On 09/06/2021 22:29, Jason Ekstrand wrote:
Instead of attempting to recycle a request in to the cache when it
retires, stuff a new one in the cache every time we allocate a request
for some other reason.
I supposed the "why?" is "simpler scheme" - but in what way it is simpler?
Signed-off-by: Jason Ekstrand <jason@xxxxxxxxxxxxxx>
Cc: Jon Bloomfield <jon.bloomfield@xxxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxxx>
Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/i915_request.c | 66 ++++++++++++++---------------
1 file changed, 31 insertions(+), 35 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index 48c5f8527854b..e531c74f0b0e2 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -128,41 +128,6 @@ static void i915_fence_release(struct dma_fence *fence)
i915_sw_fence_fini(&rq->submit);
i915_sw_fence_fini(&rq->semaphore);
- /*
- * Keep one request on each engine for reserved use under mempressure
- *
- * We do not hold a reference to the engine here and so have to be
- * very careful in what rq->engine we poke. The virtual engine is
- * referenced via the rq->context and we released that ref during
- * i915_request_retire(), ergo we must not dereference a virtual
- * engine here. Not that we would want to, as the only consumer of
- * the reserved engine->request_pool is the power management parking,
- * which must-not-fail, and that is only run on the physical engines.
- *
- * Since the request must have been executed to be have completed,
- * we know that it will have been processed by the HW and will
- * not be unsubmitted again, so rq->engine and rq->execution_mask
- * at this point is stable. rq->execution_mask will be a single
- * bit if the last and _only_ engine it could execution on was a
- * physical engine, if it's multiple bits then it started on and
- * could still be on a virtual engine. Thus if the mask is not a
- * power-of-two we assume that rq->engine may still be a virtual
- * engine and so a dangling invalid pointer that we cannot dereference
- *
- * For example, consider the flow of a bonded request through a virtual
- * engine. The request is created with a wide engine mask (all engines
- * that we might execute on). On processing the bond, the request mask
- * is reduced to one or more engines. If the request is subsequently
- * bound to a single engine, it will then be constrained to only
- * execute on that engine and never returned to the virtual engine
- * after timeslicing away, see __unwind_incomplete_requests(). Thus we
- * know that if the rq->execution_mask is a single bit, rq->engine
- * can be a physical engine with the exact corresponding mask.
- */
- if (is_power_of_2(rq->execution_mask) &&
- !cmpxchg(&rq->engine->request_pool, NULL, rq))
- return;
-
kmem_cache_free(global.slab_requests, rq);
}
@@ -869,6 +834,29 @@ static void retire_requests(struct intel_timeline *tl)
break;
}
+static void
+ensure_cached_request(struct i915_request **rsvd, gfp_t gfp)
+{
+ struct i915_request *rq;
+
+ /* Don't try to add to the cache if we don't allow blocking. That
+ * just increases the chance that the actual allocation will fail.
+ */
Linus has been known to rant passionately against this comment style so
we actively try to never use it.
Rega4rds,
Tvrtko
+ if (gfpflags_allow_blocking(gfp))
+ return;
+
+ if (READ_ONCE(rsvd))
+ return;
+
+ rq = kmem_cache_alloc(global.slab_requests,
+ gfp | __GFP_RETRY_MAYFAIL | __GFP_NOWARN);
+ if (!rq)
+ return; /* Oops but nothing we can do */
+
+ if (cmpxchg(rsvd, NULL, rq))
+ kmem_cache_free(global.slab_requests, rq);
+}
+
static noinline struct i915_request *
request_alloc_slow(struct intel_timeline *tl,
struct i915_request **rsvd,
@@ -937,6 +925,14 @@ __i915_request_create(struct intel_context *ce, gfp_t gfp)
/* Check that the caller provided an already pinned context */
__intel_context_pin(ce);
+ /* Before we do anything, try to make sure we have at least one
+ * request in the engine's cache. If we get here with GPF_NOWAIT
+ * (this can happen when switching to a kernel context), we we want
+ * to try very hard to not fail and we fall back to this cache.
+ * Top it off with a fresh request whenever it's empty.
+ */
+ ensure_cached_request(&ce->engine->request_pool, gfp);
+
/*
* Beware: Dragons be flying overhead.
*
