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)" <j...@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. 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? 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? > So I don't think more complexity is needed to handle the case > you're bringing up. > > thanks! > > - Joel > > [1] https://lkml.org/lkml/2018/5/17/668 > -- #include <best/regards.h> Patrick Bellasi
