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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2011-10-14 at 14:37 +0200, Stephane Eranian wrote:
> + * When sampling the branck stack in system-wide, it may be necessary
> + * to flush the stack on context switch. This happens when the branch
> + * stack does not tag its entries with the pid of the current task.
> + * Otherwise it becomes impossible to associate a branch entry with a
> + * task. This ambiguity is more likely to appear when the branch stack
> + * supports priv level filtering and the user sets it to monitor only
> + * at the user level (which could be a useful measurement in system-wide
> + * mode). In that case, the risk is high of having a branch stack with
> + * branch from multiple tasks. Flushing may mean dropping the existing
> + * entries or stashing them somewhere in the PMU specific code layer. 

It doesn't need to tag stuff with PID to solve that problem, making the
TOS a full 64bit wide counter will work equally well, we'd simply record
the TOS value at context switch time and discard everything prior to the
last switch-in.

But yeah, we need to flush this stuff under the current scheme.