linux-kernel - Re: [RFC PATCH v2 00/17] Core scheduling v2

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Date:   Tue, 30 Apr 2019 06:42:50 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Aubrey Li <aubrey.intel@...il.com>
Cc:     "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Vineeth Remanan Pillai <vpillai@...italocean.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Turner <pjt@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Subhra Mazumdar <subhra.mazumdar@...cle.com>,
        Frédéric Weisbecker <fweisbec@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kerr <kerrnel@...gle.com>, Phil Auld <pauld@...hat.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC PATCH v2 00/17] Core scheduling v2


* Aubrey Li <aubrey.intel@...il.com> wrote:

> On Tue, Apr 30, 2019 at 12:01 AM Ingo Molnar <mingo@...nel.org> wrote:
> > * Li, Aubrey <aubrey.li@...ux.intel.com> wrote:
> >
> > > > I.e. showing the approximate CPU thread-load figure column would be
> > > > very useful too, where '50%' shows half-loaded, '100%' fully-loaded,
> > > > '200%' over-saturated, etc. - for each row?
> > >
> > > See below, hope this helps.
> > > .--------------------------------------------------------------------------------------------------------------------------------------.
> > > |NA/AVX vanilla-SMT     [std% / sem%]     cpu% |coresched-SMT   [std% / sem%]     +/-     cpu% |  no-SMT [std% / sem%]   +/-      cpu% |
> > > |--------------------------------------------------------------------------------------------------------------------------------------|
> > > |  1/1        508.5     [ 0.2%/ 0.0%]     2.1% |        504.7   [ 1.1%/ 0.1%]    -0.8%    2.1% |   509.0 [ 0.2%/ 0.0%]   0.1%     4.3% |
> > > |  2/2       1000.2     [ 1.4%/ 0.1%]     4.1% |       1004.1   [ 1.6%/ 0.2%]     0.4%    4.1% |   997.6 [ 1.2%/ 0.1%]  -0.3%     8.1% |
> > > |  4/4       1912.1     [ 1.0%/ 0.1%]     7.9% |       1904.2   [ 1.1%/ 0.1%]    -0.4%    7.9% |  1914.9 [ 1.3%/ 0.1%]   0.1%    15.1% |
> > > |  8/8       3753.5     [ 0.3%/ 0.0%]    14.9% |       3748.2   [ 0.3%/ 0.0%]    -0.1%   14.9% |  3751.3 [ 0.4%/ 0.0%]  -0.1%    30.5% |
> > > | 16/16      7139.3     [ 2.4%/ 0.2%]    30.3% |       7137.9   [ 1.8%/ 0.2%]    -0.0%   30.3% |  7049.2 [ 2.4%/ 0.2%]  -1.3%    60.4% |
> > > | 32/32     10899.0     [ 4.2%/ 0.4%]    60.3% |      10780.3   [ 4.4%/ 0.4%]    -1.1%   55.9% | 10339.2 [ 9.6%/ 0.9%]  -5.1%    97.7% |
> > > | 64/64     15086.1     [11.5%/ 1.2%]    97.7% |      14262.0   [ 8.2%/ 0.8%]    -5.5%   82.0% | 11168.7 [22.2%/ 1.7%] -26.0%   100.0% |
> > > |128/128    15371.9     [22.0%/ 2.2%]   100.0% |      14675.8   [14.4%/ 1.4%]    -4.5%   82.8% | 10963.9 [18.5%/ 1.4%] -28.7%   100.0% |
> > > |256/256    15990.8     [22.0%/ 2.2%]   100.0% |      12227.9   [10.3%/ 1.0%]   -23.5%   73.2% | 10469.9 [19.6%/ 1.7%] -34.5%   100.0% |
> > > '--------------------------------------------------------------------------------------------------------------------------------------'
> >
> > Very nice, thank you!
> >
> > What's interesting is how in the over-saturated case (the last three
> > rows: 128, 256 and 512 total threads) coresched-SMT leaves 20-30% CPU
> > performance on the floor according to the load figures.
> 
> Yeah, I found the next focus.
> 
> > Is this true idle time (which shows up as 'id' during 'top'), or some 
> > load average artifact?
> 
> vmstat periodically reported intermediate CPU utilization in one 
> second, it was running simultaneously when the benchmarks run. The cpu% 
> is computed by the average of (100-idle) series.

Ok - so 'vmstat' uses /proc/stat, which uses cpustat[CPUTIME_IDLE] (or 
its NOHZ work-alike), so this should be true idle time - to the extent 
the HZ process clock's sampling is accurate.

So I guess the answer to my question is "yes". ;-)

BTW., for robustness sake you might want to add iowait to idle time (it's 
the 'wa' field of vmstat) - it shouldn't matter for this particular 
benchmark which doesn't do much IO, but it might for others.

Both CPUTIME_IDLE and CPUTIME_IOWAIT are idle states when a CPU is not 
utilized.

[ Side note: we should really implement precise idle time accounting when 
  CONFIG_IRQ_TIME_ACCOUNTING=y is enabled. We pay all the costs of the 
  timestamps, but AFAICS we don't propagate that into the idle cputime
  metrics. ]

Thanks,

	Ingo