linux-kernel - Re: [BUG] perf: multiplexing and hotplug CPU problem

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Date:	Wed, 12 Sep 2012 16:50:17 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Robert Richter <robert.richter@....com>
Subject: Re: [BUG] perf: multiplexing and hotplug CPU problem

On Wed, Sep 12, 2012 at 4:47 PM, Peter Zijlstra <peterz@...radead.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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