Messages in this thread | | | Date | Mon, 21 May 2018 18:41:42 +0100 | From | Patrick Bellasi <> | Subject | Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even when kthread kicked |
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On 21-May 10:20, Joel Fernandes wrote: > Hi Patrick, > > On Mon, May 21, 2018 at 06:00:50PM +0100, Patrick Bellasi wrote: > > On 21-May 08:49, Joel Fernandes wrote: > > > On Mon, May 21, 2018 at 11:50:55AM +0100, Patrick Bellasi wrote: > > > > On 18-May 11:55, Joel Fernandes (Google.) wrote: > > > > > From: "Joel Fernandes (Google)" <joel@joelfernandes.org> > > > > > > > > > > Currently there is a chance of a schedutil cpufreq update request to be > > > > > dropped if there is a pending update request. This pending request can > > > > > be delayed if there is a scheduling delay of the irq_work and the wake > > > > > up of the schedutil governor kthread. > > > > > > > > > > A very bad scenario is when a schedutil request was already just made, > > > > > such as to reduce the CPU frequency, then a newer request to increase > > > > > CPU frequency (even sched deadline urgent frequency increase requests) > > > > > can be dropped, even though the rate limits suggest that its Ok to > > > > > process a request. This is because of the way the work_in_progress flag > > > > > is used. > > > > > > > > > > This patch improves the situation by allowing new requests to happen > > > > > even though the old one is still being processed. Note that in this > > > > > approach, if an irq_work was already issued, we just update next_freq > > > > > and don't bother to queue another request so there's no extra work being > > > > > done to make this happen. > > > > > > > > Maybe I'm missing something but... is not this patch just a partial > > > > mitigation of the issue you descrive above? > > > > > > > > If a DL freq increase is queued, with this patch we store the request > > > > but we don't actually increase the frequency until the next schedutil > > > > update, which can be one tick away... isn't it? > > > > > > > > If that's the case, maybe something like the following can complete > > > > the cure? > > > > > > We already discussed this and thought of this case, I think you missed a > > > previous thread [1]. The outer loop in the kthread_work subsystem will take > > > care of calling sugov_work again incase another request was queued which we > > > happen to miss. > > > > Ok, I missed that thread... my bad. > > Sure no problem, sorry I was just pointing out the thread, not blaming you > for not reading it ;)
Sure, np here too ;)
> > However, [1] made me noticing that your solution works under the > > assumption that we keep queuing a new kworker job for each request we > > get, isn't it? > > Not at each request, but each request after work_in_progress was cleared by the > sugov_work. Any requests that happen between work_in_progress is set and > cleared only result in updating of the next_freq.
I see, so we enqueue for the time of:
mutex_lock(&sg_policy->work_lock); __cpufreq_driver_target(sg_policy->policy, freq, CPUFREQ_RELATION_L); mutex_unlock(&sg_policy->work_lock);
> > If that's the case, this means that if, for example, during a > > frequency switch you get a request to reduce the frequency (e.g. > > deadline task passing the 0-lag time) and right after a request to > > increase the frequency (e.g. the current FAIR task tick)... you will > > enqueue a freq drop followed by a freq increase and actually do two > > frequency hops? > > Yes possibly,
Not sure about the time window above, I can try to get some measurements tomorrow.
> I see your point but I'm not sure if the tight loop around that > is worth the complexity, or atleast is within the scope of my patch. > Perhaps the problem you describe can be looked at as a future enhancement?
Sure, I already have it as a patch on top of your. I can post it afterwards and we can discuss whether it makes sense or not.
Still have to better check, but I think that maybe we can skip the queueing altogether if some work is already pending... in case we wanna go for a dedicated inner loop like the one I was proposing.
Apart that, I think that your patch is already fixing 90% of the issue we have now.
> thanks, > > - Joel
-- #include <best/regards.h>
Patrick Bellasi
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