[PATCH v2] Revert "drm/scheduler: Avoid accessing freed bad job."
Daniel Vetter
daniel at ffwll.ch
Tue Sep 7 08:47:04 UTC 2021
On Thu, Sep 02, 2021 at 11:36:34AM -0400, Andrey Grodzovsky wrote:
>
> On 2021-09-02 10:28 a.m., Daniel Vetter wrote:
> > On Tue, Aug 31, 2021 at 02:24:52PM -0400, Andrey Grodzovsky wrote:
> > > On 2021-08-31 9:11 a.m., Daniel Vetter wrote:
> > > > On Thu, Aug 26, 2021 at 11:04:14AM +0200, Daniel Vetter wrote:
> > > > > On Thu, Aug 19, 2021 at 11:25:09AM -0400, Andrey Grodzovsky wrote:
> > > > > > On 2021-08-19 5:30 a.m., Daniel Vetter wrote:
> > > > > > > On Wed, Aug 18, 2021 at 10:51:00AM -0400, Andrey Grodzovsky wrote:
> > > > > > > > On 2021-08-18 10:42 a.m., Daniel Vetter wrote:
> > > > > > > > > On Wed, Aug 18, 2021 at 10:36:32AM -0400, Andrey Grodzovsky wrote:
> > > > > > > > > > On 2021-08-18 10:32 a.m., Daniel Vetter wrote:
> > > > > > > > > > > On Wed, Aug 18, 2021 at 10:26:25AM -0400, Andrey Grodzovsky wrote:
> > > > > > > > > > > > On 2021-08-18 10:02 a.m., Alex Deucher wrote:
> > > > > > > > > > > >
> > > > > > > > > > > > > + dri-devel
> > > > > > > > > > > > >
> > > > > > > > > > > > > Since scheduler is a shared component, please add dri-devel on all
> > > > > > > > > > > > > scheduler patches.
> > > > > > > > > > > > >
> > > > > > > > > > > > > On Wed, Aug 18, 2021 at 7:21 AM Jingwen Chen <Jingwen.Chen2 at amd.com> wrote:
> > > > > > > > > > > > > > [Why]
> > > > > > > > > > > > > > for bailing job, this commit will delete it from pending list thus the
> > > > > > > > > > > > > > bailing job will never have a chance to be resubmitted even in advance
> > > > > > > > > > > > > > tdr mode.
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > [How]
> > > > > > > > > > > > > > after embeded hw_fence into amdgpu_job is done, the race condition that
> > > > > > > > > > > > > > this commit tries to work around is completely solved.So revert this
> > > > > > > > > > > > > > commit.
> > > > > > > > > > > > > > This reverts commit 135517d3565b48f4def3b1b82008bc17eb5d1c90.
> > > > > > > > > > > > > > v2:
> > > > > > > > > > > > > > add dma_fence_get/put() around timedout_job to avoid concurrent delete
> > > > > > > > > > > > > > during processing timedout_job
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > Signed-off-by: Jingwen Chen <Jingwen.Chen2 at amd.com>
> > > > > > > > > > > > > > ---
> > > > > > > > > > > > > > drivers/gpu/drm/scheduler/sched_main.c | 23 +++++------------------
> > > > > > > > > > > > > > 1 file changed, 5 insertions(+), 18 deletions(-)
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> > > > > > > > > > > > > > index a2a953693b45..f9b9b3aefc4a 100644
> > > > > > > > > > > > > > --- a/drivers/gpu/drm/scheduler/sched_main.c
> > > > > > > > > > > > > > +++ b/drivers/gpu/drm/scheduler/sched_main.c
> > > > > > > > > > > > > > @@ -314,6 +314,7 @@ static void drm_sched_job_timedout(struct work_struct *work)
> > > > > > > > > > > > > > {
> > > > > > > > > > > > > > struct drm_gpu_scheduler *sched;
> > > > > > > > > > > > > > struct drm_sched_job *job;
> > > > > > > > > > > > > > + struct dma_fence *fence;
> > > > > > > > > > > > > > enum drm_gpu_sched_stat status = DRM_GPU_SCHED_STAT_NOMINAL;
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > sched = container_of(work, struct drm_gpu_scheduler, work_tdr.work);
> > > > > > > > > > > > > > @@ -325,11 +326,10 @@ static void drm_sched_job_timedout(struct work_struct *work)
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > 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.
> > > > > > > > > > > > > > + * Get job->s_fence->parent here to avoid concurrent delete during
> > > > > > > > > > > > > > + * processing timedout_job
> > > > > > > > > > > > > > */
> > > > > > > > > > > > > > - list_del_init(&job->list);
> > > > > > > > > > > > > > + fence = dma_fence_get(job->s_fence->parent);
> > > > > > > > > > > > While this is true for amdgpu, it has no meaning for other drivers for whom
> > > > > > > > > > > > we haven't
> > > > > > > > > > > > done the refactoring of embedding HW fence (parent) into the job structure.
