Re: [PATCH 1/2] drm/i915: Break busywaiting for requests on pending signals

[Tvrtko Ursulin]

Hi,

On 15/11/15 13:32, Chris Wilson wrote:
> The busywait in __i915_spin_request() does not respect pending signals
> and so may consume the entire timeslice for the task instead of
> returning to userspace to handle the signal.

Obviously correct to break the spin, but if spending a jiffie to react 
to signals was the only problem then it is not too severe.

Add something to the commit message about how it was found/reported and 
about the severity of impact, etc?

Otherwise,

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

> Fixes regression from
> commit 2def4ad99befa25775dd2f714fdd4d92faec6e34 [v4.2]
> Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Date:   Tue Apr 7 16:20:41 2015 +0100
>
>       drm/i915: Optimistically spin for the request completion
>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Jens Axboe <axboe@xxxxxxxxx>
> Cc; "Rogozhkin, Dmitry V" <dmitry.v.rogozhkin@xxxxxxxxx>
> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
> Cc: Eero Tamminen <eero.t.tamminen@xxxxxxxxx>
> Cc: "Rantala, Valtteri" <valtteri.rantala@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>   drivers/gpu/drm/i915/i915_gem.c | 13 ++++++++-----
>   1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 5cf4a1998273..740530c571d1 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1146,7 +1146,7 @@ static bool missed_irq(struct drm_i915_private *dev_priv,
>   	return test_bit(ring->id, &dev_priv->gpu_error.missed_irq_rings);
>   }
>
> -static int __i915_spin_request(struct drm_i915_gem_request *req)
> +static int __i915_spin_request(struct drm_i915_gem_request *req, int state)
>   {
>   	unsigned long timeout;
>
> @@ -1158,6 +1158,9 @@ static int __i915_spin_request(struct drm_i915_gem_request *req)
>   		if (i915_gem_request_completed(req, true))
>   			return 0;
>
> +		if (signal_pending_state(state, current))
> +			break;
> +
>   		if (time_after_eq(jiffies, timeout))
>   			break;
>
> @@ -1197,6 +1200,7 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
>   	struct drm_i915_private *dev_priv = dev->dev_private;
>   	const bool irq_test_in_progress =
>   		ACCESS_ONCE(dev_priv->gpu_error.test_irq_rings) & intel_ring_flag(ring);
> +	int state = interruptible ? TASK_INTERRUPTIBLE : TASK_UNINTERRUPTIBLE;
>   	DEFINE_WAIT(wait);
>   	unsigned long timeout_expire;
>   	s64 before, now;
> @@ -1221,7 +1225,7 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
>   	before = ktime_get_raw_ns();
>
>   	/* Optimistic spin for the next jiffie before touching IRQs */
> -	ret = __i915_spin_request(req);
> +	ret = __i915_spin_request(req, state);
>   	if (ret == 0)
>   		goto out;
>
> @@ -1233,8 +1237,7 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
>   	for (;;) {
>   		struct timer_list timer;
>
> -		prepare_to_wait(&ring->irq_queue, &wait,
> -				interruptible ? TASK_INTERRUPTIBLE : TASK_UNINTERRUPTIBLE);
> +		prepare_to_wait(&ring->irq_queue, &wait, state);
>
>   		/* We need to check whether any gpu reset happened in between
>   		 * the caller grabbing the seqno and now ... */
> @@ -1252,7 +1255,7 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
>   			break;
>   		}
>
> -		if (interruptible && signal_pending(current)) {
> +		if (signal_pending_state(state, current)) {
>   			ret = -ERESTARTSYS;
>   			break;
>   		}
--
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