Re: [PATCH V7 13/17] perf, x86: enable LBR callstack when recording callchain

[Andi Kleen]

> The reason why using LBR call stack mode is restricted to user level
> only is because of
> a bug in the LBR call stack hardware which forces the kernel to drop
> LBR_FREEZE_PMI.

It works with PEBS events, just not with non PEBS (the patch currently
does not implement this distinction and I'm not sure it's worth it)

> In other words ,the LBR does not stop on counter overflow, thus it
> will be wiped out by
> execution of kernel code leading to the PMU interrupt handler.

It stops, but it takes some time to stop so the call-return stack
can get out of sync.

-Andi

-- 
ak@xxxxxxxxxxxxxxx -- Speaking for myself only
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