linux-kernel - Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods

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Message-ID: <20090615195514.GA18436@elte.hu>
Date:	Mon, 15 Jun 2009 21:55:14 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>, mingo@...hat.com,
	hpa@...or.com, paulus@...ba.org, acme@...hat.com,
	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl,
	penberg@...helsinki.fi, vegard.nossum@...il.com, efault@....de,
	jeremy@...p.org, npiggin@...e.de, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain
	support to use NMI-safe methods


btw., here's the cost analysis of cr2 reading and writing (in a 
tight loop). I've executed cr2 read+write instructions 1 billion 
times on a Nehalem box:

static long cr2_test(void)
{
	unsigned long tmp = 0;
	int i;

	for (i = 0; i < 1000000000; i++)
		asm("movq %0, %%cr2; movq %%cr2, %0" : : "r" (tmp));

	return 0;
}

Which gave these overall stats:

 Performance counter stats for './prctl 0 0':

   28414.696319  task-clock-msecs     #      0.997 CPUs 
              3  context-switches     #      0.000 M/sec
              1  CPU-migrations       #      0.000 M/sec
            149  page-faults          #      0.000 M/sec
    87254432334  cycles               #   3070.750 M/sec
     5078691161  instructions         #      0.058 IPC  
         304144  cache-references     #      0.011 M/sec
          28760  cache-misses         #      0.001 M/sec

   28.501962853  seconds time elapsed.

87254432334/1000000000 ~== 87, so we have 87 cycles cost per 
iteration.

The annotated output shows:

 aldebaran:~> perf annotate sys_prctl | grep -A 2 cr2

    0.42 :	ffffffff81053131:	0f 22 d1             	mov    %rcx,%cr2
   96.56 :	ffffffff81053134:	0f 20 d1             	mov    %cr2,%rcx
    3.02 :	ffffffff81053137:	ff c0                	inc    %eax
    0.00 :	ffffffff81053139:	39 d0                	cmp    %edx,%eax

the read/write cost ratio is 3%:96.5% (with skidding taken into 
account), that suggests that the reading cost of cr2 is about 2-3 
cycles, the writing cost is about 85 cycles.

Which makes sense - reading cr2 is in the pagefault critical path, 
so that's optimized. Writing it is allowed but not optimized at all. 
(especially in such a tight loop where it could easily have some 
back-to-back additional latency that would not be there in an NMI 
handler save/restore path which has other instructions inbetween.)

	Ingo
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