Messages in this thread | | | Date | Mon, 15 Jun 2009 17:12:07 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods |
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* Ingo Molnar (mingo@elte.hu) wrote: > > * H. Peter Anvin <hpa@zytor.com> wrote: > > > Mathieu Desnoyers wrote: > > >>> > > >> Writing control registers is serializing, so it's a lot more expensive > > >> than writing a normal register; my *guess* is that it will be on the > > >> order of 100-200 cycles. > > >> > > >> That is not based on any actual information. > > >> > > > > > > Then how about just writing to the cr2 register *if* it has changed > > > while the NMI handler was running ? > > > > > > if (unlikely(read_cr2() != saved_cr2))) > > > write_cr2(saved_cr2) > > > > > > Mathieu > > > > > > > That works fine, obviously, and although it's probably overkill > > it's also a trivially cheap optimization. > > Writing cr2 costs 84 cycles so it's not overkill at all - it's a > nice optimization! > > Btw., we dont have to re-read it - we actually _know_ when we got a > fault (the fault handler gives us back an error code). >
Just for the sake of making NMI handlers less tricky, supporting page faults caused by faulting kernel instructions (rather than only supporting explicit faulting from get_user_pages_inatomic) would be rather nice design-wise if it only costs 2-3 cycles.
And I would not want to touch the page fault handler itself to write the saved cr2 value before the handler exits, because this would add a branch on a very hot path.
Mathieu
> So we can do this common optimization and avoid the cr2 write > usually. We only need the cr2 read. > > Hm, tempting ... > > Ingo
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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