Messages in this thread | | | Date | Fri, 12 Jul 2013 08:38:52 -0700 | From | Dave Hansen <> | Subject | Re: Yet more softlockups. |
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On 07/12/2013 03:31 AM, Ingo Molnar wrote: > * Dave Jones <davej@redhat.com> wrote: >> On Wed, Jul 10, 2013 at 05:20:15PM +0200, Markus Trippelsdorf wrote: >> > On 2013.07.10 at 11:13 -0400, Dave Jones wrote: >> > > I get this right after booting.. >> > > >> > > [ 114.516619] perf samples too long (4262 > 2500), lowering kernel.perf_event_max_sample_rate to 50000 >> > >> > You can disable this warning by: >> > >> > echo 0 > /proc/sys/kernel/perf_cpu_time_max_percent >> >> Yes, but why is this even being run when I'm not running perf ? >> >> The only NMI source running should be the watchdog. > > The NMI watchdog is a perf event. > > I've Cc:-ed Dave Hansen, the author of those changes - is this a false > positive or some real problem?
The warning comes from calling perf_sample_event_took(), which is only called from one place: perf_event_nmi_handler().
So we can be pretty sure that the perf NMI is firing, or at least that this handler code is running.
nmi_handle() says: /* * NMIs are edge-triggered, which means if you have enough * of them concurrently, you can lose some because only one * can be latched at any given time. Walk the whole list * to handle those situations. */
perf_event_nmi_handler() probably gets _called_ when the watchdog NMI goes off. But, it should hit this check:
if (!atomic_read(&active_events)) return NMI_DONE;
and return quickly. This is before it has a chance to call perf_sample_event_took().
Dave, for your case, my suspicion would be that it got turned on inadvertently, or that we somehow have a bug which bumped up perf_event.c's 'active_events' and we're running some perf code that we don't have to.
But, I'm suspicious. I was having all kinds of issues with perf and NMIs taking hundreds of milliseconds. I never isolated it to having a real, single, cause. I attributed it to my large NUMA system just being slow. Your description makes me wonder what I missed, though.
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