Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chainsupport to use NMI-safe methods

[Ingo Molnar]

* Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> 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.
> 
> I am tempted to think that taking, say, ~10 cycles on the 
> interrupt path worths it if we save a few tens of cycles on the 
> page fault handler fast path.
> 
> But again, this calls for benchmarks.

One absolutely non-trivial complication with such a scheme would be 
preemptability: if we enter #PF with irqs enabled then it's 
immediately preemptible on CONFIG_PREEMPT=y. The scheduler would 
switch away to another context and the cr2 value is lost before it 
has been read out.

This means an additional collateral damage to context-switch cr2. 
(which might still be worth it given that context-switches are a 
less hot codepath than pagefaults - but an additional complicaton.)

The ideal solution would be for the CPU to atomically push the cr2 
value to the #PF hardware stack, alongside the error code it already 
pushes there.

	Ingo
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