Messages in this thread | | | From | Steve Muckle <> | Date | Wed, 3 Aug 2016 19:24:18 -0700 | Subject | Re: [RFC][PATCH 5/7] cpufreq / sched: UUF_IO flag to indicate iowait condition |
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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.muckle@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.muckle@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.
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.
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.
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
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.
thanks, Steve
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