Re: [Intel-gfx] [PATCH] drm/i915/execlists: Pull tasklet interrupt-bh local to direct submission

[Francisco Jerez <currojerez@riseup.net>]

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Chris Wilson <chris@chris-wilson.co.uk> writes:

> We dropped calling process_csb prior to handling direct submission in
> order to avoid the nesting of spinlocks and lift process_csb() and the
> majority of the tasklet out of irq-off. However, we do want to avoid
> ksoftirqd latency in the fast path, so try and pull the interrupt-bh
> local to direct submission if we can acquire the tasklet's lock.
>
> v2: Tweak the balance to avoid over submitting lite-restores
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Francisco Jerez <currojerez@riseup.net>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/gt/intel_lrc.c    | 44 ++++++++++++++++++++------
>  drivers/gpu/drm/i915/gt/selftest_lrc.c |  2 +-
>  2 files changed, 36 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> index f09dd87324b9..dceb65a0088f 100644
> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> @@ -2884,17 +2884,17 @@ static void queue_request(struct intel_engine_cs *engine,
>  	set_bit(I915_FENCE_FLAG_PQUEUE, &rq->fence.flags);
>  }
>  
> -static void __submit_queue_imm(struct intel_engine_cs *engine)
> +static bool pending_csb(const struct intel_engine_execlists *el)
>  {
> -	struct intel_engine_execlists * const execlists = &engine->execlists;
> +	return READ_ONCE(*el->csb_write) != READ_ONCE(el->csb_head);
> +}
>  
> -	if (reset_in_progress(execlists))
> -		return; /* defer until we restart the engine following reset */
> +static bool skip_lite_restore(struct intel_engine_execlists *el,
> +			      const struct i915_request *rq)
> +{
> +	struct i915_request *inflight = execlists_active(el);
>  
> -	if (execlists->tasklet.func == execlists_submission_tasklet)
> -		__execlists_submission_tasklet(engine);
> -	else
> -		tasklet_hi_schedule(&execlists->tasklet);
> +	return inflight && inflight->context == rq->context;
>  }
>  
>  static void submit_queue(struct intel_engine_cs *engine,
> @@ -2905,8 +2905,34 @@ static void submit_queue(struct intel_engine_cs *engine,
>  	if (rq_prio(rq) <= execlists->queue_priority_hint)
>  		return;
>  
> +	if (reset_in_progress(execlists))
> +		return; /* defer until we restart the engine following reset */
> +
> +	/*
> +	 * Suppress immediate lite-restores, leave that to the tasklet.
> +	 *
> +	 * However, we leave the queue_priority_hint unset so that if we do
> +	 * submit a second context, we push that into ELSP[1] immediately.
> +	 */
> +	if (skip_lite_restore(execlists, rq))
> +		return;
> +
Why do you need to treat lite-restore specially here?

Anyway, trying this out now in combination with my patches now.

> +	/* Hopefully we clear execlists->pending[] to let us through */
> +	if (execlists->pending[0] && tasklet_trylock(&execlists->tasklet)) {
> +		process_csb(engine);
> +		tasklet_unlock(&execlists->tasklet);
> +		if (skip_lite_restore(execlists, rq))
> +			return;
> +	}
> +
>  	execlists->queue_priority_hint = rq_prio(rq);
> -	__submit_queue_imm(engine);
> +	__execlists_submission_tasklet(engine);
> +
> +	/* Try and pull an interrupt-bh queued on another CPU to here */
> +	if (pending_csb(execlists) && tasklet_trylock(&execlists->tasklet)) {
> +		process_csb(engine);
> +		tasklet_unlock(&execlists->tasklet);
> +	}
>  }
>  
>  static bool ancestor_on_hold(const struct intel_engine_cs *engine,
> diff --git a/drivers/gpu/drm/i915/gt/selftest_lrc.c b/drivers/gpu/drm/i915/gt/selftest_lrc.c
> index 6f06ba750a0a..c5c4b07a7d5f 100644
> --- a/drivers/gpu/drm/i915/gt/selftest_lrc.c
> +++ b/drivers/gpu/drm/i915/gt/selftest_lrc.c
> @@ -1028,7 +1028,7 @@ static int live_timeslice_rewind(void *arg)
>  		if (IS_ERR(rq[1]))
>  			goto err;
>  
> -		err = wait_for_submit(engine, rq[1], HZ / 2);
> +		err = wait_for_submit(engine, rq[0], HZ / 2);
>  		if (err) {
>  			pr_err("%s: failed to submit first context\n",
>  			       engine->name);
> -- 
> 2.20.1

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