linux-kernel - Re: [x86/signal] 3aac3ebea0: will-it-scale.per_thread

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Date:   Tue, 7 Dec 2021 22:17:25 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     "Sang, Oliver" <oliver.sang@...el.com>,
        Borislav Petkov <bp@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "lkp@...ts.01.org" <lkp@...ts.01.org>, lkp <lkp@...el.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "Tang, Feng" <feng.tang@...el.com>,
        "zhengjun.xing@...ux.intel.com" <zhengjun.xing@...ux.intel.com>,
        "Yin, Fengwei" <fengwei.yin@...el.com>
Subject: Re: [x86/signal]  3aac3ebea0:  will-it-scale.per_thread_ops -11.9%
 regression

On Dec 7, 2021, at 12:36, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Tue, Dec 07 2021 at 18:49, Chang Seok Bae wrote:
>> On Dec 7, 2021, at 05:38, Thomas Gleixner <tglx@...utronix.de> wrote:
>>> 
>>> Does that use sigaltstack() ?
>> 
>> FWIW, I was also wondering about this with:
>> 
>> $ git clone https://github.com/antonblanchard/will-it-scale.git
>> $ cd will-it-scale/
>> $ git grep sigaltstack
>> $
>> 
>> But, the test seems to use python via runtest.py. And the python code has
>> sigaltstack():
>>    https://github.com/python/cpython/blob/main/Modules/faulthandler.c#L454
> 
> But how does that affect the test written in C? Mysterious!

Indeed, I can only see the sigaltstack() trace via the python script. 

$ strace -f python3.7 ./runtest.py signal1 1>out 2>err
$ grep -r sigaltstack err
sigaltstack({ss_sp=0xe13f50, ss_flags=0, ss_size=16384}, {ss_sp=NULL,
ss_flags=SS_DISABLE, ss_size=0}) = 0
…

$ strace -f ./signal1_processes 1>out 2>err
$ grep -r sigaltstack err
$ strace -f ./signal1_threads 1>out 2>err
$ grep -r sigaltstack err
$

I don’t get how this syscall could contribute 11% degradation in this test.

BTW, the current code rejects the reported instruction here:

$ python3.7 ./runtest.py signal1 295 thread 16
Usage: runtest.py <testcase>

Thanks,
Chang