Re: [PATCH 1/7] drm/i915: Stop tracking timeline->inflight_seqnos

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 26/04/2018 18:49, Chris Wilson wrote:
> In commit 9b6586ae9f6b ("drm/i915: Keep a global seqno per-engine"), we
> moved from a global inflight counter to per-engine counters in the
> hope that will be easy to run concurrently in future. However, with the
> advent of the desire to move requests between engines, we do need a
> global counter to preserve the semantics that no engine wraps in the
> middle of a submit. (Although this semantic is now only required for gen7
> semaphore support, which only supports greater-then comparisons!)
> 
> v2: Keep a global counter of all requests ever submitted and force the
> reset when it wraps.
> 
> References: 9b6586ae9f6b ("drm/i915: Keep a global seqno per-engine")
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
>   drivers/gpu/drm/i915/i915_debugfs.c      |  5 ++--
>   drivers/gpu/drm/i915/i915_drv.h          |  1 +
>   drivers/gpu/drm/i915/i915_gem_timeline.h |  6 -----
>   drivers/gpu/drm/i915/i915_request.c      | 33 ++++++++++++------------
>   drivers/gpu/drm/i915/intel_engine_cs.c   |  5 ++--
>   5 files changed, 22 insertions(+), 28 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
> index 1c88805d3354..83c86257fe1c 100644
> --- a/drivers/gpu/drm/i915/i915_debugfs.c
> +++ b/drivers/gpu/drm/i915/i915_debugfs.c
> @@ -1340,10 +1340,9 @@ static int i915_hangcheck_info(struct seq_file *m, void *unused)
>   		struct rb_node *rb;
>   
>   		seq_printf(m, "%s:\n", engine->name);
> -		seq_printf(m, "\tseqno = %x [current %x, last %x], inflight %d\n",
> +		seq_printf(m, "\tseqno = %x [current %x, last %x]\n",
>   			   engine->hangcheck.seqno, seqno[id],
> -			   intel_engine_last_submit(engine),
> -			   engine->timeline->inflight_seqnos);
> +			   intel_engine_last_submit(engine));
>   		seq_printf(m, "\twaiters? %s, fake irq active? %s, stalled? %s\n",
>   			   yesno(intel_engine_has_waiter(engine)),
>   			   yesno(test_bit(engine->id,
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 8444ca8d5aa3..8fd9fb6efba5 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2061,6 +2061,7 @@ struct drm_i915_private {
>   		struct list_head timelines;
>   		struct i915_gem_timeline global_timeline;
>   		u32 active_requests;
> +		u32 request_serial;
>   
>   		/**
>   		 * Is the GPU currently considered idle, or busy executing
> diff --git a/drivers/gpu/drm/i915/i915_gem_timeline.h b/drivers/gpu/drm/i915/i915_gem_timeline.h
> index 33e01bf6aa36..6e82119e2cd8 100644
> --- a/drivers/gpu/drm/i915/i915_gem_timeline.h
> +++ b/drivers/gpu/drm/i915/i915_gem_timeline.h
> @@ -37,12 +37,6 @@ struct intel_timeline {
>   	u64 fence_context;
>   	u32 seqno;
>   
> -	/**
> -	 * Count of outstanding requests, from the time they are constructed
> -	 * to the moment they are retired. Loosely coupled to hardware.
> -	 */
> -	u32 inflight_seqnos;
> -
>   	spinlock_t lock;
>   
>   	/**
> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> index b692a9f7c357..b1993d4a1a53 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -241,6 +241,7 @@ static int reset_all_global_seqno(struct drm_i915_private *i915, u32 seqno)
>   			       sizeof(timeline->engine[id].global_sync));
>   	}
>   
> +	i915->gt.request_serial = seqno;
>   	return 0;
>   }
>   
> @@ -257,18 +258,22 @@ int i915_gem_set_global_seqno(struct drm_device *dev, u32 seqno)
>   	return reset_all_global_seqno(i915, seqno - 1);
>   }
>   
> -static int reserve_engine(struct intel_engine_cs *engine)
> +static int reserve_gt(struct drm_i915_private *i915)
>   {
