Messages in this thread | | | Date | Thu, 09 Aug 2012 14:55:12 +0800 | From | "Yan, Zheng" <> | Subject | Re: [BUG] perf: sharing of cpuctx between core and ibs PMU causes problems |
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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.
> > 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.
Regards Yan, Zheng.
> > If we need to share, then the perf_cgroup_switch() code > needs to change because, as it stands, it is doing the > switching twice in this case. >
> Either way something looks wrong here. > > Any idea? >
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