Re: I.6 - Group scheduling

From: Ingo Molnar
Date: Mon Jun 22 2009 - 07:53:40 EST

> 6/ Group scheduling
> Looking at the existing code, it seems to me there is a risk of
> starvation for groups, i.e., groups never scheduled on the PMU.
> My understanding of the scheduling algorithm is:
> - first try to ïschedule pinned groups. If a pinned group
> fails, put it in error mode. read() will fail until the
> group gets another chance at being scheduled.
> - then try to schedule the remaining groups. If a group fails
> just skip it.
> If the group list does not change, then certain groups may always
> fail. However, the ordering of the list changes because at every
> tick, it is rotated. The head becomes the tail. Therefore, each
> group eventually gets the first position and therefore gets the
> full PMU to assign its events.
> This works as long as there is a guarantee the list will ALWAYS
> rotate. If a thread does not run long enough for a tick, it may
> never rotate.

You need to ensure the task never runs during the tick, this is a
statistical propery that is bound to untrue for any 'normal'

This is similar to the old cputime hiding tricks. We don't think
this will be a problem, you really need to actively avoid the tick
to aggregate a significantly lower tick rate.

In any case this can also be excluded via a natural extension
mentioned above: scheduling based not on the scheduler tick but
based on one of the counters.
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