Re: [RFC 1/4] dma-fence: Add deadline awareness

[Christian König <ckoenig.leichtzumerken@xxxxxxxxx>]

Am 28.07.21 um 09:03 schrieb Christian König:
> Am 27.07.21 um 16:25 schrieb Rob Clark:
> > On Tue, Jul 27, 2021 at 12:11 AM Christian König
> > <ckoenig.leichtzumerken@xxxxxxxxx> wrote:
> > > Am 27.07.21 um 01:38 schrieb Rob Clark:
> > > > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> > > >
> > > > Add a way to hint to the fence signaler of an upcoming deadline, 
> > > > such as
> > > > vblank, which the fence waiter would prefer not to miss. This is to 
> > > > aid
> > > > the fence signaler in making power management decisions, like boosting
> > > > frequency as the deadline approaches and awareness of missing 
> > > > deadlines
> > > > so that can be factored in to the frequency scaling.
> > > >
> > > > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
> > > > ---
> > > >    drivers/dma-buf/dma-fence.c | 39 
> > > > +++++++++++++++++++++++++++++++++++++
> > > >    include/linux/dma-fence.h   | 17 ++++++++++++++++
> > > >    2 files changed, 56 insertions(+)
> > > >
> > > > diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> > > > index ce0f5eff575d..2e0d25ab457e 100644
> > > > --- a/drivers/dma-buf/dma-fence.c
> > > > +++ b/drivers/dma-buf/dma-fence.c
> > > > @@ -910,6 +910,45 @@ dma_fence_wait_any_timeout(struct dma_fence 
> > > > **fences, uint32_t count,
> > > >    }
> > > >    EXPORT_SYMBOL(dma_fence_wait_any_timeout);
> > > >
> > > > +
> > > > +/**
> > > > + * dma_fence_set_deadline - set desired fence-wait deadline
> > > > + * @fence:    the fence that is to be waited on
> > > > + * @deadline: the time by which the waiter hopes for the fence to be
> > > > + *            signaled
> > > > + *
> > > > + * Inform the fence signaler of an upcoming deadline, such as 
> > > > vblank, by
> > > > + * which point the waiter would prefer the fence to be signaled 
> > > > by.  This
> > > > + * is intended to give feedback to the fence signaler to aid in power
> > > > + * management decisions, such as boosting GPU frequency if a periodic
> > > > + * vblank deadline is approaching.
> > > > + */
> > > > +void dma_fence_set_deadline(struct dma_fence *fence, ktime_t 
> > > > deadline)
> > > > +{
> > > > +     unsigned long flags;
> > > > +
> > > > +     if (dma_fence_is_signaled(fence))
> > > > +             return;
> > > > +
> > > > +     spin_lock_irqsave(fence->lock, flags);
> > > > +
> > > > +     /* If we already have an earlier deadline, keep it: */
> > > > +     if (test_bit(DMA_FENCE_FLAG_HAS_DEADLINE_BIT, &fence->flags) &&
> > > > +         ktime_before(fence->deadline, deadline)) {
> > > > +             spin_unlock_irqrestore(fence->lock, flags);
> > > > +             return;
> > > > +     }
> > > > +
> > > > +     fence->deadline = deadline;
> > > > +     set_bit(DMA_FENCE_FLAG_HAS_DEADLINE_BIT, &fence->flags);
> > > > +
> > > > +     spin_unlock_irqrestore(fence->lock, flags);
> > > > +
> > > > +     if (fence->ops->set_deadline)
> > > > +             fence->ops->set_deadline(fence, deadline);
> > > > +}
> > > > +EXPORT_SYMBOL(dma_fence_set_deadline);
> > > > +
> > > >    /**
> > > >     * dma_fence_init - Initialize a custom fence.
> > > >     * @fence: the fence to initialize
> > > > diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> > > > index 6ffb4b2c6371..4e6cfe4e6fbc 100644
> > > > --- a/include/linux/dma-fence.h
> > > > +++ b/include/linux/dma-fence.h
> > > > @@ -88,6 +88,7 @@ struct dma_fence {
> > > >                /* @timestamp replaced by @rcu on 
> > > > dma_fence_release() */
> > > >                struct rcu_head rcu;
> > > >        };
> > > > +     ktime_t deadline;
> > > Mhm, adding the flag sounds ok to me but I'm a bit hesitating adding 
> > > the
> > > deadline as extra field here.
> > >
> > > We tuned the dma_fence structure intentionally so that it is only 64 
> > > bytes.
> > Hmm, then I guess you wouldn't be a fan of also adding an hrtimer?
> >
> > We could push the ktime_t (and timer) down into the derived fence
> > class, but I think there is going to need to be some extra storage
> > *somewhere*.. maybe the fence signaler could get away with just
> > storing the nearest upcoming deadline per fence-context instead?
>
> I would just push that into the driver instead.
>
> You most likely don't want the deadline per fence anyway in complex 
> scenarios, but rather per frame. And a frame is usually composed from 
> multiple fences.

Thinking more about it we could probably kill the spinlock pointer and 
make the flags 32bit if we absolutely need that here.

But I still don't see the need for that, especially since most drivers 
probably won't implement it.

Regards,
Christian.

> Regards,
> Christian.
>
> > BR,
> > -R
> >
> > > Regards,
> > > Christian.
> > >
> > > >        u64 context;
> > > >        u64 seqno;
> > > >        unsigned long flags;
> > > > @@ -99,6 +100,7 @@ enum dma_fence_flag_bits {
> > > >        DMA_FENCE_FLAG_SIGNALED_BIT,
> > > >        DMA_FENCE_FLAG_TIMESTAMP_BIT,
> > > >        DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
> > > > +     DMA_FENCE_FLAG_HAS_DEADLINE_BIT,
> > > >        DMA_FENCE_FLAG_USER_BITS, /* must always be last member */
> > > >    };
> > > >
> > > > @@ -261,6 +263,19 @@ struct dma_fence_ops {
> > > >         */
> > > >        void (*timeline_value_str)(struct dma_fence *fence,
> > > >                                   char *str, int size);
> > > > +
> > > > +     /**
> > > > +      * @set_deadline:
> > > > +      *
> > > > +      * Callback to allow a fence waiter to inform the fence 
> > > > signaler of an
> > > > +      * upcoming deadline, such as vblank, by which point the 
> > > > waiter would
> > > > +      * prefer the fence to be signaled by.  This is intended to 
> > > > give feedback
> > > > +      * to the fence signaler to aid in power management 
> > > > decisions, such as
> > > > +      * boosting GPU frequency.
> > > > +      *
> > > > +      * This callback is optional.
> > > > +      */
> > > > +     void (*set_deadline)(struct dma_fence *fence, ktime_t deadline);
> > > >    };
> > > >
> > > >    void dma_fence_init(struct dma_fence *fence, const struct 
> > > > dma_fence_ops *ops,
> > > > @@ -586,6 +601,8 @@ static inline signed long dma_fence_wait(struct 
> > > > dma_fence *fence, bool intr)
> > > >        return ret < 0 ? ret : 0;
> > > >    }
> > > >
> > > > +void dma_fence_set_deadline(struct dma_fence *fence, ktime_t 
> > > > deadline);
> > > > +
> > > >    struct dma_fence *dma_fence_get_stub(void);
> > > >    struct dma_fence *dma_fence_allocate_private_stub(void);
> > > >    u64 dma_fence_context_alloc(unsigned num);