Messages in this thread | | | Subject | Re: [PATCH 09/12] perf_events: add hook to flush branch_stack on context switch (v2) | From | Peter Zijlstra <> | Date | Mon, 05 Dec 2011 22:10:57 +0100 |
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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.
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