On 28/07/2022 07:32, Mauro Carvalho Chehab wrote:
On Wed, 27 Jul 2022 13:56:50 +0100
Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:
Because vma_invalidate_tlb() basically stores a TLB seqno, but the
actual invalidation is deferred to when the pages are unset, at
__i915_gem_object_unset_pages().
So, what happens is:
- on VMA sync mode, the need to invalidate TLB is marked at
__vma_put_pages(), before VMA unbind;
- on async, this is deferred to happen at ppgtt_unbind_vma(), where
it marks the need to invalidate TLBs.
On both cases, __i915_gem_object_unset_pages() is called later,
when the driver is ready to unmap the page.
Sorry still not clear to me why is the patch moving marking of the need
to invalidate (regardless if it a bit like today, or a seqno like in
this patch) from bind to unbind?
What if the seqno was stored in i915_vma_bind, where the bit is set
today, and all the hunks which touch the unbind and evict would
disappear from the patch. What wouldn't work in that case, if anything?
Ah, now I see your point.
I can't see any sense on having a sequence number at VMA bind, as the
unbind order can be different. The need of doing a full TLB invalidation
or not depends on the unbind order.
Sorry yes that was stupid from me.. What I was really thinking was the
approach I initially used for coalescing. Keeping the set_bit in bind
and then once the code enters intel_gt_invalidate_tlbs, takes a "ticket"
and waits on the mutex. Once it gets the mutex checks the ticket against
the GT copy and if two invalidations have passed since it was waiting on
the mutex it can immediately exit. That would seem like a minimal
improvement to batch things up.
But I guess it would still emit needless invalidations if there is no
contention, just a stream of serialized put pages. While the approach
from this patch can skip all but truly required.
Okay, go for it and thanks for the explanations.
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Regards,
Tvrtko
P.S. The last remaining "ugliness" is the 2nd call to invalidation from
evict. It would be nicer if there was a single common place to do it on
vma unbind but okay, I do not plan to dig into it so fine.
The way the current algorithm works is that drm_i915_gem_object can be
created on any order, and, at unbind/evict, they receive a seqno.
The seqno is incremented at intel_gt_invalidate_tlb():
void intel_gt_invalidate_tlb(struct intel_gt *gt, u32 seqno)
{
with_intel_gt_pm_if_awake(gt, wakeref) {
mutex_lock(>->tlb.invalidate_lock);
if (tlb_seqno_passed(gt, seqno))
goto unlock;
mmio_invalidate_full(gt);
write_seqcount_invalidate(>->tlb.seqno); // increment seqno
So, let's say 3 objects were created, on this order:
obj1
obj2
obj3
They would be unbind/evict on a different order. On that time,
the mm.tlb will be stamped with a seqno, using the number from the
last TLB flush, plus 1.
As different threads can be used to handle TLB flushes, let's imagine
two threads (just for the sake of having an example). On such case,
what we would have is:
seqno Thread 0 Thread 1
seqno=2 unbind/evict event
obj3.mm.tlb = seqno | 1
seqno=2 unbind/evict event
obj1.mm.tlb = seqno | 1
__i915_gem_object_unset_pages()
called for obj3, TLB flush happened,
invalidating both obj1 and obj2.
seqno += 2
seqno=4 unbind/evict event
obj1.mm.tlb = seqno | 1
__i915_gem_object_unset_pages()
called for obj1, don't flush.
...
__i915_gem_object_unset_pages() called for obj2, TLB flush happened
seqno += 2
seqno=6
So, basically the seqno is used to track when the object data stopped
being updated, because of an unbind/evict event, being later used by
intel_gt_invalidate_tlb() when called from __i915_gem_object_unset_pages(),
in order to check if a previous invalidation call was enough to invalidate
the object, or if a new call is needed.
Now, if seqno is stored at bind, data can still leak, as the assumption
made by intel_gt_invalidate_tlb() that the data stopped being used at
seqno is not true anymore.
Still, I agree that this logic is complex and should be better
documented. So, if you're now OK with this patch, I'll add the above
explanation inside a kernel-doc comment.
Regards,
Mauro
