On Wed, Sep 12, 2012 at 4:47 PM, Peter Zijlstra <pet...@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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
