Messages in this thread | | | Date | Fri, 8 Jul 2011 14:44:35 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH -tip, final] perf, x86: Add hw_watchdog_set_attr() in a sake of nmi-watchdog on P4 |
| |
* Cyrill Gorcunov <gorcunov@gmail.com> wrote:
> On Tue, Jul 05, 2011 at 03:31:05PM +0200, Ingo Molnar wrote: > > > > * Peter Zijlstra <a.p.zijlstra@chello.nl> wrote: > > > > > > So the question is, why does the NMI watchdog prevent 'perf top' > > > > from working on a P4? > > > > > > because the NMI watchdog is a pinned event, you don't want to share > > > the counter, that would be very bad, suppose you lock up when the > > > NMI watchdog was scheduled out. Unreliably debug tools are worse > > > than no tools. > > > > Yeah, indeed that explains the symptom. > > > > Firstly, we should fix/enhance perf top to print out an error message > > in this case, not just hang there doing nothing. > > It waits for event to be scheduled, so seems first the kernel > should have top level context-schedule-in functions changed from > void to int (so I have admit I might be missin something here). > > > > > Secondly, the proper solution would be to allow the multiplexing of > > like-minded hw events. Here if we have two events: > > > > - pinned NMI watchdog, set to a period of 2 billion cycles > > - perf top with a default of 1 khz auto-freq cycles > > > > We should first change the NMI watchdog to use auto-freq samples - > > the hw_nmi_get_sample_period() looks unnecessary - if we set the NMI > > watchdog to 1 Hz by default it should be more than enough. > > Should we drop tunability of watchdog period as well? At moment > there is a way to setup period via /sys.
Well, the tunable can remain - there's some real usecases that want to increased/decrease the frequency.
Thanks,
Ingo
| |