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@jlekstrand.net Cc: Jon Bloomfield jon.bloomfield@intel.com Cc: Daniel Vetter daniel.vetter@intel.com Cc: Matthew Auld matthew.auld@intel.com Cc: Maarten Lankhorst maarten.lankhorst@linux.intel.com
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.