> > > > > > > > > > > > In fact thinking
> > > > > > > > > > > > about it, unless you do the HW fence embedding for all the drivers using the
> > > > > > > > > > > > scheduler you cannot
> > > > > > > > > > > > revert this patch or you will just break them.
> > > > > > > > > > > btw, why did you do that embedding? I do still have my patches with
> > > > > > > > > > > dma_fence annotations floating around, but my idea at least was to fix
> > > > > > > > > > > that issue with a mempool, not with embeddeding. What was the motivation
> > > > > > > > > > > for embedding the wh fence?
> > > > > > > > > > > -Daniel
> > > > > > > > > > The motivation was 2 fold, avoid memory allocation during jobs submissions
> > > > > > > > > > (HW fence allocation) because as Christian explained this leads to deadlock
> > > > > > > > > > with
> > > > > > > > > > mm code during evictions due to memory pressure (Christian can clarify if I
> > > > > > > > > > messed
> > > > > > > > > Yeah that's the exact same thing I've chased with my dma_fence
> > > > > > > > > annotations, but thus far zero to none interested in getting it sorted. I
> > > > > > > > > think it'd be good to have some cross-driver agreement on how this should
> > > > > > > > > be solved before someone just charges ahead ...
> > > > > > > > >
> > > > > > > > > > this explanation). Second is to exactly revert this patch because while it
> > > > > > > > > > solved the issue
> > > > > > > > > > described in the patch it created another with drivers who baildc out early
> > > > > > > > > > during TDR handling
> > > > > > > > > > for various reason and the job would just leak because it was already
> > > > > > > > > > removed form pending list.
> > > > > > > > > Can't we reinsert it before we restart the scheduler thread? It might need
> > > > > > > > > a separate list for that due to the lockless queue tricks. Or am I
> > > > > > > > > thinking about the wrong kind of "we lost the job"?
> > > > > > > > > -Danile
> > > > > > > > If you look at the original patch it would reinsert it even earlier - right
> > > > > > > > after stopping the SW scheduler thread, and even then it was to late for
> > > > > > > > some drivers as they would decide to return back from their TDR handler even
> > > > > > > > before that. It is solvable but in an ugly way as far as I see, you need to
> > > > > > > > require each driver in his code to put the job back in the list if they do
> > > > > > > > it before reaching the place where scheduler framework does it. Kind of
> > > > > > > > spaghetti code seems to me.
> > > > > > > Hm yeah I didn't realize this all happens before we stop the scheduler
> > > > > > > thread.
> > > > > > >
> > > > > > > Why can't we stop the scheduler thread first, so that there's guaranteed
> > > > > > > no race? I've recently had a lot of discussions with panfrost folks about
> > > > > > > their reset that spawns across engines, and without stopping the scheduler
> > > > > > > thread first before you touch anything it's just plain impossible.
> > > > > > Talked with Christian on that, for each TDR we actually stop all the
> > > > > > schedulers for all the rings and not only the hanged ring since
> > > > > > ASIC reset will impact all the rings anyway. So we cannot allow
> > > > > > other timeout handlers for other rings run in parallel to ours
> > > > > > as they will stop/restart the threads we just stopped and rely
> > > > > > on them being stopped. So it's all done with device wide lock
> > > > > > inside the amdgpu tTDR handler. Only inside the locked
> > > > > > section then we may stop/restart the scheduler threads.
> > > > > > Christian also mentioned that you proposed at some point
> > > > > > to serialize all TDR handling into single threading for all rings - this
> > > > > > seems
> > > > > > like something that could be used - we then don't need any
> > > > > > locking against TDR handlers from other rings and then we may
> > > > > > stop the scheduler thread as first step
> > > > > >
> > > > > >
> > > > > > > I'm also still not understanding what exactly you guys have done,
> > > > > > > can someone please dig out the the amdgpu patches that motivate all this
> > > > > > > maybe that's clearer? A full explanation would still be good since I've
> > > > > > > only started in scheduler stuff.
> > > > > > https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.freedesktop.org%2Fagd5f%2Flinux%2F-%2Fcommit%2Fde7515d43659f852590645a688f8d493e4a18141&data=04%7C01%7Candrey.grodzovsky%40amd.com%7Ca5d9bacd4415453ba6c308d96e1de455%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637661896953391179%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=T3WC4%2B3BcWBy9gnjCMLRJjPM%2BWXfmN4GfR2Ojn8P3qc%3D&reserved=0
> > > > > Uh, it would have been really good if this was discussed a bit wider
> > > > > beforehand. Now we have rather diverging approaches to this. Also would be
> > > > > really good to resurrect the dma_fence annotations too.
