Re: [PATCH 09/12] perf_events: add hook to flush branch_stack oncontext switch (v2)

[Stephane Eranian]

On Thu, Dec 8, 2011 at 10:13 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, 2011-12-08 at 10:04 -0800, Stephane Eranian wrote:
>> The whole motivation behind the flush_branch_stack is explained in the
>> Changelog of the patch. In summary, we need to flush the LBR (regardless
>> of TOS) because in system-wide we need to be able to associate the content
>> of the LBR with a specific task. Given that the HW does not capture the PID
>> in the LBR buffer, the kernel has to intervene.
>
> That's not regardless of the TOS. If the TOS was a full u64 you wouldn't
> need the TID (which would be good, since the hardware has no such
> concept).
>
Maybe I missed the trick but I don't quite see how a 64-bit TOS would
solve the TID
problem. It's not about the wraparound issue, i.e., not like the
sampling buffer indexes.
Could you describe the trick again?

>> Why don't we have this already?
>> Because we are capturing at all priv levels. But with this patchset, it becomes
>> possible to filter taken branches based on priv levels. Thus, if you only sample
>> at the user level and run in system-wide mode, it is more likely you could end
>> up with branches belonging to two different tasks in the LBR buffer. But you'd
>> have no way of determining this just by looking at the content of the buffer.
>> So instead, we need to flush the LBR on context switch to associate a PID
>> with them.
>
> Yeah, I get that.
>
>> Because this is an expensive operation, we want to do this only when we
>> sample on LBR. That's what the ctx->nr_branch_stack is about. We could
>> refine that some more by checking for system-wide events with only
>> user priv level on the branch stack. But I did not do that yet.
>>
>> Does this make more sense now?
>
> It already did. The only thing I wanted to do was get rid of that method
> check. Initially I overlooked the fact that its optional, even if you
> support the branch stack. My reply from today argued for it, since
> installing a dummy method would still have the needless ctx_lock &&
> pmu_disable overhead.
>
>
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