Re: [Intel-gfx] [PATCH v2 06/21] drm/i915/gt: Batch TLB invalidations

[Mauro Carvalho Chehab <mauro.chehab@linux.intel.com>]

On Thu, 28 Jul 2022 08:32:32 +0200
Mauro Carvalho Chehab <mauro.chehab@linux.intel.com> wrote:

> On Wed, 27 Jul 2022 13:56:50 +0100
> Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> 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.
> 
> 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(&gt->tlb.invalidate_lock);
> 		if (tlb_seqno_passed(gt, seqno))
> 				goto unlock;
> 
> 		mmio_invalidate_full(gt);
> 
> 		write_seqcount_invalidate(&gt->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

cut-and-paste typo. it should be, instead:

 		obj2.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.

I'm enclosing the kernel-doc patch (to be applied after moving the code into
its own files: intel_tlb.c/intel_tlb.h):

[PATCH] drm/i915/gt: document TLB cache invalidation functions

Add a description for the TLB cache invalidation algorithm and for
the related kAPI functions.

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

diff --git a/drivers/gpu/drm/i915/gt/intel_tlb.c b/drivers/gpu/drm/i915/gt/intel_tlb.c
index af8cae979489..8eda0743da74 100644
--- a/drivers/gpu/drm/i915/gt/intel_tlb.c
+++ b/drivers/gpu/drm/i915/gt/intel_tlb.c
@@ -145,6 +145,18 @@ static void mmio_invalidate_full(struct intel_gt *gt)
 	intel_uncore_forcewake_put_delayed(uncore, FORCEWAKE_ALL);
 }
 
+/**
+ * intel_gt_invalidate_tlb_full - do full TLB cache invalidation
+ * @gt: GT structure
+ * @seqno: sequence number
+ *
+ * Do a full TLB cache invalidation if the @seqno is bigger than the last
+ * full TLB cache invalidation.
+ *
+ * Note:
+ * The TLB cache invalidation logic depends on GEN-specific registers.
+ * It currently supports GEN8 to GEN12 and GuC-based TLB cache invalidation.
+ */
 void intel_gt_invalidate_tlb_full(struct intel_gt *gt, u32 seqno)
 {
 	intel_wakeref_t wakeref;
@@ -177,6 +189,12 @@ void intel_gt_init_tlb(struct intel_gt *gt)
 	seqcount_mutex_init(&gt->tlb.seqno, &gt->tlb.invalidate_lock);
 }
 
+/**
+ * intel_gt_fini_tlb - initialize TLB-specific vars
+ * @gt: GT structure
+ *
+ * Frees any resources needed by TLB cache invalidation logic.
+ */
 void intel_gt_fini_tlb(struct intel_gt *gt)
 {
 	mutex_destroy(&gt->tlb.invalidate_lock);
diff --git a/drivers/gpu/drm/i915/gt/intel_tlb.h b/drivers/gpu/drm/i915/gt/intel_tlb.h
index 46ce25bf5afe..d186f5d5901f 100644
--- a/drivers/gpu/drm/i915/gt/intel_tlb.h
+++ b/drivers/gpu/drm/i915/gt/intel_tlb.h
@@ -11,16 +11,99 @@
 
 #include "intel_gt_types.h"
 
+/**
+ * DOC: TLB cache invalidation logic
+ *
+ * The way the current algorithm works is that drm_i915_gem_object can be
+ * created on any order. At unbind/evict time, the object is warranted that
+ * it won't be used anymore. So, they store a sequence number provided by
+ * intel_gt_next_invalidate_tlb_full().This can happen either at
+ * __vma_put_pages(), for VMA sync unbind, or at ppgtt_unbind_vma(), for
+ * VMA async VMA bind.
+ *
+ * At __i915_gem_object_unset_pages(), intel_gt_invalidate_tlb() is called,
+ * where it checks if the sequence number of the object was already invalidated
+ * or not. If not, it increments it::
+ *
+ *   void intel_gt_invalidate_tlb(struct intel_gt *gt, u32 seqno)
+ *   {
+ *   ...
+ * 	with_intel_gt_pm_if_awake(gt, wakeref) {
+ * 		mutex_lock(&gt->tlb.invalidate_lock);
+ * 		if (tlb_seqno_passed(gt, seqno))
+ * 				goto unlock;
+ *
+ * 		mmio_invalidate_full(gt);
+ *
+ * 		write_seqcount_invalidate(&gt->tlb.seqno); // increment seqno
+ *    ...
+ *
+ * So, let's say the current seqno is 2 and 3 new objects were created,
+ * on this order:
+ *
+ * 	obj1
+ * 	obj2
+ * 	obj3
+ *
+ * They can be unbind/evict on a different order. At unbind/evict time,
+ * the mm.tlb will be stamped with the sequence number, using the number
+ * from the last TLB flush, plus 1.
+ *
+ * Different threads may be used on unbind/evict and/or unset pages.
+ *
+ * As the logic at void intel_gt_invalidate_tlb() is protected by a mutex,
+ * for simplicity, let's consider just two threads::
+ *
+ *   sequence number	Thread 0		Thread 1
+ *
+ *   seqno=2
+ *			unbind/evict event
+ * 			obj3.mm.tlb = seqno | 1
+ *
+ *			unbind/evict event
+ * 			obj1.mm.tlb = seqno | 1
+ * 						__i915_gem_object_unset_pages()
+ * 						called for obj3 => TLB flush
+ * 						invalidating both obj1 and obj2.
+ * 						seqno += 2
+ *   seqno=4
+ *			unbind/evict event
+ * 			obj2.mm.tlb = seqno | 1
+ * 						__i915_gem_object_unset_pages()
+ * 						called for obj1, don't flush,
+ *						as past flush invalidated obj1
+ *
+ * 						__i915_gem_object_unset_pages()
+ *						called for obj2 => TLB flush
+ * 						seqno += 2
+ *   seqno=6
+ */
+
 void intel_gt_invalidate_tlb_full(struct intel_gt *gt, u32 seqno);
 
 void intel_gt_init_tlb(struct intel_gt *gt);
 void intel_gt_fini_tlb(struct intel_gt *gt);
 
+/**
+ * intel_gt_tlb_seqno - Returns the current TLB invlidation sequence number
+ *
+ * @gt: GT structure
+ *
+ * There's no need to lock while calling it, as seqprop_sequence is thread-safe
+ */
 static inline u32 intel_gt_tlb_seqno(const struct intel_gt *gt)
 {
 	return seqprop_sequence(&gt->tlb.seqno);
 }
 
+/**
+ * intel_gt_next_invalidate_tlb_full - Returns the next TLB full invalidation
+ *	sequence number
+ *
+ * @gt: GT structure
+ *
+ * There's no need to lock while calling it, as seqprop_sequence is thread-safe
+ */
 static inline u32 intel_gt_next_invalidate_tlb_full(const struct intel_gt *gt)
 {
 	return intel_gt_tlb_seqno(gt) | 1;