drm/i915: Keep a per-engine request pool

Add a tiny per-engine request mempool so that we should always have a
request available for powermanagement allocations from tricky
contexts. This reserve is expected to be only used for kernel
contexts when barriers must be emitted [almost] without fail.

The main consumer for this reserved request is expected to be engine-pm,
for which we know that there will always be at least the previous pm
request that we can reuse under mempressure (so there should always be
a spare request for engine_park()).

This is an alternative to using a comparatively bulky mempool, which
requires custom handling for both our reserved allocation requirement
and to protect our TYPESAFE_BY_RCU slab cache. The advantage of mempool
would be that it would allow us to keep a larger per-engine request
pool. However, converting over to mempool is straightforward should the
need arise.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-and-tested-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200402184037.21630-1-chris@chris-wilson.co.uk

diff --git a/drivers/gpu/drm/i915/gt/intel_engine_cs.c b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
index 843cb6f..5f45c82 100644 (file)
--- a/drivers/gpu/drm/i915/gt/intel_engine_cs.c
+++ b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
@@ -431,7 +431,14 @@ void intel_engines_free(struct intel_gt *gt)
        struct intel_engine_cs *engine;
        enum intel_engine_id id;
 
+       /* Free the requests! dma-resv keeps fences around for an eternity */
+       rcu_barrier();
+
        for_each_engine(engine, gt, id) {
+               if (engine->request_pool)
+                       kmem_cache_free(i915_request_slab_cache(),
+                                       engine->request_pool);
+
                kfree(engine);
                gt->engine[id] = NULL;
        }

diff --git a/drivers/gpu/drm/i915/gt/intel_engine_types.h b/drivers/gpu/drm/i915/gt/intel_engine_types.h
index 80cdde7..de8e6ed 100644 (file)
--- a/drivers/gpu/drm/i915/gt/intel_engine_types.h
+++ b/drivers/gpu/drm/i915/gt/intel_engine_types.h
@@ -308,6 +308,9 @@ struct intel_engine_cs {
                struct list_head hold; /* ready requests, but on hold */
        } active;
 
+       /* keep a request in reserve for a [pm] barrier under oom */
+       struct i915_request *request_pool;
+
        struct llist_head barrier_tasks;
 
        struct intel_context *kernel_context; /* pinned */

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index 3388c5b..22635bb 100644 (file)
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -101,6 +101,11 @@ static signed long i915_fence_wait(struct dma_fence *fence,
                                 timeout);
 }
 
+struct kmem_cache *i915_request_slab_cache(void)
+{
+       return global.slab_requests;
+}
+
 static void i915_fence_release(struct dma_fence *fence)
 {
        struct i915_request *rq = to_request(fence);
@@ -115,6 +120,10 @@ 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 */
+       if (!cmpxchg(&rq->engine->request_pool, NULL, rq))
+               return;
+
        kmem_cache_free(global.slab_requests, rq);
 }
 
@@ -629,14 +638,22 @@ static void retire_requests(struct intel_timeline *tl)
 }
 
 static noinline struct i915_request *
-request_alloc_slow(struct intel_timeline *tl, gfp_t gfp)
+request_alloc_slow(struct intel_timeline *tl,
+                  struct i915_request **rsvd,
+                  gfp_t gfp)
 {
        struct i915_request *rq;
 
-       if (list_empty(&tl->requests))
-               goto out;
+       /* If we cannot wait, dip into our reserves */
+       if (!gfpflags_allow_blocking(gfp)) {
+               rq = xchg(rsvd, NULL);
+               if (!rq) /* Use the normal failure path for one final WARN */
+                       goto out;
 
-       if (!gfpflags_allow_blocking(gfp))
+               return rq;
+       }
+
+       if (list_empty(&tl->requests))
                goto out;
 
        /* Move our oldest request to the slab-cache (if not in use!) */
@@ -721,7 +738,7 @@ __i915_request_create(struct intel_context *ce, gfp_t gfp)
        rq = kmem_cache_alloc(global.slab_requests,
                              gfp | __GFP_RETRY_MAYFAIL | __GFP_NOWARN);
        if (unlikely(!rq)) {
-               rq = request_alloc_slow(tl, gfp);
+               rq = request_alloc_slow(tl, &ce->engine->request_pool, gfp);
                if (!rq) {
                        ret = -ENOMEM;
                        goto err_unreserve;

diff --git a/drivers/gpu/drm/i915/i915_request.h b/drivers/gpu/drm/i915/i915_request.h
index 3c552bf..d8ce908 100644 (file)
--- a/drivers/gpu/drm/i915/i915_request.h
+++ b/drivers/gpu/drm/i915/i915_request.h
@@ -300,6 +300,8 @@ static inline bool dma_fence_is_i915(const struct dma_fence *fence)
        return fence->ops == &i915_fence_ops;
 }
 
+struct kmem_cache *i915_request_slab_cache(void);
+
 struct i915_request * __must_check
 __i915_request_create(struct intel_context *ce, gfp_t gfp);
 struct i915_request * __must_check