On 07/05/2024 00:23, Matthew Brost wrote:
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.
Simply for extra paranoia or you remember if there was a reason identified?
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.
I wonder what is different between that and having a single shared
unbound queue and let kernel manage the concurrency? Both this..
So anyway.. ordered vs unordered - drm sched dictated or at driver's choice?
Default ordered, driver can override with unordered.
.. and this, go back to my original question - whether the default queue
must be ordered or not, or under which circustmances can drivers choose
unordered. I think in drm_sched_init, where kerneldoc says it will
create an ordered queue, it would be good to document the rules.
Regards,
Tvrtko
