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

["Yan, Zheng" <zheng.z.yan@intel.com>]

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?
>