Re: [PATCH v2 2/7] drm/i915: Disable tasklet scheduling across initial scheduling

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 07/05/2018 14:57, Chris Wilson wrote:
> During request submission, we call the engine->schedule() function so
> that we may reorder the active requests as required for inheriting the
> new request's priority. This may schedule several tasklets to run on the
> local CPU, but we will need to schedule the tasklets again for the new
> request. Delay all the local tasklets until the end, so that we only
> have to process the queue just once.
> 
> v2: Beware PREEMPT_RCU, as then local_bh_disable() is then not a
> superset of rcu_read_lock().
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
> ---
>   drivers/gpu/drm/i915/i915_request.c | 5 ++---
>   1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> index e4cf76ec14a6..f336942229cf 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -1110,12 +1110,11 @@ void __i915_request_add(struct i915_request *request, bool flush_caches)
>   	 * decide whether to preempt the entire chain so that it is ready to
>   	 * run at the earliest possible convenience.
>   	 */
> -	rcu_read_lock();
> +	local_bh_disable();
> +	rcu_read_lock(); /* RCU serialisation for set-wedged protection */
>   	if (engine->schedule)
>   		engine->schedule(request, &request->ctx->sched);
>   	rcu_read_unlock();
> -
> -	local_bh_disable();
>   	i915_sw_fence_commit(&request->submit);
>   	local_bh_enable(); /* Kick the execlists tasklet if just scheduled */
>   
> 

Before I wasn't sure about this one, since it lengthens the softirq off 
section and still doesn't make the re-schedule atomic. But today I am 
thinking that in normal use dependency chains should not be that deep 
for it to become an issue. In the light of that, go for it:

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

Regards,

Tvrtko




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