Re: drm scheduler and wq flavours

[Matthew Brost <matthew.brost@intel.com>]

On Thu, May 02, 2024 at 03:33:50PM +0100, Tvrtko Ursulin wrote:
> 
> Hi all,
> 
> Continuing after the brief IRC discussion yesterday regarding work queues
> being prone to deadlocks or not, I had a browse around the code base and
> ended up a bit confused.
> 
> When drm_sched_init documents and allocates an *ordered* wq, if no custom
> one was provided, could someone remind me was the ordered property
> fundamental for something to work correctly? Like run_job vs free_job
> ordering?
> 

Before the work queue (kthread design), run_job & free_job were ordered.
It was decided to not break this existing behavior.

> I ask because it appears different drivers to different things and at the
> moment it looks we have all possible combos or ordered/unordered, bound and
> unbound, shared or not shared with the timeout wq, or even unbound for the
> timeout wq.
> 
> The drivers worth looking at in this respect are probably nouveau, panthor,
> pvr and xe.
> 
> Nouveau also talks about a depency betwen run_job and free_job and goes to
> create two unordered wqs.
> 
> Then xe looks a bit funky with the workaround/hack for lockep where it
> creates 512 work queues and hands them over to user queues in round-robin
> fashion. (Instead of default 1:1.) Which I suspect is a problem which should
> be applicable for any 1:1 driver given a thorough enough test suite.
> 

I think lockdep ran out of chains or something when executing some wild
IGT with 1:1. Yes, any driver with a wild enough test would likely hit
this lockdep splat too. Using a pool probably is not bad idea either.

> So anyway.. ordered vs unordered - drm sched dictated or at driver's choice?
>

Default ordered, driver can override with unordered.

Matt
 
> Regards,
> 
> Tvrtko