[PATCH 2/2] drm/sched: serialize job_timeout and scheduler
Daniel Vetter
daniel at ffwll.ch
Wed Nov 10 09:50:01 UTC 2021
On Tue, Nov 09, 2021 at 08:17:01AM -0800, Rob Clark wrote:
> On Tue, Nov 9, 2021 at 1:07 AM Daniel Vetter <daniel at ffwll.ch> wrote:
> >
> > On Mon, Nov 08, 2021 at 03:39:17PM -0800, Rob Clark wrote:
> > > I stumbled across this thread when I ran into the same issue, while
> > > working out how to move drm/msm to use scheduler's retire +
> > > timeout/recovery (and get rid of our own mirror list of in-flight
> > > jobs). We already have hw error detection enabled, and it can signal
> > > quite fast, so assuming the first job on the list is the guilty job
> > > just won't work.
> > >
> > > But I was considering a slightly different approach to fixing this,
> > > instead just handling it all in drm_sched_main() and getting rid of
> > > the complicated kthread parking gymnastics. Ie. something along the
> > > lines of:
> >
> > So handling timeouts in the main sched thread wont work as soon as you
> > have multiple engines and reset that impacts across engines:
> >
> > - Nothing is simplified since you still need to stop the other scheduler
> > threads.
> >
> > - You get deadlocks if 2 schedulers time out at the same time, and both
> > want to stop the other one.
> >
> > Hence workqueue. Now the rule for the wq is that you can only have one per
> > reset domain, so
> > - single engine you just take the one drm/sched provides
> > - if reset affects all your engines in the chip, then you allocate on in
> > the drm_device and pass that to all
> > - if you have a complex of gpus all interconnected (e.g. xgmi hive for
> > amd), then it's one wq for the entire hive
> >
> > _All_ reset related things must be run on that workqueue or things breaks,
> > which means if you get hw fault that also needs to be run there. I guess
> > we should either patch drm/sched to check you call that function from the
> > right workqueue, or just handle it internally.
>
> Hmm, ok.. I guess it would be useful to better document the reasoning
> for the current design, that would have steered me more towards the
> approach taken in this patch.
Maybe this was because you worked on an old kernel? Boris did update the
kerneldoc as part of making gpu reset work for panfrost, which has this
multi-engine reset problem. If that's not yet clear then we need to
improve the docs further.
AMD's problem is even worse, because their reset domain is the entire xgmi
hive, so multiple pci devices.
Also there might more issues in drm/sched ofc, e.g. I've looked a bit at
ordering/barriers and I'm pretty sure a lot are still missing. Or at least
we should have comments in the code explaining why it all works.
-Daniel
>
> BR,
> -R
>
> > -Daniel
> >
> > >
> > > ---------------------
> > > diff --git a/drivers/gpu/drm/scheduler/sched_main.c
> > > b/drivers/gpu/drm/scheduler/sched_main.c
> > > index 67382621b429..4d6ce775c316 100644
> > > --- a/drivers/gpu/drm/scheduler/sched_main.c
> > > +++ b/drivers/gpu/drm/scheduler/sched_main.c
> > > @@ -764,6 +764,45 @@ static bool drm_sched_blocked(struct
> > > drm_gpu_scheduler *sched)
> > > return false;
> > > }
> > >
> > > +static bool handle_timeout(struct drm_gpu_scheduler *sched)
> > > +{
> > > + struct drm_sched_job *bad;
> > > +
> > > + if (!sched->has_timeout)
> > > + return false;
> > > +
> > > + sched->has_timeout = false;
> > > +
> > > + spin_lock(&sched->job_list_lock);
> > > + bad = list_first_entry_or_null(&sched->pending_list,
> > > + struct drm_sched_job, list);
> > > +
> > > + if (!bad) {
> > > + spin_unlock(&sched->job_list_lock);
> > > + return false;
> > > + }
> > > +
> > > + spin_unlock(&sched->job_list_lock);
> > > +
> > > + if (sched->timeout_wq == system_wq) {
> > > + /*
> > > + * If driver has no specific requirements about serializing
> > > + * reset wrt. other engines, just call timedout_job() directly
> > > + */
> > > + sched->ops->timedout_job(job);
> > > + } else {
> > > + /*
> > > + * Otherwise queue it on timeout_wq and wait for it to complete
> > > + */
> > > + ... more typing needed here ...
