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SubjectRe: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods
* Ingo Molnar (mingo@elte.hu) wrote:
>
> * Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
>
> > * Ingo Molnar (mingo@elte.hu) wrote:
> > >
> > > * Mathieu Desnoyers <mathieu.desnoyers@polymtl.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

I fully agree with what you say here Ingo, but then I think I must make
my main point a bit more clear :

Trap handlers are currently defined as "interrupt gates" rather than
trap gates, so interrupts are disabled starting from the moment the page
fault is generated. This is done, as Linus said, to protect the content
of the cr2 register from being messed up by interrupts. However, if we
choose to save the cr2 register around irq handler execution, we could
turn the page fault handler into a "real" trap gate (with interrupts
on).

Given I think, just like you, that we must save cycles on the PF handler
path, it would be interesting to see if there is a performance gain to
get by switching the pf handler from interrupt gate to trap gate.

So the test would be :

traps.c: set_intr_gate(14, &page_fault);

changed for something like a set_trap_gate.

But we should make sure to save the cr2 register upon interrupt/NMI
entry and restore it upon int/NMI exit.

Mathieu

--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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