On 19 February 2015 at 18:18, Morten Rasmussen <morten.rasmus...@arm.com> wrote: > On Thu, Feb 19, 2015 at 04:52:41PM +0000, Peter Zijlstra wrote: >> On Thu, Jan 15, 2015 at 11:09:25AM +0100, Vincent Guittot wrote: >> > The average running time of RT tasks is used to estimate the remaining >> > compute >> > capacity for CFS tasks. This remaining capacity is the original capacity >> > scaled >> > down by a factor (aka scale_rt_capacity). This estimation of available >> > capacity >> > must also be invariant with frequency scaling. >> > >> > A frequency scaling factor is applied on the running time of the RT tasks >> > for >> > computing scale_rt_capacity. >> > >> > In sched_rt_avg_update, we scale the RT execution time like below: >> > rq->rt_avg += rt_delta * arch_scale_freq_capacity() >> SCHED_CAPACITY_SHIFT >> > >> > Then, scale_rt_capacity can be summarized by: >> > scale_rt_capacity = SCHED_CAPACITY_SCALE - >> > ((rq->rt_avg << SCHED_CAPACITY_SHIFT) / period) >> > >> > We can optimize by removing right and left shift in the computation of >> > rq->rt_avg >> > and scale_rt_capacity >> >> So far so good.. >> >> > The call to arch_scale_frequency_capacity in the rt scheduling path might >> > be >> > a concern for RT folks because I'm not sure whether we can rely on >> > arch_scale_freq_capacity to be short and efficient ? >> >> No, that is, arch_scale_frequency_capacity() _must_ be short and >> efficient, event for the fair class, its called in very hot paths. > > ... and very frequently too. > >> I think we've talked about this before; this function should basically >> only return a cached value, which is periodically updated through some >> means. > > Agreed. I think it is reasonable to assume that the arch code > implementing arch_scale_freq_capacity() does it's best to make it fast > for the particular architecture. Since the scaling factor to be returned > by the function may be obtained in different ways for different > architectures the caching should be done on the arch side. > >> But lets see, I've yet to see an actual implementation of it; and its >> got that sd argument, curious what you're going to do with that. > > So we do have an RFC implementation for ARM already which I posted in > December and is also included in the rather large RFC posting I did some > weeks ago. That one basically reads two atomic variables and returns the > ratio between the two. I have yet to benchmark how horribly expensive it > is though. The sd argument is ignored. We might actually not need it at > all?
For consistency across all arch_scale_xx_capacity, i would prefer to keep the same prototype interface (struct sched_domain *sd, int cpu) even if it's not used ofr now > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/