linux-kernel - Re: [PATCH] drm/msm: Avoid rounding down to zero jiffies

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Date:   Tue, 28 Mar 2023 08:56:00 -0700
From:   Rob Clark <robdclark@...il.com>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc:     dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
        linux-arm-msm@...r.kernel.org, Rob Clark <robdclark@...omium.org>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/msm: Avoid rounding down to zero jiffies

On Tue, Mar 28, 2023 at 8:28 AM Dmitry Baryshkov
<dmitry.baryshkov@...aro.org> wrote:
>
> On 25/03/2023 00:00, Rob Clark wrote:
> > From: Rob Clark <robdclark@...omium.org>
> >
> > If userspace asked for a timeout greater than zero, but less than a
> > jiffy, they clearly weren't planning on spinning.  So it is better
> > to round up to one.
> >
> > This fixes an issue with supertuxkart that was (for some reason)
> > spinning on a gl sync with 1ms timeout.  CPU time for a demo lap
> > drops from:
> >
> >    15.83user 20.98system 0:47.46elapsed 77%CPU
> >
> > to:
> >
> >    8.84user 2.30system 0:46.67elapsed 23%CPU
>
> Interesting. We potentially increased the timeout, but the overall
> (elapsed) time has decreased. Nevertheless:

There is some randomness from run to run so small variations in total
--profile-laps=N time are normal.  So I wouldn't read too much into
that, compared to %CPU.  This shouldn't really change how long it
takes for the fence to signal, as much as just prevent the CPU from
busy looping until the fence does signal ;-)

(In theory the CPU busy looping could cause GPU thermal throttling,
but I don't think that was happening in this case.)

> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>

thx

> >
> > Signed-off-by: Rob Clark <robdclark@...omium.org>
> > ---
> >   drivers/gpu/drm/msm/msm_drv.h | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/msm/msm_drv.h b/drivers/gpu/drm/msm/msm_drv.h
> > index 9f0c184b02a0..7936aa6cad03 100644
> > --- a/drivers/gpu/drm/msm/msm_drv.h
> > +++ b/drivers/gpu/drm/msm/msm_drv.h
> > @@ -548,7 +548,7 @@ static inline unsigned long timeout_to_jiffies(const ktime_t *timeout)
> >               remaining_jiffies = ktime_divns(rem, NSEC_PER_SEC / HZ);
> >       }
> >
> > -     return clamp(remaining_jiffies, 0LL, (s64)INT_MAX);
> > +     return clamp(remaining_jiffies, 1LL, (s64)INT_MAX);
> >   }
> >
> >   /* Driver helpers */
>
> --
> With best wishes
> Dmitry
>