On Wednesday, August 03, 2016 07:24:18 PM Steve Muckle wrote: > On Wed, Aug 03, 2016 at 12:38:20AM +0200, Rafael J. Wysocki wrote: > > On Wed, Aug 3, 2016 at 12:02 AM, Steve Muckle <steve.muc...@linaro.org> > > wrote: > > > On Tue, Aug 02, 2016 at 03:37:02AM +0200, Rafael J. Wysocki wrote: > > >> On Tue, Aug 2, 2016 at 3:22 AM, Steve Muckle <steve.muc...@linaro.org> > > >> wrote: > > >> > On Mon, Aug 01, 2016 at 01:37:23AM +0200, Rafael J. Wysocki wrote: > > >> > ... > > >> >> For this purpose, define a new cpufreq_update_util() flag > > >> >> UUF_IO and modify enqueue_task_fair() to pass that flag to > > >> >> cpufreq_update_util() in the in_iowait case. That generally > > >> >> requires cpufreq_update_util() to be called directly from there, > > >> >> because update_load_avg() is not likely to be invoked in that > > >> >> case. > > >> > > > >> > I didn't follow why the cpufreq hook won't likely be called if > > >> > in_iowait is set? AFAICS update_load_avg() gets called in the second > > >> > loop > > >> > and calls update_cfs_rq_load_avg (triggers the hook). > > >> > > >> In practice it turns out that in the majority of cases when in_iowait > > >> is set the second loop will not run. > > > > > > My understanding of enqueue_task_fair() is that the first loop walks up > > > the portion of the sched_entity hierarchy that needs to be enqueued, and > > > the second loop updates the rest of the hierarchy that was already > > > enqueued. > > > > > > Even if the se corresponding to the root cfs_rq needs to be enqueued > > > (meaning the whole hierarchy is traversed in the first loop and the > > > second loop does nothing), enqueue_entity() on the root cfs_rq should > > > result in the cpufreq hook being called, via enqueue_entity() -> > > > enqueue_entity_load_avg() -> update_cfs_rq_load_avg(). > > > > But then it's rather difficult to pass the IO flag to this one, isn't it? > > > > Essentially, the problem is to pass "IO" to cpufreq_update_util() when > > p->in_iowait is set. > > > > If you can find a clever way to do it without adding an extra call > > site, that's fine by me, but in any case the extra > > cpufreq_update_util() invocation should not be too expensive. > > I was under the impression that function pointer calls were more > expensive, and in the shared policy case there is a nontrivial amount of > code that is run in schedutil (including taking a spinlock) before we'd > see sugov_should_update_freq() return false and bail.
That's correct in principle, but we only do that if p->in_iowait is set, which is somewhat special anyway and doesn't happen every time for sure. So while there is overhead theoretically, I'm not even sure if it is measurable. > Agreed that getting knowledge of p->in_iowait down to the existing hook > is not easy. I spent some time fiddling with that. It seemed doable but > somewhat gross due to the required flag passing and modifications > to enqueue_entity, update_load_avg, etc. If it is decided that it is worth > pursuing I can keep working on it and post a draft. Well, that's a Peter's call. :-) > But I also wonder if the hooks are in the best location. They are > currently deep in the PELT code. This may make sense from a theoretical > standpoint, calling them whenever a root cfs_rq utilization changes, but > it also makes the hooks difficult to correlate (for policy purposes such > as this iowait change) with higher level logical events like a task > wakeup. Or load balance where we probably want to call the hook just > once after a load balance is complete. I generally agree. We still need to ensure that the hools will be invoked frequently enough, though, even if HZ is 100. > This is also an issue for the remote wakeup case where I currently have > another invocation of the hook in check_preempt_curr(), so I can know if > preemption was triggered and skip a remote schedutil update in that case > to avoid a duplicate IPI. > > It seems to me worth evaluating if a higher level set of hook locations > could be used. One possibility is higher up in CFS: > - enqueue_task_fair, dequeue_task_fair > - scheduler_tick > - active_load_balance_cpu_stop, load_balance Agreed, that's worth checking. > Though this wouldn't solve my issue with check_preempt_curr. That would > probably require going further up the stack to try_to_wake_up() etc. Not > yet sure what the other hook locations would be at that level. That's probably too far away from the root cfs_rq utilization changes IMO. Thanks, Rafael
