[Mesa-dev] [PATCH 2/9] i965: Check last known busy status on bo before asking the kernel

[Kenneth Graunke]

On Thursday, June 15, 2017 1:41:39 AM PDT Chris Wilson wrote:
> Quoting Kenneth Graunke (2017-06-14 22:49:01)
> > On Friday, June 9, 2017 6:01:33 AM PDT Chris Wilson wrote:
> > > If we know the bo is idle (that is we have no submitted a command buffer
> > > referencing this bo since the last query) we can skip asking the kernel.
> > > Note this may report a false negative if the target is being shared
> > > between processes (exported via dmabuf or flink). To allow the caller
> > > control over using the last known flag, the query is split into two.
> > 
> > I'm not crazy about exposing __brw_bo_busy and brw_bo_busy, with slightly
> > different semantics.  Why not just make brw_bo_busy do:
> > 
> >    if (bo->idle && bo->reusable)
> >       return false;
> > 
> >    /* otherwise query the kernel */
> > 
> > These days, it appears that bo->reusable is false for any buffers that
> > have been imported/exported via dmabuf or flink, and true otherwise.
> > (We might want to rename it to bo->foreign or such.)
> 
> I set bo->reusable to false on snooped buffers as we don't yet handle
> mixed modes in the bo cache. To offset that I was thinking of having a
> specific cache for qbo. We do have flags we pass to bo_alloc, so
> supporting a cache of snoopable, or even uncached on llc, within bufmgr
> isn't hard.
> 
> An alternative is to have bo->exported, which is useful if we ever want
> to disregard our state tracking.

Imported is a problem too, right?  Whoever gave us the buffer could still
do things with it, and we wouldn't be able to track that.

That's why I was thinking bo->foreign, bo->external, bo->shared, or the
like.  Adding a new flag seems reasonable - bo->reusable isn't quite what
we want.

> > With that change, brw_bo_busy should bypass the ioctl for most BOs, but
> > would still work for foreign BOs, without the caller having to worry
> > about it.
> 
> Just not qbo right now :)

Good point.  Adding a new flag would be better.

> > >  /**
> > > - * Returns 1 if mapping the buffer for write could cause the process
> > > - * to block, due to the object being active in the GPU.
> > > + * Returns 0 if mapping the buffer is not in active use by the gpu.
> > > + * If non-zero, any mapping for for write could cause the process
> > > + * to block, due to the object being active in the GPU. If the lower
> > > + * 16 bits are zero, then we can map for read without stalling.
> > > + *
> > > + * The last-known busy status of the brw_bo is checked first. This may be
> > > + * stale if the brw_bo has been exported to a foriegn process. If used on an
> > > + * exported bo, call __brw_bo_busy() directly to bypass the local check.
> > >   */
> > > -int brw_bo_busy(struct brw_bo *bo);
> > > +static inline int brw_bo_busy(struct brw_bo *bo)
> > > +{
> > > +   if (bo->idle) /* Note this may be stale if the bo is exported */
> > > +      return 0;
> > > +
> > > +   return __brw_bo_busy(bo);
> > > +}
> > 
> > I'd rather keep this as a boolean result, rather than an integer with
> > certain bits having particular meanings.  Bonus points for changing the
> > return type to "bool".
> 
> The different bits are significant, especially in the case where you do
> want to distinguish ready to read vs ready to write. Such as reading
> back a qbo that might also be still in use for GPU queries.
> -Chris

Right, so different bits are how the kernel exposes that...but IMO we
should just have two boolean functions, one for "ready to read" and one
for "ready to write"...

I sort of thought that's what you were doing.  Or do we need three?
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