Messages in this thread | | | Date | Mon, 11 Nov 2013 18:18:05 +0000 | From | Catalin Marinas <> | Subject | Re: [RFC][PATCH v5 00/14] sched: packing tasks |
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On Mon, Nov 11, 2013 at 04:39:45PM +0000, Arjan van de Ven wrote: > > I think the scheduler simply wants to say: we expect to go idle for X > > ns, we want a guaranteed wakeup latency of Y ns -- go do your thing. > > as long as Y normally is "large" or "infinity" that is ok ;-) > (a smaller Y will increase power consumption and decrease system performance)
Cpuidle already takes a latency into account via pm_qos. The scheduler could pass this information down to the hardware driver or the cpuidle driver could use pm_qos directly (as it's currently done in governors).
The scheduler may have its own requirements in terms of latency (e.g. some real-time thread) and we could extend the pm_qos API with per-thread information. But so far we don't have a way to pass such per-thread requirements from user space (unless we assume that any real-time thread has some fixed latency requirements). I suggest we ignore this per-thread part until we find an actual need.
> > I think you also raised the point in that we do want some feedback as to > > the cost of waking up particular cores to better make decisions on which > > to wake. That is indeed so. > > having a hardware driver give a prefered CPU ordering for wakes can indeed be useful. > (I'm doubtful that changing the recommendation for each idle is going to pay off, > but proof is in the pudding; there are certainly long term effects where this can help)
The ordering is based on the actual C-state, so a simple way is to wake up the CPU in the shallowest C-state. With asymmetric configurations (big.LITTLE) we have different costs for the same C-state, so this would come in handy.
Even for symmetric configuration, the cost of moving a task to a CPU includes wake-up cost plus the run-time cost which depends on the P-state after wake-up (that's much trickier since we can't easily estimate the cost of a P-state and it may change once you place a task on it).
-- Catalin
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