[Intel-gfx] [PATCH] drm/i915: Limit the backpressure for i915_request allocation

[Tvrtko Ursulin]

On 12/09/2018 12:11, Chris Wilson wrote:
> If we try and fail to allocate a i915_request, we apply some
> backpressure on the clients to throttle the memory allocations coming
> from i915.ko. Currently, we wait until completely idle, but this is far
> too heavy and leads to some situations where the only escape is to
> declare a client hung and reset the GPU. The intent is to only ratelimit
> the allocation requests, so we need only wait for a jiffie before using
> the normal direct reclaim.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106680
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> ---
>   drivers/gpu/drm/i915/i915_request.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> index 09ed48833b54..588bc5a4d18b 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -736,7 +736,7 @@ i915_request_alloc(struct intel_engine_cs *engine, struct i915_gem_context *ctx)
>   		ret = i915_gem_wait_for_idle(i915,
>   					     I915_WAIT_LOCKED |
>   					     I915_WAIT_INTERRUPTIBLE,
> -					     MAX_SCHEDULE_TIMEOUT);
> +					     1);
>   		if (ret)
>   			goto err_unreserve;
>   
> 

What is the remaining value of even trying to wait for idle, instead of 
maybe just i915_request_retire and sleep for a jiffie? The intention 
would potentially read clearer since it is questionable there is any 
relationship with idle and rate limiting clients. In fact, now that I 
think of it, waiting for idle is a nice way to starve an unlucky client 
forever.

Regards,

Tvrtko