Re: [Intel-gfx] [PATCH] drm/i915/selftests: Bump the scheduling threshold for fast heartbeats

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

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

> Since we are system_highpri_wq, we expected the heartbeat to be
> scheduled promptly. However, we see delays of over 10ms upsetting our
> assertions. Accept this as inevitable and bump the error threshold to
> 20ms (from 6ms).
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/gt/selftest_engine_heartbeat.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/selftest_engine_heartbeat.c b/drivers/gpu/drm/i915/gt/selftest_engine_heartbeat.c
> index b88aa35ad75b..e88a01390dc5 100644
> --- a/drivers/gpu/drm/i915/gt/selftest_engine_heartbeat.c
> +++ b/drivers/gpu/drm/i915/gt/selftest_engine_heartbeat.c
> @@ -197,6 +197,7 @@ static int cmp_u32(const void *_a, const void *_b)
>  
>  static int __live_heartbeat_fast(struct intel_engine_cs *engine)
>  {
> +	const int error_threshold = max(20000, jffies_to_usecs(6));

s/jffies/jiffies

Also for the commit message, 6 jiffies are not 6ms so it needs
some mending.

-Mika

>  	struct intel_context *ce;
>  	struct i915_request *rq;
>  	ktime_t t0, t1;
> @@ -254,12 +255,18 @@ static int __live_heartbeat_fast(struct intel_engine_cs *engine)
>  		times[0],
>  		times[ARRAY_SIZE(times) - 1]);
>  
> -	/* Min work delay is 2 * 2 (worst), +1 for scheduling, +1 for slack */
> -	if (times[ARRAY_SIZE(times) / 2] > jiffies_to_usecs(6)) {
> +	/*
> +	 * Ideally, the upper bound on min work delay would be something like
> +	 * 2 * 2 (worst), +1 for scheduling, +1 for slack. In practice, we
> +	 * are, even with system_wq_highpri, at the mercy of the CPU scheduler
> +	 * and may be stuck behind some slow work for many millisecond. Such
> +	 * as our very own display workers.
> +	 */
> +	if (times[ARRAY_SIZE(times) / 2] > error_threshold) {
>  		pr_err("%s: Heartbeat delay was %uus, expected less than %dus\n",
>  		       engine->name,
>  		       times[ARRAY_SIZE(times) / 2],
> -		       jiffies_to_usecs(6));
> +		       error_threshold);
>  		err = -EINVAL;
>  	}
>  
> -- 
> 2.20.1
>
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