Quoting Thomas Hellström (Intel) (2020-07-22 12:19:28)
>
> On 2020-07-15 13:50, Chris Wilson wrote:
> > Rather than require the next timeline after idling to match the MRU
> > before idling, reset the index on the node and allow it to match the
> > first request. However, this requires cmpxchg(u64) and so is not trivial
> > on 32b, so for compatibility we just fallback to keeping the cached node
> > pointing to the MRU timeline.
> >
> > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > ---
> > drivers/gpu/drm/i915/i915_active.c | 21 +++++++++++++++++++--
> > 1 file changed, 19 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
> > index 0854b1552bc1..6737b5615c0c 100644
> > --- a/drivers/gpu/drm/i915/i915_active.c
> > +++ b/drivers/gpu/drm/i915/i915_active.c
> > @@ -157,6 +157,10 @@ __active_retire(struct i915_active *ref)
> > rb_link_node(&ref->cache->node, NULL, &ref->tree.rb_node);
> > rb_insert_color(&ref->cache->node, &ref->tree);
> > GEM_BUG_ON(ref->tree.rb_node != &ref->cache->node);
> > +
> > + /* Make the cached node available for reuse with any timeline */
> > + if (IS_ENABLED(CONFIG_64BIT))
> > + ref->cache->timeline = 0; /* needs cmpxchg(u64) */
> > }
> >
> > spin_unlock_irqrestore(&ref->tree_lock, flags);
> > @@ -235,9 +239,22 @@ static struct active_node *__active_lookup(struct i915_active *ref, u64 idx)
> > {
> > struct active_node *it;
> >
> > + GEM_BUG_ON(idx == 0); /* 0 is the unordered timeline, rsvd for cache */
> > +
> > it = READ_ONCE(ref->cache);
> > - if (it && it->timeline == idx)
> > - return it;
> > + if (it) {
> > + u64 cached = READ_ONCE(it->timeline);
> > +
> > + if (cached == idx)
> > + return it;
> > +
> > +#ifdef CONFIG_64BIT /* for cmpxchg(u64) */
> > + if (!cached && !cmpxchg(&it->timeline, 0, idx)) {
>
> Doesn't cmpxchg() already do an unlocked compare before attempting the
> locked cycle?
It goes straight to the locked instruction, as it's usually used at
the end of a loop that looks at the old value (and now try_cmpxchg). You
can see the difference in perf, as the cmpxchg stands out like a sore
thumb.
-Chris
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