> -	struct drm_i915_private *i915 = engine->i915;
> -	u32 active = ++engine->timeline->inflight_seqnos;
> -	u32 seqno = engine->timeline->seqno;
>   	int ret;
>   
> -	/* Reservation is fine until we need to wrap around */
> -	if (unlikely(add_overflows(seqno, active))) {
> +	/*
> +	 * Reservation is fine until we may need to wrap around
> +	 *
> +	 * By incrementing the serial for every request, we know that no
> +	 * individual engine may exceed that serial (as each is reset to 0
> +	 * on any wrap). This protects even the most pessimistic of migrations
> +	 * of every request from all engines onto just one.
> +	 */
> +	while (unlikely(++i915->gt.request_serial == 0)) {
>   		ret = reset_all_global_seqno(i915, 0);
>   		if (ret) {
> -			engine->timeline->inflight_seqnos--;
> +			i915->gt.request_serial--;
>   			return ret;
>   		}
>   	}
> @@ -279,15 +284,10 @@ static int reserve_engine(struct intel_engine_cs *engine)
>   	return 0;
>   }
>   
> -static void unreserve_engine(struct intel_engine_cs *engine)
> +static void unreserve_gt(struct drm_i915_private *i915)
>   {
> -	struct drm_i915_private *i915 = engine->i915;
> -
>   	if (!--i915->gt.active_requests)
>   		i915_gem_park(i915);
> -
> -	GEM_BUG_ON(!engine->timeline->inflight_seqnos);
> -	engine->timeline->inflight_seqnos--;
>   }
>   
>   void i915_gem_retire_noop(struct i915_gem_active *active,
> @@ -362,7 +362,6 @@ static void i915_request_retire(struct i915_request *request)
>   	list_del_init(&request->link);
>   	spin_unlock_irq(&engine->timeline->lock);
>   
> -	unreserve_engine(request->engine);
>   	advance_ring(request);
>   
>   	free_capture_list(request);
> @@ -424,6 +423,8 @@ static void i915_request_retire(struct i915_request *request)
>   	}
>   	spin_unlock_irq(&request->lock);
>   
> +	unreserve_gt(request->i915);
> +
>   	i915_sched_node_fini(request->i915, &request->sched);
>   	i915_request_put(request);
>   }
> @@ -642,7 +643,7 @@ i915_request_alloc(struct intel_engine_cs *engine, struct i915_gem_context *ctx)
>   		return ERR_CAST(ring);
>   	GEM_BUG_ON(!ring);
>   
> -	ret = reserve_engine(engine);
> +	ret = reserve_gt(i915);
>   	if (ret)
>   		goto err_unpin;
>   
> @@ -784,7 +785,7 @@ i915_request_alloc(struct intel_engine_cs *engine, struct i915_gem_context *ctx)
>   
>   	kmem_cache_free(i915->requests, rq);
>   err_unreserve:
> -	unreserve_engine(engine);
> +	unreserve_gt(i915);
>   err_unpin:
>   	engine->context_unpin(engine, ctx);
>   	return ERR_PTR(ret);
> diff --git a/drivers/gpu/drm/i915/intel_engine_cs.c b/drivers/gpu/drm/i915/intel_engine_cs.c
> index ac009f10c948..eba81d55dc3a 100644
> --- a/drivers/gpu/drm/i915/intel_engine_cs.c
> +++ b/drivers/gpu/drm/i915/intel_engine_cs.c
> @@ -1321,12 +1321,11 @@ void intel_engine_dump(struct intel_engine_cs *engine,
>   	if (i915_terminally_wedged(&engine->i915->gpu_error))
>   		drm_printf(m, "*** WEDGED ***\n");
>   
> -	drm_printf(m, "\tcurrent seqno %x, last %x, hangcheck %x [%d ms], inflight %d\n",
> +	drm_printf(m, "\tcurrent seqno %x, last %x, hangcheck %x [%d ms]\n",
>   		   intel_engine_get_seqno(engine),
>   		   intel_engine_last_submit(engine),
>   		   engine->hangcheck.seqno,
> -		   jiffies_to_msecs(jiffies - engine->hangcheck.action_timestamp),
> -		   engine->timeline->inflight_seqnos);
> +		   jiffies_to_msecs(jiffies - engine->hangcheck.action_timestamp));
>   	drm_printf(m, "\tReset count: %d (global %d)\n",
>   		   i915_reset_engine_count(error, engine),
>   		   i915_reset_count(error));
> 

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko

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