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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:
    >
    > > 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).

    Mathieu



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


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