Messages in this thread |  | | | Date | Thu, 27 Nov 2008 13:28:28 +0100 | | From | "stephane eranian" <> | | Subject | Re: [patch 05/24] perfmon: X86 generic code (x86) |
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On Thu, Nov 27, 2008 at 1:32 PM, Andi Kleen <andi@firstfloor.org> wrote: > On Thu, Nov 27, 2008 at 12:35:54PM +0100, stephane eranian wrote: >> On Thu, Nov 27, 2008 at 12:31 PM, Andi Kleen <andi@firstfloor.org> wrote: >> >> The only reason why I have to deal with NMI is not so much to allow >> >> for profiling irq-off regions but because I have to share the PMU with >> >> the NMI watchdog. Otherwise I'd have to fail or disable the NMI watchdog >> >> on the fly. >> > >> > The NMI watchdog is now off by default so failing with it enabled >> > is fine. >> >> Yes, but most likely it is on in distro kernels. > > Really? Why? > To make sure they can get a crash dump off of production systems.
> Old distros of course do it but only because they run old > kernels. > >> You have to handle the case where the NMI fires while you are holding >> a perfmon lock. What you have in the patch (and the the fully-featured version) >> is that we get the NMI and we stop the PMU WITHOUT grabbing any perfmon >> lock, and the we repost the interrupt with the regular vector. We also make sure >> we grab the RIP at NMI. That is the one we want to see reported in the sampling >> buffer. >> >> I am still wondering how Oprofile handles the case where multiple processes or >> threads access the same file descriptor. > > It uses per CPU buffers (so no races on the writer) and readers can > of course use a lock to coordinate between themselves. > What if a threads reprograms the counters while another is reading them? How is the buffer reset?
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