lkml.org 
[lkml]   [2018]   [May]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    Date
    SubjectRe: [PATCH v2] schedutil: Allow cpufreq requests to be made even when kthread kicked
    On Tue, May 22, 2018 at 5:30 PM, Rafael J. Wysocki <rafael@kernel.org> wrote:
    > On Tue, May 22, 2018 at 12:02 PM, Rafael J. Wysocki <rafael@kernel.org> wrote:
    >> On Mon, May 21, 2018 at 6:13 PM, Joel Fernandes <joel@joelfernandes.org> wrote:
    >>> On Mon, May 21, 2018 at 10:29:52AM +0200, Rafael J. Wysocki wrote:
    >>>> On Mon, May 21, 2018 at 7:14 AM, Viresh Kumar <viresh.kumar@linaro.org> wrote:
    >>>> > On 18-05-18, 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.
    >>>> >
    >>>> > Now that this isn't an RFC anymore, you shouldn't have added below
    >>>> > paragraph here. It could go to the comments section though.
    >>>> >
    >>>> >> I had brought up this issue at the OSPM conference and Claudio had a
    >>>> >> discussion RFC with an alternate approach [1]. I prefer the approach as
    >>>> >> done in the patch below since it doesn't need any new flags and doesn't
    >>>> >> cause any other extra overhead.
    >>>> >>
    >>>> >> [1] https://patchwork.kernel.org/patch/10384261/
    >>>> >>
    >>>> >> LGTMed-by: Viresh Kumar <viresh.kumar@linaro.org>
    >>>> >> LGTMed-by: Juri Lelli <juri.lelli@redhat.com>
    >>>> >
    >>>> > Looks like a Tag you just invented ? :)
    >>>>
    >>>> Yeah.
    >>>>
    >>>> The LGTM from Juri can be converned into an ACK silently IMO. That
    >>>> said I have added Looks-good-to: tags to a couple of commits. :-)
    >>>
    >>> Cool, I'll covert them to Acks :-)
    >>
    >> So it looks like I should expect an update of this patch, right?
    >>
    >> Or do you prefer the current one to be applied and work on top of it?
    >>
    >
    > [cut]
    >
    >>>
    >>> I just realized that on a single policy switch that uses the governor thread,
    >>> there will be 1 thread per-CPU. The sugov_update_single will be called on the
    >>> same CPU with interrupts disabled.
    >>
    >> sugov_update_single() doesn't have to run on the target CPU.
    >
    > Which sadly is a bug IMO. :-/

    My bad.

    sugov_update_single() runs under rq->lock, so it need not run on a
    target CPU so long as the CPU running it can update the frequency for
    the target and there is the requisite check for that in
    sugov_should_update_freq().

    That means that sugov_update_single() will not run concurrently on two
    different CPUs for the same target, but it may be running concurrently
    with the kthread (as pointed out by Viresh).

    >>> In sugov_work, we are doing a
    >>> raw_spin_lock_irqsave which also disables interrupts. So I don't think
    >>> there's any possibility of a race happening on the same CPU between the
    >>> frequency update request and the sugov_work executing. In other words, I feel
    >>> we can drop the above if (..) statement for single policies completely and
    >>> only keep the changes for the shared policy. Viresh since you brought up the
    >>> single policy issue initially which made me add this if statememnt, could you
    >>> let me know if you agree with what I just said?
    >>
    >> Which is why you need the spinlock too.
    >
    > And you totally have a point.

    Not really, sorry about that.

    It is necessary to take the spinlock in the non-fast-switch case,
    because of the possible race with the kthread, so something like my
    patch at https://patchwork.kernel.org/patch/10418551/ is needed after
    all.

    \
     
     \ /
      Last update: 2018-05-22 19:07    [W:6.381 / U:0.008 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site