Messages in this thread |  | | | Date | Tue, 5 Jul 2011 15:10:05 +0200 | | From | Ingo Molnar <> | | Subject | Re: [PATCH -tip, final] perf, x86: Add hw_watchdog_set_attr() in a sake of nmi-watchdog on P4 |
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* Cyrill Gorcunov <gorcunov@gmail.com> wrote:
> perf, x86: P4 PMU - Add hw_watchdog_set_attr helper to simulate cpu-cycles counting in nmi-watchdog > > Because of constraints existed in Netburst PMU counting > cpu cycles is allowed for one consumer only. > > If the kernel is booted up with nmi-watchdog enabled > the watchdog become a consumer of such event and there > is no more room left for "perf top" and friends (ie any > attempts to count cpu cycles simultaneously with nmi-watchdog > doomed to fail).
Hm, what is the symptom - 'perf top' reports nothing?
If multiple users request cycles then perf will time-share them - this is what happens if you run many 'perf top' or 'perf stat' sessions in parallel for example. For example i just tried to run six separate 'perf top' in parallel - and all six worked fine.
So the question is, why does the NMI watchdog prevent 'perf top' from working on a P4?
Thanks,
Ingo
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