Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even when kthread kicked

[Joel Fernandes]

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@xxxxxxxxxxxxxxxxx>
> > > > 
> > > > 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 ;)

> 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.

> 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, 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?

thanks,

 - Joel