang@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Jiri Olsa <jolsa@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
kernel test robot <rong.a.chen@intel.com>,
Ingo Molnar <mingo@kernel.org>,
Vince Weaver <vincent.weaver@maine.edu>,
Jiri Olsa <jolsa@kernel.org>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
"Arnaldo Carvalho de Melo" <acme@kernel.org>,
Arnaldo Carvalho de Melo <acme@redhat.com>,
"Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
Ravi Bangoria <ravi.bangoria@linux.ibm.com>,
Stephane Eranian <eranian@google.com>,
"Thomas Gleixner" <tglx@linutronix.de>,
LKML <linux-kernel@vger.kernel.org>, <lkp@lists.01.org>,
<andi.kleen@intel.com>
Subject: Re: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops -5.5% regression
Date: Mon, 24 Feb 2020 09:58:09 +0800 [thread overview]
Message-ID: <87lfosd9vy.fsf@yhuang-dev.intel.com> (raw)
In-Reply-To: <CAHk-=whi87NNOnNXJ6CvyyedmhnS8dZA2YkQQSajvBArH5XOeA@mail.gmail.com> (Linus Torvalds's message of "Sun, 23 Feb 2020 17:06:33 -0800")
Linus Torvalds <torvalds@linux-foundation.org> writes:
> On Sun, Feb 23, 2020 at 4:33 PM Feng Tang <feng.tang@intel.com> wrote:
>>
>> 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.
>
> That's odd.
>
> Why? Because those two would be guaranteed to be in the same cacheline
> _after_ you've aligned that user_struct.
>
> So if it were a false sharing issue between those two, it would
> actually get _worse_ with alignment. Those two fields are basically
> next to each other.
>
> But maybe it was straddling a cacheline before, and it caused two
> cache accesses each time?
>
> I find this as confusing as you do.
>
> If it's sigpending vs the __refcount, then we almost always change
> them together. sigpending gets incremented by __sigqueue_alloc() -
> which also does a "get_uid()", and then we decrement it in
> __sigqueue_free() - which also does a "free_uid().
>
One way to verify this is to change the layout of user_struct (or
root_user) to make __count and sigpending fields to be in 2 separate
cache lines explicitly.
Best Regards,
Huang, Ying
WARNING: multiple messages have this Message-ID (diff)
From: Huang, Ying <ying.huang@intel.com>
To: lkp@lists.01.org
Subject: Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops -5.5% regression
Date: Mon, 24 Feb 2020 09:58:09 +0800 [thread overview]
Message-ID: <87lfosd9vy.fsf@yhuang-dev.intel.com> (raw)
In-Reply-To: <CAHk-=whi87NNOnNXJ6CvyyedmhnS8dZA2YkQQSajvBArH5XOeA@mail.gmail.com>
[-- Attachment #1: Type: text/plain, Size: 1268 bytes --]
Linus Torvalds <torvalds@linux-foundation.org> writes:
> On Sun, Feb 23, 2020 at 4:33 PM Feng Tang <feng.tang@intel.com> wrote:
>>
>> 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.
>
> That's odd.
>
> Why? Because those two would be guaranteed to be in the same cacheline
> _after_ you've aligned that user_struct.
>
> So if it were a false sharing issue between those two, it would
> actually get _worse_ with alignment. Those two fields are basically
> next to each other.
>
> But maybe it was straddling a cacheline before, and it caused two
> cache accesses each time?
>
> I find this as confusing as you do.
>
> If it's sigpending vs the __refcount, then we almost always change
> them together. sigpending gets incremented by __sigqueue_alloc() -
> which also does a "get_uid()", and then we decrement it in
> __sigqueue_free() - which also does a "free_uid().
>
One way to verify this is to change the layout of user_struct (or
root_user) to make __count and sigpending fields to be in 2 separate
cache lines explicitly.
Best Regards,
Huang, Ying
next prev parent reply other threads:[~2020-02-24 1:58 UTC|newest]
Thread overview: 57+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-05 12:32 [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops -5.5% regression kernel test robot
2020-02-05 12:32 ` kernel test robot
2020-02-05 12:58 ` Peter Zijlstra
2020-02-05 12:58 ` Peter Zijlstra
2020-02-06 3:04 ` [LKP] " Li, Philip
2020-02-06 3:04 ` Li, Philip
2020-02-21 8:03 ` [LKP] " Feng Tang
2020-02-21 8:03 ` Feng Tang
2020-02-21 10:58 ` [LKP] " Peter Zijlstra
2020-02-21 10:58 ` Peter Zijlstra
2020-02-21 13:20 ` [LKP] " Jiri Olsa
2020-02-21 13:20 ` Jiri Olsa
2020-02-23 14:11 ` [LKP] " Feng Tang
2020-02-23 14:11 ` Feng Tang
2020-02-23 17:37 ` [LKP] " Linus Torvalds
2020-02-23 17:37 ` Linus Torvalds
2020-02-24 0:33 ` [LKP] " Feng Tang
2020-02-24 0:33 ` Feng Tang
2020-02-24 1:06 ` [LKP] " Linus Torvalds
2020-02-24 1:06 ` Linus Torvalds
2020-02-24 1:58 ` Huang, Ying [this message]
2020-02-24 1:58 ` Huang, Ying
2020-02-24 2:19 ` [LKP] " Feng Tang
2020-02-24 2:19 ` Feng Tang
2020-02-24 13:20 ` [LKP] " Feng Tang
2020-02-24 13:20 ` Feng Tang
2020-02-24 19:24 ` [LKP] " Linus Torvalds
2020-02-24 19:24 ` Linus Torvalds
2020-02-24 19:42 ` [LKP] " Kleen, Andi
2020-02-24 19:42 ` Kleen, Andi
2020-02-24 20:09 ` [LKP] " Linus Torvalds
2020-02-24 20:09 ` Linus Torvalds
2020-02-24 20:47 ` [LKP] " Linus Torvalds
2020-02-24 20:47 ` Linus Torvalds
2020-02-24 21:20 ` [LKP] " Eric W. Biederman
2020-02-24 21:20 ` Eric W. Biederman
2020-02-24 21:43 ` [LKP] " Linus Torvalds
2020-02-24 21:43 ` Linus Torvalds
2020-02-24 21:59 ` [LKP] " Eric W. Biederman
2020-02-24 21:59 ` Eric W. Biederman
2020-02-24 22:12 ` [LKP] " Linus Torvalds
2020-02-24 22:12 ` Linus Torvalds
2020-02-25 2:57 ` [LKP] " Feng Tang
2020-02-25 2:57 ` Feng Tang
2020-02-25 3:15 ` [LKP] " Linus Torvalds
2020-02-25 3:15 ` Linus Torvalds
2020-02-25 4:53 ` [LKP] " Feng Tang
2020-02-25 4:53 ` Feng Tang
2020-02-23 19:36 ` [LKP] " Jiri Olsa
2020-02-23 19:36 ` Jiri Olsa
2020-02-24 1:14 ` Feng Tang
2020-02-21 18:05 ` [LKP] " Kleen, Andi
2020-02-21 18:05 ` Kleen, Andi
2020-02-22 12:43 ` [LKP] " Feng Tang
2020-02-22 12:43 ` Feng Tang
2020-02-22 17:08 ` [LKP] " Kleen, Andi
2020-02-22 17:08 ` Kleen, Andi
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