[PATCH] nouveau: rip out busy fence waits

[Daniel Vetter]

On Mon, Jun 17, 2024 at 05:11:34PM +0200, Danilo Krummrich wrote:
> On 4/17/24 07:40, Dave Airlie wrote:
> > From: Dave Airlie <airlied at redhat.com>
> > 
> > I'm pretty sure this optimisation is actually not a great idea,
> > and is racy with other things waiting for fences.
> 
> Yes, I tried to use it in the past on scheduler tear down, to have an
> indicator whether all jobs had the chance to finish.
> 
> However, it happened that using a CPU busy loop saw the fence as signaled,
> while an (event based) dma_fence was still seen as unsignaled.

If the busy loop goes through dma_fence_is_signalled this kind of stuff
shouldn't happen. But if you open-code it, it can.
-Sima

> 
> > 
> > Just nuke it, there should be no need to do fence waits in a
> > busy CPU loop.
> > 
> > Signed-off-by: Dave Airlie <airlied at redhat.com>
> 
> Applied to drm-misc-next.
> 
> > ---
> >   drivers/gpu/drm/nouveau/nouveau_bo.c    |  2 +-
> >   drivers/gpu/drm/nouveau/nouveau_chan.c  |  2 +-
> >   drivers/gpu/drm/nouveau/nouveau_dmem.c  |  2 +-
> >   drivers/gpu/drm/nouveau/nouveau_fence.c | 30 +------------------------
> >   drivers/gpu/drm/nouveau/nouveau_fence.h |  2 +-
> >   drivers/gpu/drm/nouveau/nouveau_gem.c   |  2 +-
> >   6 files changed, 6 insertions(+), 34 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/nouveau/nouveau_bo.c b/drivers/gpu/drm/nouveau/nouveau_bo.c
> > index 8a30f5a0525b..a4e8f625fce6 100644
> > --- a/drivers/gpu/drm/nouveau/nouveau_bo.c
> > +++ b/drivers/gpu/drm/nouveau/nouveau_bo.c
> > @@ -902,7 +902,7 @@ nouveau_bo_move_m2mf(struct ttm_buffer_object *bo, int evict,
> >   	 * Without this the operation can timeout and we'll fallback to a
> >   	 * software copy, which might take several minutes to finish.
> >   	 */
> > -	nouveau_fence_wait(fence, false, false);
> > +	nouveau_fence_wait(fence, false);
> >   	ret = ttm_bo_move_accel_cleanup(bo, &fence->base, evict, false,
> >   					new_reg);
> >   	nouveau_fence_unref(&fence);
> > diff --git a/drivers/gpu/drm/nouveau/nouveau_chan.c b/drivers/gpu/drm/nouveau/nouveau_chan.c
> > index 7c97b2886807..66fca95c10c7 100644
> > --- a/drivers/gpu/drm/nouveau/nouveau_chan.c
> > +++ b/drivers/gpu/drm/nouveau/nouveau_chan.c
> > @@ -72,7 +72,7 @@ nouveau_channel_idle(struct nouveau_channel *chan)
> >   		ret = nouveau_fence_new(&fence, chan);
> >   		if (!ret) {
> > -			ret = nouveau_fence_wait(fence, false, false);
> > +			ret = nouveau_fence_wait(fence, false);
> >   			nouveau_fence_unref(&fence);
> >   		}
> > diff --git a/drivers/gpu/drm/nouveau/nouveau_dmem.c b/drivers/gpu/drm/nouveau/nouveau_dmem.c
> > index 12feecf71e75..033a09cd3c8f 100644
> > --- a/drivers/gpu/drm/nouveau/nouveau_dmem.c
> > +++ b/drivers/gpu/drm/nouveau/nouveau_dmem.c
> > @@ -128,7 +128,7 @@ static void nouveau_dmem_page_free(struct page *page)
> >   static void nouveau_dmem_fence_done(struct nouveau_fence **fence)
> >   {
> >   	if (fence) {
> > -		nouveau_fence_wait(*fence, true, false);
> > +		nouveau_fence_wait(*fence, false);
> >   		nouveau_fence_unref(fence);
> >   	} else {
> >   		/*
> > diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c b/drivers/gpu/drm/nouveau/nouveau_fence.c
> > index c3ea3cd933cd..8de941379324 100644
> > --- a/drivers/gpu/drm/nouveau/nouveau_fence.c
> > +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c
> > @@ -312,39 +312,11 @@ nouveau_fence_wait_legacy(struct dma_fence *f, bool intr, long wait)
> >   	return timeout - t;
> >   }
> > -static int
> > -nouveau_fence_wait_busy(struct nouveau_fence *fence, bool intr)
> > -{
> > -	int ret = 0;
> > -
> > -	while (!nouveau_fence_done(fence)) {
> > -		if (time_after_eq(jiffies, fence->timeout)) {
> > -			ret = -EBUSY;
> > -			break;
> > -		}
> > -
> > -		__set_current_state(intr ?
> > -				    TASK_INTERRUPTIBLE :
> > -				    TASK_UNINTERRUPTIBLE);
> > -
> > -		if (intr && signal_pending(current)) {
> > -			ret = -ERESTARTSYS;
> > -			break;
> > -		}
> > -	}
> > -
> > -	__set_current_state(TASK_RUNNING);
> > -	return ret;
> > -}
> > -
> >   int
> > -nouveau_fence_wait(struct nouveau_fence *fence, bool lazy, bool intr)
> > +nouveau_fence_wait(struct nouveau_fence *fence, bool intr)
> >   {
> >   	long ret;
> > -	if (!lazy)
> > -		return nouveau_fence_wait_busy(fence, intr);
> > -
> >   	ret = dma_fence_wait_timeout(&fence->base, intr, 15 * HZ);
> >   	if (ret < 0)
> >   		return ret;
> > diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.h b/drivers/gpu/drm/nouveau/nouveau_fence.h
> > index bc13110bdfa4..88213014b675 100644
> > --- a/drivers/gpu/drm/nouveau/nouveau_fence.h
> > +++ b/drivers/gpu/drm/nouveau/nouveau_fence.h
> > @@ -23,7 +23,7 @@ void nouveau_fence_unref(struct nouveau_fence **);
> >   int  nouveau_fence_emit(struct nouveau_fence *);
> >   bool nouveau_fence_done(struct nouveau_fence *);
> > -int  nouveau_fence_wait(struct nouveau_fence *, bool lazy, bool intr);
> > +int  nouveau_fence_wait(struct nouveau_fence *, bool intr);
> >   int  nouveau_fence_sync(struct nouveau_bo *, struct nouveau_channel *, bool exclusive, bool intr);
> >   struct nouveau_fence_chan {
> > diff --git a/drivers/gpu/drm/nouveau/nouveau_gem.c b/drivers/gpu/drm/nouveau/nouveau_gem.c
> > index 49c2bcbef129..f715e381da69 100644
> > --- a/drivers/gpu/drm/nouveau/nouveau_gem.c
> > +++ b/drivers/gpu/drm/nouveau/nouveau_gem.c
> > @@ -928,7 +928,7 @@ nouveau_gem_ioctl_pushbuf(struct drm_device *dev, void *data,
> >   	}
> >   	if (sync) {
> > -		if (!(ret = nouveau_fence_wait(fence, false, false))) {
> > +		if (!(ret = nouveau_fence_wait(fence, false))) {
> >   			if ((ret = dma_fence_get_status(&fence->base)) == 1)
> >   				ret = 0;
> >   		}
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch