Messages in this thread | | | Date | Wed, 12 Sep 2012 16:50:17 +0200 | Subject | Re: [BUG] perf: multiplexing and hotplug CPU problem | From | Stephane Eranian <> |
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On Wed, Sep 12, 2012 at 4:47 PM, Peter Zijlstra <peterz@infradead.org> wrote: > On Wed, 2012-09-12 at 16:40 +0200, Stephane Eranian wrote: >> Hi, >> >> As I was debugging my hrtimer patch, I ran a few tests >> with hotplug CPU. In others words, I offline a CPU while >> there is an active monitoring session which causes multiplexing. >> >> When the CPU goes down, all is well. But when it comes back, >> things go wrong. No kernel crashes but wrong results and multiplexing >> does not work anymore. >> >> I investigated this some more and found out there is an issue >> on re-activation. >> >> During shutdown, system-wide events are scheduled out AND removed >> from the event lists. Consequently, ctx->nr_events and ctx->nr_active >> go to zero. >> >> When the CPU is brought back online and tools do start/stop on the events >> they can be scheduled back in, and therefore increment ctx->nr_active. >> Because list_add_event() is not called again, you may end up with >> ctx->nr_events < ctx->nr_active which is wrong. Events may not >> be a lists and therefore they cannot get multiplexed again. >> >> It is not clear to me why we need to remove the events from any >> list (list_del_event) when the CPU goes down. >> >> Why isn't calling event_sched_out() enough? >> If events are kept on lists, why not try to schedule them back in when >> the CPU is brought back online? > > This might be never, I think we should put the events in error state > instead of disabling them, that should avoid the re-activation and > provide a stronger hint to userspace that something went funny.
Yeah, that's probably a better approach. Counters should be put in error state such that IOC_ENABLE or read() return errors.
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