linux-kernel - Re: perf-stat changes after "Use hrtimers for event multiplexing"

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Date:	Sat, 4 Jan 2014 20:02:28 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	fengguang.wu@...el.com
Cc:	Stephane Eranian <eranian@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, lkp@...ux.intel.com
Subject: Re: perf-stat changes after "Use hrtimers for event multiplexing"

On Thu, Jan 02, 2014 at 02:12:42PM +0800, fengguang.wu@...el.com wrote:
> Greetings,
> 
> We noticed many perf-stat changes between commit 9e6302056f ("perf: Use
> hrtimers for event multiplexing") and its parent commit ab573844e.
> Are these expected changes?
> 
> ab573844e3058ee  9e6302056f8029f438e853432
> ---------------  -------------------------
>     152917         +842.9%    1441897       TOTAL interrupts.0:IO-APIC-edge.timer
>     545996         +478.0%    3155637       TOTAL interrupts.LOC
>     182281          +12.3%     204718       TOTAL softirqs.SCHED
>  1.986e+08          -96.4%    7105919       TOTAL perf-stat.node-store-misses
>  107241719          -99.7%     317525       TOTAL perf-stat.node-prefetch-misses
>  1.938e+08          -90.7%   17930426       TOTAL perf-stat.node-load-misses
>       2590         +247.8%       9009       TOTAL vmstat.system.in
>  4.549e+12         +158.3%  1.175e+13       TOTAL perf-stat.stalled-cycles-backend
>  6.807e+12         +149.1%  1.696e+13       TOTAL perf-stat.stalled-cycles-frontend
>  1.753e+08          -50.8%   86339289       TOTAL perf-stat.node-prefetches
>  8.326e+11          +45.0%  1.207e+12       TOTAL perf-stat.cpu-cycles
>   37932143          +32.2%   50146025       TOTAL perf-stat.iTLB-load-misses
>  4.738e+11          +30.1%  6.165e+11       TOTAL perf-stat.iTLB-loads
>   2.56e+11          +30.1%   3.33e+11       TOTAL perf-stat.L1-icache-loads
>  4.951e+11          +24.6%  6.169e+11       TOTAL perf-stat.instructions
>   7.85e+08           +7.5%  8.439e+08       TOTAL perf-stat.LLC-prefetch-misses
>  1.891e+12          +22.8%  2.322e+12       TOTAL perf-stat.ref-cycles
>  4.344e+08          -20.3%  3.462e+08       TOTAL perf-stat.node-loads
>  2.836e+11          +17.4%  3.328e+11       TOTAL perf-stat.branch-loads
>  9.506e+10          +24.5%  1.183e+11       TOTAL perf-stat.branch-load-misses
>  2.803e+11          +18.4%  3.319e+11       TOTAL perf-stat.branch-instructions
>  7.988e+10          +20.9%  9.658e+10       TOTAL perf-stat.bus-cycles
>  2.041e+09          +22.2%  2.495e+09       TOTAL perf-stat.branch-misses
>     229145          -17.3%     189601       TOTAL perf-stat.cpu-migrations
>  1.782e+11          +17.9%    2.1e+11       TOTAL perf-stat.dTLB-loads
>  4.702e+08          -14.8%  4.006e+08       TOTAL perf-stat.LLC-load-misses
>  1.418e+11          +17.4%  1.666e+11       TOTAL perf-stat.L1-dcache-loads
>  1.838e+09          +16.1%  2.133e+09       TOTAL perf-stat.LLC-stores
>  2.428e+09          +11.3%  2.702e+09       TOTAL perf-stat.LLC-loads
>  2.788e+11           +8.6%  3.029e+11       TOTAL perf-stat.dTLB-stores
>   8.66e+08          +10.8%  9.594e+08       TOTAL perf-stat.LLC-prefetches
>  1.117e+09          +10.5%  1.234e+09       TOTAL perf-stat.dTLB-store-misses
>  1.705e+09           +5.3%  1.796e+09       TOTAL perf-stat.L1-dcache-store-misses
>  5.671e+09           +6.1%  6.015e+09       TOTAL perf-stat.L1-dcache-load-misses
>  8.794e+10           +3.6%  9.109e+10       TOTAL perf-stat.L1-dcache-stores
>   3.46e+09           +4.6%  3.618e+09       TOTAL perf-stat.cache-references
>  8.696e+08           +1.8%  8.849e+08       TOTAL perf-stat.cache-misses
>    1613129           +2.6%    1655724       TOTAL perf-stat.context-switches
> 
> All of the changes happen in one of our test box, which has a DX58SO
> baseboard and 4-core CPU. The boot dmesg and kconfig are attached.
> We can test more boxes if necessary.

How do you run perf stat? Curious that you notice this now, its a fairly
old commit.

IIRC we did have a few wobbles with that, but I cannot remember much
detail.

The biggest difference between before and after that patch is that we'd
rotate while the core is 'idle'. So if you do something like 'perf stat
-a' and have significant idle time it does indeed make a difference.
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