linux-kernel - Re: [BUG] perf: sharing of cpuctx between core and ibs PMU causes problems

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Date:	Thu, 9 Aug 2012 16:05:09 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	"Yan, Zheng" <zheng.z.yan@...el.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Robert Richter <robert.richter@....com>, mingo@...e.hu
Subject: Re: [BUG] perf: sharing of cpuctx between core and ibs PMU causes problems

On Thu, Aug 9, 2012 at 8:55 AM, Yan, Zheng <zheng.z.yan@...el.com> wrote:
> On 08/09/2012 08:51 AM, Stephane Eranian wrote:
>> Hi,
>>
>> I ran into a problem on my AMD box whereby I would hit the
>> WARN_ON_ONCE(cpuctx->cgrp) in perf_cgroup_switch().
>>
>> It took me a while to track this down. It turns out that the
>> list_for_each_entry_rcu() loop had multiple iterations. That's
>> normal, we have CPU PMU and IBS PMU.  But what caused
>> the warning to fire is that both the core and IBS PMU were
>> pointing to the same cpuctx struct. Thus, the cpuctx->cgrp
>> was already set  in the second iteration.
>>
>> Is the warning a false positive?
>
> I think it's a false positive, I'm not sure.
>
Well, but then you're doing the same work twice.

>>
>> In perf_pmu_register(), there is a search for a matching
>> pmu->task_ctx_nr. Given that the field is pointing to
>> perf_hw_context for both cpu and IBS PMU, there is
>> a match and therefore the cpuctx are shared.
>>
>> The question is: why do we have to share the cpuctx?
>>
>> Note that the same issue probably exists with the Intel
>> uncore PMU.
>
> uncore PMU does not have this issue because uncore_pmu->task_ctx_nr
> is 'perf_invalid_context'. find_pmu_context() always return NULL in
> that case.
>
Yes, I think IBS should do the same and that should fix the problem
there too. Will try that.
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