On Sat, Nov 05, 2016 at 10:35:59AM +0200, Imre Deak wrote:
> There is a small race where a new request can be submitted and retired
> after the idle worker started to run which leads to idling the GPU too
> early. Fix this by deferring the idling to the pending instance of the
> worker.
>
> This scenario was pointed out by Chris.
>
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Imre Deak <imre.deak@xxxxxxxxx>
> ---
> drivers/gpu/drm/i915/i915_gem.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 0dbf38c..36a16df 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2766,6 +2766,13 @@ i915_gem_idle_work_handler(struct work_struct *work)
> goto out_rearm;
> }
>
> + /*
> + * New request retired after this work handler started, extend active
> + * period until next instance of the work.
> + */
> + if (work_pending(work))
> + goto out_unlock;
Ok, it took some digging around inside kernel/workqueue.c to come to
agreement that this works.
WORK_STRUCT_PENDING_BIT is set when we queue the work and released just
prior to execution of the work->func. A race on active_requests before
we take the struct_mutex will leave this set.
Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
For bonus points
diff --git a/drivers/gpu/drm/i915/i915_gem_request.c b/drivers/gpu/drm/i915/i915_gem_request.c
index cfda095..5d349e0 100644
--- a/drivers/gpu/drm/i915/i915_gem_request.c
+++ b/drivers/gpu/drm/i915/i915_gem_request.c
@@ -1146,7 +1146,7 @@ void i915_gem_retire_requests(struct drm_i915_private *dev_priv)
engine_retire_requests(engine);
if (!dev_priv->gt.active_requests)
- queue_delayed_work(dev_priv->wq,
- &dev_priv->gt.idle_work,
- msecs_to_jiffies(100));
+ mod_delayed_work(dev_priv->wq,
+ &dev_priv->gt.idle_work,
+ msecs_to_jiffies(100));
}
--
Chris Wilson, Intel Open Source Technology Centre
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