[RFC PATCH] drm/scheduler: Set the FIFO schedulig policy as the default

[Christian König]

I've seen that one, but couldn't figure out what was actually changed 
between the two.

If you just fixed a typo feel free to add my R-B to this one as well.

Christian.

Am 24.10.22 um 19:25 schrieb Luben Tuikov:
> Hi Christian,
>
> Can you add an R-B to V1 of this patch, as the v1 is what I'd like pushed.
> I sent it right after this one.
>
> Regards,
> Luben
>
> On 2022-10-24 06:42, Christian König wrote:
>> Am 22.10.22 um 03:09 schrieb Luben Tuikov:
>>> The currently default Round-Robin GPU scheduling can result in starvation
>>> of entities which have a large number of jobs, over entities which have
>>> a very small number of jobs (single digit).
>>>
>>> This can be illustrated in the following diagram, where jobs are
>>> alphabetized to show their chronological order of arrival, where job A is
>>> the oldest, B is the second oldest, and so on, to J, the most recent job to
>>> arrive.
>>>
>>>       ---> entities
>>> j | H-F-----A--E--I--
>>> o | --G-----B-----J--
>>> b | --------C--------
>>> s\/ --------D--------
>>>
>>> WLOG, asuming all jobs are "ready", then a R-R scheduling will execute them
>>> in the following order (a slice off of the top of the entities' list),
>>>
>>> H, F, A, E, I, G, B, J, C, D.
>>>
>>> However, to mitigate job starvation, we'd rather execute C and D before E,
>>> and so on, given, of course, that they're all ready to be executed.
>>>
>>> So, if all jobs are ready at this instant, the order of execution for this
>>> and the next 9 instances of picking the next job to execute, should really
>>> be,
>>>
>>> A, B, C, D, E, F, G, H, I, J,
>>>
>>> which is their chronological order. The only reason for this order to be
>>> broken, is if an older job is not yet ready, but a younger job is ready, at
>>> an instant of picking a new job to execute. For instance if job C wasn't
>>> ready at time 2, but job D was ready, then we'd pick job D, like this:
>>>
>>> 0 +1 +2  ...
>>> A, B, D, ...
>>>
>>> And from then on, C would be preferred before all other jobs, if it is ready
>>> at the time when a new job for execution is picked. So, if C became ready
>>> two steps later, the execution order would look like this:
>>>
>>> ......0 +1 +2  ...
>>> A, B, D, E, C, F, G, H, I, J
>>>
>>> This is what the FIFO GPU scheduling algorithm achieves. It uses a
>>> Red-Black tree to keep jobs sorted in chronological order, where picking
>>> the oldest job is O(1) (we use the "cached" structure), and balancing the
>>> tree is O(log n). IOW, it picks the *oldest ready* job to execute now.
>>>
>>> The implemntation is already in the kernel, and this commit only changes
>>> the default GPU scheduling algorithm to use.
>>>
>>> This was tested and achieves about 1% faster performance over the Round
>>> Robin algorithm.
>>>
>>> Cc: Christian König <christian.koenig at amd.com>
>>> Cc: Alex Deucher <Alexander.Deucher at amd.com>
>>> Cc: Direct Rendering Infrastructure - Development <dri-devel at lists.freedesktop.org>
>>> Signed-off-by: Luben Tuikov <luben.tuikov at amd.com>
>> Reviewed-by: Christian König <christian.koenig at amd.com>
>>
>>> ---
>>>    drivers/gpu/drm/scheduler/sched_main.c | 4 ++--
>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>> index 2fab218d708279..d0ff9e11cb69fa 100644
>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>> @@ -62,13 +62,13 @@
>>>    #define to_drm_sched_job(sched_job)		\
>>>    		container_of((sched_job), struct drm_sched_job, queue_node)
>>>    
>>> -int drm_sched_policy = DRM_SCHED_POLICY_RR;
>>> +int drm_sched_policy = DRM_SCHED_POLICY_FIFO;
>>>    
>>>    /**
>>>     * DOC: sched_policy (int)
>>>     * Used to override default entities scheduling policy in a run queue.
>>>     */
>>> -MODULE_PARM_DESC(sched_policy, "Specify schedule policy for entities on a runqueue, " __stringify(DRM_SCHED_POLICY_RR) " = Round Robin (default), " __stringify(DRM_SCHED_POLICY_FIFO) " = use FIFO.");
>>> +MODULE_PARM_DESC(sched_policy, "Specify the scheduling policy for entities on a run-queue, " __stringify(DRM_SCHED_POLICY_RR) " = Round Robin, " __stringify(DRM_SCHED_POLICY_FIFO) " = FIFO (default).");
>>>    module_param_named(sched_policy, drm_sched_policy, int, 0444);
>>>    
>>>    static __always_inline bool drm_sched_entity_compare_before(struct rb_node *a,
>>>
>>> base-commit: 16d2a3f2ad1d2b95bf9122c910c63b0efe74179d