Re: [Intel-gfx] [PATCH v5 2/3] drm/i915: Move on the new pm runtime interface

[Tvrtko Ursulin]

On 21/12/2018 13:26, Vincent Guittot wrote:
> On Fri, 21 Dec 2018 at 12:33, Tvrtko Ursulin
> <tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:
> > Hi,
> >
> > On 21/12/2018 10:33, Vincent Guittot wrote:
> > > Use the new pm runtime interface to get the accounted suspended time:
> > > pm_runtime_suspended_time().
> > > This new interface helps to simplify and cleanup the code that computes
> > > __I915_SAMPLE_RC6_ESTIMATED and to remove direct access to internals of
> > > PM runtime.
> > >
> > > Reviewed-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
> > > Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > > ---
> > >    drivers/gpu/drm/i915/i915_pmu.c | 16 ++++++----------
> > >    drivers/gpu/drm/i915/i915_pmu.h |  4 ++--
> > >    2 files changed, 8 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
> > > index d6c8f8f..3f76f60 100644
> > > --- a/drivers/gpu/drm/i915/i915_pmu.c
> > > +++ b/drivers/gpu/drm/i915/i915_pmu.c
> > > @@ -5,6 +5,7 @@
> > >     */
> > >
> > >    #include <linux/irq.h>
> > > +#include <linux/pm_runtime.h>
> > >    #include "i915_pmu.h"
> > >    #include "intel_ringbuffer.h"
> > >    #include "i915_drv.h"
> > > @@ -478,7 +479,6 @@ static u64 get_rc6(struct drm_i915_private *i915)
> > >                 * counter value.
> > >                 */
> > >                spin_lock_irqsave(&i915->pmu.lock, flags);
> > > -             spin_lock(&kdev->power.lock);
> > >
> > >                /*
> > >                 * After the above branch intel_runtime_pm_get_if_in_use failed
> > > @@ -491,16 +491,13 @@ static u64 get_rc6(struct drm_i915_private *i915)
> > >                 * suspended and if not we cannot do better than report the last
> > >                 * known RC6 value.
> > >                 */
> > > -             if (kdev->power.runtime_status == RPM_SUSPENDED) {
> > > -                     if (!i915->pmu.sample[__I915_SAMPLE_RC6_ESTIMATED].cur)
> > > -                             i915->pmu.suspended_jiffies_last =
> > > -                                               kdev->power.suspended_jiffies;
> > > +             if (pm_runtime_status_suspended(kdev)) {
> > > +                     val = pm_runtime_suspended_time(kdev);
> >
> > There is a race condition between the status check and timestamp access
> > which the existing code solves by holding the power.lock over it. But I
> > don't exactly remember how this issue was manifesting. Is
> > kdev->power.suspended_jiffies perhaps reset on exit from runtime
> > suspend, which was then underflowing the val, not sure.
> >
> > Anyways, is the new way of doing this safe with regards to this race? In
>
> AFAICT it is safe.
> The current version does:
> 1-take lock,
> 2-test if dev is suspended
> 3-read some internals field to computed an up-to-date suspended time
> 4-update __I915_SAMPLE_RC6_ESTIMATED
> 5-release lock
>
> The new version does:
> 1-test if dev is suspended
> 2-get an up-to-date  suspended time with pm_runtime_suspended_time.
> This is atomic and monotonic
> 3-update __I915_SAMPLE_RC6_ESTIMATED
>
> A change from suspended to another states that happens just before
> step 1 is ok for both as we will run the else if
> No change of the state can happen after step 1 in current code and the
> estimated suspended time will be the time up to step2. In parallel,
> Any state change will have to wait step5 to continue
> If a change from suspended to another state happens after step 1 in
> new code, the suspended time return by PM core will be the time up to
> this change. So I would say you don't delay state transition and you
> get a more accurate estimated suspended time (even if the difference
> should be small).
> If a change from suspended to another state happens after step 2 in
> new code, the suspended time return by PM core will be the time up to
> step 2 so there is no changes
>
>
> > other words is the value pm_runtime_suspended_time always monotonic,
> > even when not suspended? If not we have to handle the race somehow.
>
> Yes pm_runtime_suspended_time is monotonic and stays unchanged when
> not suspended
>
> > If it is always monotonic, then worst case we report one wrong sample,
> > which I guess is still not ideal since someone could be querying the PMU
> > with quite low frequency.
> >
> > There are tests which probably can hit this, but to run them
> > automatically your patches would need to be rebased on drm-tip and maybe
> > sent to our trybot. I can do that after the holiday break if you are
> > okay with having the series waiting until then.
>
> yes looks good to me

Looks good to me as well. And our CI agrees with it. So:

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

I assume you will take the patch through some power related tree and we 
will eventually pull it back to drm-tip.

Regards,

Tvrtko