[Intel-gfx] [PATCH 2/5] drm/i915: Use a simpler scheme for caching i915_request

[Tvrtko Ursulin]

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 at jlekstrand.net>
> Cc: Jon Bloomfield <jon.bloomfield at intel.com>
> Cc: Daniel Vetter <daniel.vetter at intel.com>
> Cc: Matthew Auld <matthew.auld at intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst at 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.
>   	 *
>