linux-kernel - Re: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process

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Date:   Mon, 24 Feb 2020 10:19:15 +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

On Sun, Feb 23, 2020 at 05:06:33PM -0800, Linus Torvalds wrote:
 
> >      ffffffff8225b580 d types__ptrace
> >      ffffffff8225b5c0 D root_user
> >      ffffffff8225b680 D init_user_ns
> 
> I'm assuming this is after the alignment patch (since that's 64-byte
> aligned there).
> 
> What was it without the alignment?

For 5.0-rc6: 
	ffffffff8225b4c0 d types__ptrace
	ffffffff8225b4e0 D root_user
	ffffffff8225b580 D init_user_ns

For 5.0-rc6 + 81ec3f3c4c4
	ffffffff8225b580 d types__ptrace
	ffffffff8225b5a0 D root_user
	ffffffff8225b640 D init_user_ns

The sigpending and __count are in the same cachline.

> 
> > 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 :)
> 
> Hmm.
> 
> Humor me - what  happens if you turn off SMT on that Cascade Lake
> system?  Maybe it's about the thread ID bit in the L1? Although again,
> I'd have expected things to get _worse_ if it's the two fields that
> are now in the same cachline thanks to alignment.

I'll try it and report back.
 
> The Xeon Phi is the small-core setup, right? They may be slow enough
> to not show the issue as clearly despite having more cores. And it
> wouldn't show effects of some out-of-order speculative cache accesses.

Yes, seems the Xeon Phi is using 72 Silvermont cores. And the less bigger
platform I tested was a 2 sockets 48C/96T Cascadelake platform which
doesn't reproduce the regression.

Thanks,
Feng