Re: [BUG] perf: multiplexing and hotplug CPU problem

[Peter Zijlstra <peterz@infradead.org>]

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.