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

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