> > > + }
> > > +
> > > + if (sched->free_guilty) {
> > > + sched->ops->free_job(job);
> > > + sched->free_guilty = false;
> > > + }
> > > +}
> > > +
> > > /**
> > > * drm_sched_main - main scheduler thread
> > > *
> > > @@ -787,6 +826,7 @@ static int drm_sched_main(void *param)
> > >
> > > wait_event_interruptible(sched->wake_up_worker,
> > > (cleanup_job =
> > > drm_sched_get_cleanup_job(sched)) ||
> > > + handle_timeout(sched) ||
> > > (!drm_sched_blocked(sched) &&
> > > (entity =
> > > drm_sched_select_entity(sched))) ||
> > > kthread_should_stop());
> > > ---------------------
> > >
> > > drm_sched_fault() and the sw timeout handler would just set
> > > sched->has_timeout and kick sched->wake_up_worker.
> > >
> > > And since we handle the timeout case after
> > > drm_sched_get_cleanup_job(), we know that all of the successfully
> > > completed jobs have already been popped off the list, and won't be
> > > unfairly maligned.
> > >
> > > BR,
> > > -R
> > >
> > > On Tue, Aug 31, 2021 at 6:29 PM Liu, Monk <Monk.Liu at amd.com> wrote:
> > > >
> > > > [AMD Official Use Only]
> > > >
> > > > Okay, I will reprepare this patch
> > > >
> > > > Thanks
> > > >
> > > > ------------------------------------------
> > > > Monk Liu | Cloud-GPU Core team
> > > > ------------------------------------------
> > > >
> > > > -----Original Message-----
> > > > From: Daniel Vetter <daniel at ffwll.ch>
> > > > Sent: Tuesday, August 31, 2021 9:02 PM
> > > > To: Liu, Monk <Monk.Liu at amd.com>
> > > > Cc: amd-gfx at lists.freedesktop.org; dri-devel at lists.freedesktop.org; Chen, Jingwen <Jingwen.Chen at amd.com>
> > > > Subject: Re: [PATCH 2/2] drm/sched: serialize job_timeout and scheduler
> > > >
> > > > On Tue, Aug 31, 2021 at 02:59:02PM +0200, Daniel Vetter wrote:
> > > > > Can we please have some actual commit message here, with detailed
> > > > > explanation of the race/bug/whatever, how you fix it and why this is
> > > > > the best option?
> > > > >
> > > > > On Tue, Aug 31, 2021 at 06:35:39PM +0800, Monk Liu wrote:
> > > > > > tested-by: jingwen chen <jingwen.chen at amd.com>
> > > > > > Signed-off-by: Monk Liu <Monk.Liu at amd.com>
> > > > > > Signed-off-by: jingwen chen <jingwen.chen at amd.com>
> > > > > > ---
> > > > > > drivers/gpu/drm/scheduler/sched_main.c | 24
> > > > > > ++++--------------------
> > > > > > 1 file changed, 4 insertions(+), 20 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/gpu/drm/scheduler/sched_main.c
> > > > > > b/drivers/gpu/drm/scheduler/sched_main.c
> > > > > > index ecf8140..894fdb24 100644
> > > > > > --- a/drivers/gpu/drm/scheduler/sched_main.c
> > > > > > +++ b/drivers/gpu/drm/scheduler/sched_main.c
> > > > > > @@ -319,19 +319,17 @@ static void drm_sched_job_timedout(struct work_struct *work)
> > > > > > sched = container_of(work, struct drm_gpu_scheduler,
> > > > > > work_tdr.work);
> > > > > >
> > > > > > /* Protects against concurrent deletion in
> > > > > > drm_sched_get_cleanup_job */
> > > > > > + if (!__kthread_should_park(sched->thread))
> > > > >
> > > > > This is a __ function, i.e. considered internal, and it's lockless
> > > > > atomic, i.e. unordered. And you're not explaining why this works.
