Re: [PATCH 4/8] drm/sched: Add generic scheduler message interface

["Christian König" <christian.koenig@amd.com>]

Am 03.08.23 um 10:58 schrieb Daniel Vetter:
> On Thu, 3 Aug 2023 at 10:53, Christian König <christian.koenig@amd.com> wrote:
>> Am 01.08.23 um 22:50 schrieb Matthew Brost:
>>> Add generic schedule message interface which sends messages to backend
>>> from the drm_gpu_scheduler main submission thread. The idea is some of
>>> these messages modify some state in drm_sched_entity which is also
>>> modified during submission. By scheduling these messages and submission
>>> in the same thread their is not race changing states in
>>> drm_sched_entity.
>>>
>>> This interface will be used in XE, new Intel GPU driver, to cleanup,
>>> suspend, resume, and change scheduling properties of a drm_sched_entity.
>>>
>>> The interface is designed to be generic and extendable with only the
>>> backend understanding the messages.
>> I'm still extremely frowned on this.
>>
>> If you need this functionality then let the drivers decide which
>> runqueue the scheduler should use.
>>
>> When you then create a single threaded runqueue you can just submit work
>> to it and serialize this with the scheduler work.
>>
>> This way we wouldn't duplicate this core kernel function inside the
>> scheduler.
> Yeah that's essentially the design we picked for the tdr workers,
> where some drivers have requirements that all tdr work must be done on
> the same thread (because of cross-engine coordination issues). But
> that would require that we rework the scheduler as a pile of
> self-submitting work items, and I'm not sure that actually fits all
> that well into the core workqueue interfaces either.

There were already patches floating around which did exactly that.

Last time I checked those were actually looking pretty good.

Additional to message passing advantage the real big issue with the 
scheduler and 1 to 1 mapping is that we create a kernel thread for each 
instance, which results in tons on overhead.

Just using a work item which is submitted to a work queue completely 
avoids that.

Regards,
Christian.

