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

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Date:	Mon, 15 Jun 2009 23:12:09 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>, 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


* Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:

> * Ingo Molnar (mingo@...e.hu) wrote:
> > 
> > * Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:
> > 
> > > In the category "crazy ideas one should never express out loud", I 
> > > could add the following. We could choose to save/restore the cr2 
> > > register on the local stack at every interrupt entry/exit, and 
> > > therefore allow the page fault handler to execute with interrupts 
> > > enabled.
> > > 
> > > I have not benchmarked the interrupt disabling overhead of the 
> > > page fault handler handled by starting an interrupt-gated handler 
> > > rather than trap-gated handler, but cli/sti instructions are known 
> > > to take quite a few cycles on some architectures. e.g. 131 cycles 
> > > for the pair on P4, 23 cycles on AMD Athlon X2 64, 43 cycles on 
> > > Intel Core2.
> > 
> > The cost on Nehalem (1 billion local_irq_save()+restore() pairs):
> > 
> >  aldebaran:~> perf stat --repeat 5 ./prctl 0 0
> > 
> >  Performance counter stats for './prctl 0 0' (5 runs):
> > 
> >    10950.813461  task-clock-msecs     #      0.997 CPUs    ( +-   1.594% )
> >               3  context-switches     #      0.000 M/sec   ( +-   0.000% )
> >               1  CPU-migrations       #      0.000 M/sec   ( +-   0.000% )
> >             145  page-faults          #      0.000 M/sec   ( +-   0.000% )
> >     33946294720  cycles               #   3099.888 M/sec   ( +-   1.132% )
> >      8030365827  instructions         #      0.237 IPC     ( +-   0.006% )
> >          100933  cache-references     #      0.009 M/sec   ( +-  12.568% )
> >           27250  cache-misses         #      0.002 M/sec   ( +-   3.897% )
> > 
> >    10.985768499  seconds time elapsed.
> > 
> > That's 33.9 cycles per iteration, with a 1.1% confidence factor.
> > 
> > Annotation gives this result:
> > 
> >     2.24 :      ffffffff810535e5:       9c                      pushfq 
> >     8.58 :      ffffffff810535e6:       58                      pop    %rax
> >    10.99 :      ffffffff810535e7:       fa                      cli    
> >    20.38 :      ffffffff810535e8:       50                      push   %rax
> >     0.00 :      ffffffff810535e9:       9d                      popfq  
> >    46.71 :      ffffffff810535ea:       ff c6                   inc    %esi
> >     0.42 :      ffffffff810535ec:       3b 35 72 31 76 00       cmp    0x763172(%rip),%e
> >    10.69 :      ffffffff810535f2:       7c f1                   jl     ffffffff810535e5 
> >     0.00 :      ffffffff810535f4:       e9 7c 01 00 00          jmpq   ffffffff81053775 
> > 
> > i.e. pushfq+cli is roughly 42.19% or 14 cycles, the popfq is 46.71 
> > or 16 cycles. So the combo cost is 30 cycles, +- 1 cycle.
> > 
> > (Actual effective cost in a real critical section can be better than 
> > this, dependent on surrounding instructions.)
> > 
> > It got quite a bit faster than Core2 - but still not as fast as AMD.
> > 
> > 	Ingo
> 
> Interesting, but in our specific case, what would be even more 
> interesting to know is how many trap gates/s vs interrupt gates/s 
> can be called. This would allow us to see if it's worth trying to 
> make the page fault handler interrupt-safe by mean of atomicity 
> and context save/restore by interrupt handlers (which would let us 
> run the PF handler with interrupts enabled).

See the numbers in the other mail: about 33 million pagefaults 
happen in a typical kernel build - that's ~400K/sec - and that is 
not a particularly really pagefault-heavy workload.

OTOH, interrupt gates, if above 10K/second, do get noticed and get 
reduced. Above 100K/sec combined they are really painful. In 
practice, a combo limit of 10K is healthy.

So there's about an order of magnitude difference in the frequency 
of IRQs versus the frequency of pagefaults.

In the worst-case, we have 10K irqs/sec and almost zero pagefaults - 
every 10 cycles overhead in irq entry+exit cost causes a 0.003% 
total slowdown.

So i'd say that it's pretty safe to say that the shuffling of 
overhead from the pagefault path into the irq path, even if it's a 
zero-sum game as per cycles, is an overall win - or even in the 
worst-case, a negligible overhead.

Syscalls are even more critical: it's easy to have a 'good' workload 
with millions of syscalls per second - so getting even a single 
cycle off the syscall entry+exit path is worth the pain.

	Ingo
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