> > > > >
> > > > > Iow it's probably buggy, and an just unconditionally parking the
> > > > > kthread is probably the right thing to do. If it's not the right thing
> > > > > to do, there's a bug here for sure.
> > > >
> > > > Also why don't we reuse the function drivers already have to stop a scheduler thread? We seem to have two kthread_park now, that's probably one too much.
> > > > -Daniel
> > > >
> > > > > > + kthread_park(sched->thread);
> > > > > > +
> > > > > > spin_lock(&sched->job_list_lock);
> > > > > > job = list_first_entry_or_null(&sched->pending_list,
> > > > > > struct drm_sched_job, list);
> > > > > >
> > > > > > if (job) {
> > > > > > - /*
> > > > > > - * Remove the bad job so it cannot be freed by concurrent
> > > > > > - * drm_sched_cleanup_jobs. It will be reinserted back after sched->thread
> > > > > > - * is parked at which point it's safe.
> > > > > > - */
> > > > > > - list_del_init(&job->list);
> > > > > > spin_unlock(&sched->job_list_lock);
> > > > > >
> > > > > > + /* vendor's timeout_job should call drm_sched_start() */
> > > > > > status = job->sched->ops->timedout_job(job);
> > > > > >
> > > > > > /*
> > > > > > @@ -393,20 +391,6 @@ void drm_sched_stop(struct drm_gpu_scheduler *sched, struct drm_sched_job *bad)
> > > > > > kthread_park(sched->thread);
> > > > > >
> > > > > > /*
> > > > > > - * Reinsert back the bad job here - now it's safe as
> > > > > > - * drm_sched_get_cleanup_job cannot race against us and release the
> > > > > > - * bad job at this point - we parked (waited for) any in progress
> > > > > > - * (earlier) cleanups and drm_sched_get_cleanup_job will not be called
> > > > > > - * now until the scheduler thread is unparked.
> > > > > > - */
> > > > > > - if (bad && bad->sched == sched)
> > > > > > - /*
> > > > > > - * Add at the head of the queue to reflect it was the earliest
> > > > > > - * job extracted.
> > > > > > - */
> > > > > > - list_add(&bad->list, &sched->pending_list);
> > > > > > -
> > > > > > - /*
> > > > > > * Iterate the job list from later to earlier one and either deactive
> > > > > > * their HW callbacks or remove them from pending list if they already
> > > > > > * signaled.
> > > > > > --
> > > > > > 2.7.4
> > > > > >
> > > > >
> > > > > --
> > > > > Daniel Vetter
> > > > > Software Engineer, Intel Corporation
> > > > > https://nam11.safelinks.protection.outlook.com/?url=http%3A%2F%2Fblog.
> > > > > ffwll.ch%2F&data=04%7C01%7CMonk.Liu%40amd.com%7C298815bea18f4fbf76
> > > > > b308d96c7f7a8b%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C6376601170
> > > > > 51194614%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiL
> > > > > CJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=QzgCU7%2BPdA0aWL5%2BJLg
> > > > > KeKbGaMMGqeGI9KE0P0LXlN4%3D&reserved=0
> > > >
> > > > --
> > > > Daniel Vetter
> > > > Software Engineer, Intel Corporation
> > > > https://nam11.safelinks.protection.outlook.com/?url=http%3A%2F%2Fblog.ffwll.ch%2F&data=04%7C01%7CMonk.Liu%40amd.com%7C298815bea18f4fbf76b308d96c7f7a8b%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637660117051194614%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=QzgCU7%2BPdA0aWL5%2BJLgKeKbGaMMGqeGI9KE0P0LXlN4%3D&reserved=0
> >
> > --
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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