>
> Worst case I think this isn't a dead-end and can be refactored to
> internally use the workqueue services, with the new functions here
> just being dumb wrappers until everyone is converted over. So it
> doesn't look like an expensive mistake, if it turns out to be a
> mistake.
> -Daniel
>
>
>> Regards,
>> Christian.
>>
>>> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
>>> ---
>>>    drivers/gpu/drm/scheduler/sched_main.c | 52 +++++++++++++++++++++++++-
>>>    include/drm/gpu_scheduler.h            | 29 +++++++++++++-
>>>    2 files changed, 78 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>> index 2597fb298733..84821a124ca2 100644
>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>> @@ -1049,6 +1049,49 @@ drm_sched_pick_best(struct drm_gpu_scheduler **sched_list,
>>>    }
>>>    EXPORT_SYMBOL(drm_sched_pick_best);
>>>
>>> +/**
>>> + * drm_sched_add_msg - add scheduler message
>>> + *
>>> + * @sched: scheduler instance
>>> + * @msg: message to be added
>>> + *
>>> + * Can and will pass an jobs waiting on dependencies or in a runnable queue.
>>> + * Messages processing will stop if schedule run wq is stopped and resume when
>>> + * run wq is started.
>>> + */
>>> +void drm_sched_add_msg(struct drm_gpu_scheduler *sched,
>>> +                    struct drm_sched_msg *msg)
>>> +{
>>> +     spin_lock(&sched->job_list_lock);
>>> +     list_add_tail(&msg->link, &sched->msgs);
>>> +     spin_unlock(&sched->job_list_lock);
>>> +
>>> +     drm_sched_run_wq_queue(sched);
>>> +}
>>> +EXPORT_SYMBOL(drm_sched_add_msg);
>>> +
>>> +/**
>>> + * drm_sched_get_msg - get scheduler message
>>> + *
>>> + * @sched: scheduler instance
>>> + *
>>> + * Returns NULL or message
>>> + */
>>> +static struct drm_sched_msg *
>>> +drm_sched_get_msg(struct drm_gpu_scheduler *sched)
>>> +{
>>> +     struct drm_sched_msg *msg;
>>> +
>>> +     spin_lock(&sched->job_list_lock);
>>> +     msg = list_first_entry_or_null(&sched->msgs,
>>> +                                    struct drm_sched_msg, link);
>>> +     if (msg)
>>> +             list_del(&msg->link);
>>> +     spin_unlock(&sched->job_list_lock);
>>> +
>>> +     return msg;
>>> +}
>>> +
>>>    /**
>>>     * drm_sched_main - main scheduler thread
>>>     *
>>> @@ -1060,6 +1103,7 @@ static void drm_sched_main(struct work_struct *w)
>>>                container_of(w, struct drm_gpu_scheduler, work_run);
>>>        struct drm_sched_entity *entity;
>>>        struct drm_sched_job *cleanup_job;
>>> +     struct drm_sched_msg *msg;
>>>        int r;
>>>
>>>        if (READ_ONCE(sched->pause_run_wq))
>>> @@ -1067,12 +1111,15 @@ static void drm_sched_main(struct work_struct *w)
>>>
>>>        cleanup_job = drm_sched_get_cleanup_job(sched);
>>>        entity = drm_sched_select_entity(sched);
>>> +     msg = drm_sched_get_msg(sched);
>>>
>>> -     if (!entity && !cleanup_job)
>>> +     if (!entity && !cleanup_job && !msg)
>>>                return; /* No more work */
>>>
>>>        if (cleanup_job)
>>>                sched->ops->free_job(cleanup_job);
>>> +     if (msg)
>>> +             sched->ops->process_msg(msg);
>>>
>>>        if (entity) {
>>>                struct dma_fence *fence;
>>> @@ -1082,7 +1129,7 @@ static void drm_sched_main(struct work_struct *w)
>>>                sched_job = drm_sched_entity_pop_job(entity);
>>>                if (!sched_job) {
>>>                        complete_all(&entity->entity_idle);
>>> -                     if (!cleanup_job)
>>> +                     if (!cleanup_job && !msg)
>>>                                return; /* No more work */
>>>                        goto again;
>>>                }
>>> @@ -1177,6 +1224,7 @@ int drm_sched_init(struct drm_gpu_scheduler *sched,
>>>
>>>        init_waitqueue_head(&sched->job_scheduled);
>>>        INIT_LIST_HEAD(&sched->pending_list);
>>> +     INIT_LIST_HEAD(&sched->msgs);
>>>        spin_lock_init(&sched->job_list_lock);
>>>        atomic_set(&sched->hw_rq_count, 0);
>>>        INIT_DELAYED_WORK(&sched->work_tdr, drm_sched_job_timedout);
>>> diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
>>> index df1993dd44ae..267bd060d178 100644
>>> --- a/include/drm/gpu_scheduler.h
>>> +++ b/include/drm/gpu_scheduler.h
>>> @@ -394,6 +394,23 @@ enum drm_gpu_sched_stat {
>>>        DRM_GPU_SCHED_STAT_ENODEV,
>>>    };
>>>
>>> +/**
>>> + * struct drm_sched_msg - an in-band (relative to GPU scheduler run queue)
>>> + * message
>>> + *
>>> + * Generic enough for backend defined messages, backend can expand if needed.
>>> + */
>>> +struct drm_sched_msg {
>>> +     /** @link: list link into the gpu scheduler list of messages */
>>> +     struct list_head                link;
>>> +     /**
>>> +      * @private_data: opaque pointer to message private data (backend defined)
>>> +      */
>>> +     void                            *private_data;
>>> +     /** @opcode: opcode of message (backend defined) */
>>> +     unsigned int                    opcode;
>>> +};
>>> +
>>>    /**
>>>     * struct drm_sched_backend_ops - Define the backend operations
>>>     *  called by the scheduler
>>> @@ -471,6 +488,12 @@ struct drm_sched_backend_ops {
>>>             * and it's time to clean it up.
>>>         */
>>>        void (*free_job)(struct drm_sched_job *sched_job);
>>> +
>>> +     /**
>>> +      * @process_msg: Process a message. Allowed to block, it is this
>>> +      * function's responsibility to free message if dynamically allocated.
>>> +      */
>>> +     void (*process_msg)(struct drm_sched_msg *msg);
>>>    };
>>>
>>>    /**
>>> @@ -482,6 +505,7 @@ struct drm_sched_backend_ops {
>>>     * @timeout: the time after which a job is removed from the scheduler.
>>>     * @name: name of the ring for which this scheduler is being used.
>>>     * @sched_rq: priority wise array of run queues.
>>> + * @msgs: list of messages to be processed in @work_run
>>>     * @job_scheduled: once @drm_sched_entity_do_release is called the scheduler
>>>     *                 waits on this wait queue until all the scheduled jobs are
>>>     *                 finished.
>>> @@ -489,7 +513,7 @@ struct drm_sched_backend_ops {
>>>     * @job_id_count: used to assign unique id to the each job.
>>>     * @run_wq: workqueue used to queue @work_run
>>>     * @timeout_wq: workqueue used to queue @work_tdr
>>> - * @work_run: schedules jobs and cleans up entities
>>> + * @work_run: schedules jobs, cleans up jobs, and processes messages
>>>     * @work_tdr: schedules a delayed call to @drm_sched_job_timedout after the
>>>     *            timeout interval is over.
>>>     * @pending_list: the list of jobs which are currently in the job queue.
>>> @@ -513,6 +537,7 @@ struct drm_gpu_scheduler {
>>>        long                            timeout;
>>>        const char                      *name;
>>>        struct drm_sched_rq             sched_rq[DRM_SCHED_PRIORITY_COUNT];
>>> +     struct list_head                msgs;
>>>        wait_queue_head_t               job_scheduled;
>>>        atomic_t                        hw_rq_count;
>>>        atomic64_t                      job_id_count;
>>> @@ -566,6 +591,8 @@ void drm_sched_entity_modify_sched(struct drm_sched_entity *entity,
>>>
>>>    void drm_sched_job_cleanup(struct drm_sched_job *job);
>>>    void drm_sched_wakeup(struct drm_gpu_scheduler *sched);
>>> +void drm_sched_add_msg(struct drm_gpu_scheduler *sched,
>>> +                    struct drm_sched_msg *msg);
>>>    void drm_sched_run_wq_stop(struct drm_gpu_scheduler *sched);
>>>    void drm_sched_run_wq_start(struct drm_gpu_scheduler *sched);
>>>    void drm_sched_stop(struct drm_gpu_scheduler *sched, struct drm_sched_job *bad);
>