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

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Date:   Mon, 21 May 2018 18:00:50 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Joel Fernandes (Google.)" <joelaf@...gle.com>,
        linux-kernel@...r.kernel.org,
        Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Todd Kjos <tkjos@...gle.com>, claudio@...dence.eu.com,
        kernel-team@...roid.com, linux-pm@...r.kernel.org
Subject: Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even
 when kthread kicked

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@...lfernandes.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