> > > > >
> > > > > Can you guys pls spend a bit of time on this? Shouldn't be to hard to type
> > > > > up rfc conversion patches for the other drivers.
> > > > Ping for this. Currently the hw fence is returned from the ->run_job
> > > > callback, and that's not great design.
> > >
> > > What's the problem you see there ?
> > For one, all other drivers work like that, and it's not great to be
> > inconsistent. And it allows that inconsistent/wrong pattern to continue.
> >
> > Second I'm not even sure you can embed the hw fence, because there's this
> > job restarting going on. Which at least thus far allocated a new hw fence.
> > So this needs considerations.
>
>
> There is a solution to this at least at the amdgou level, see here -
> https://www.spinics.net/lists/amd-gfx/msg66614.html So we would
> reset the embedded fence seqno for this purpose (see amdgpu_fence_emit).
I think stuff like this really should be lifted into standard behaviour. I
have no idea whether this is doable across the board in all drivers, and
having incompatible solutions here without understanding the constraints
across drivers is no good at all.
> > > > If we embed it, then I think it should start existing latest from
> > > > drm_sched_job_arm. Maybe not yet initialized, but at least allocated. So
> > > > the right thing to do here is to have the hw fence as a pointer in
> > > > struct drm_sched_job. And check in drm_sched_job_arm() that it's at least
> > > > allocated.
> > >
> > > Why we need to allocate the HW fence if it's embedded within a job struct ?
> > the hw fence is a refcounted struct, and the drm_sched_job is a different
> > struct. And we didn't have a dri-devel discussion about whether it's
> > correct to conflate these two lifetimes, amdgpu folks simply hacked
> > something together.
>
>
> Obviously scheduler level changes must be discussed at dri-devel forum
> level.
> What happened here and as Monk already mentioned - we had internal
> discussion
> about how to fix the problem in the header of this thread - avoiding
> accessing feed job
> from TDR handler without the current hack in place of removal and back
> insertion
> into pending list. It's there we we came up (I think Christian first
> mentioned this) with the
> idea of embedding the HW fence into amdgpu job - both for avoiding memory
> allocations
> but also - because this allows to use the HW fence's recounting as a
> solution to the above
> problem by simply grabbing a reference to the next fence in the pending list
> as a first step
> in the TDR handler. What we didn't take into account at the time is that
> indeed this change
> cannot be limited to amdgpu level - this we noticed much later during final
> code reviews.
Not sure where this fell through cracks, but imo at least changing where
the hw fence is allocated is a very fundamental change, and latest then
you should have discussed this on dri-devel.
But even the tdr races would probably have been good to start on
dri-devel. Now it looks like Monk&team have lost 6 months for nothing.
-Daniel
>
> Andrey
>
>
> >
> > > > Otherwise we're just diverging across drivers and tempting them to do the
> > > > wrong thing with the current ->run_job callback interface.
> > >
> > > Maybe we should switch from embedding in driver level job struct as it's now
> > > to drm_sched_job and just leave the fence initialization to driver specific
> > > code ?
> > Maybe? Like I've not been involved in these discussion ont he amd side at
> > all, I'm just noticing that we do have a now rather inconsistently used
> > inteface across drivers. Which is no good.
> > -Daniel
> >
> > > Andrey
> > >
> > >
> > > > Can you guys look into this?
> > > > -Daniel
> > > >
> > > > > > > Another thing I recently pondered for tdr races looking at i915 code is
> > > > > > > whether the tdr should first block the completion fence for that job. My
> > > > > > > motivation is to have a race-free error capture (if the completion races
> > > > > > > then we might start evicting memory and everything goes boom), but maybe
> > > > > > > that helps here too. Some kind of atomic "block this fence from
> > > > > > > completing thing.
> > > > > > >
> > > > > > > Or I'm I completely guessing in the wrong direction?
> > > > > > I think we already do it here - https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Felixir.bootlin.com%2Flinux%2Fv5.14-rc1%2Fsource%2Fdrivers%2Fgpu%2Fdrm%2Fscheduler%2Fsched_main.c%23L410&data=04%7C01%7Candrey.grodzovsky%40amd.com%7Ca5d9bacd4415453ba6c308d96e1de455%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637661896953391179%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=1xqr4XCqY%2FCYJjAzT3GI8MyBi15tJQmOt6sB79COsmc%3D&reserved=0
> > > > > Ah yes this works becase drm/sched has separate hw fence from the logical
> > > > > job fence.
> > > > > -Daniel
> > > > >
> > > > > > Andrey
> > > > > >
> > > > > >
> > > > > > > -Daniel
> > > > > > >
> > > > > > > > Andrey
> > > > > > > >
> > > > > > > >
> > > > > > > > > > Andrey
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > > > Andrey
> > > > > > > > > > > >
> > > > > > > > > > > >
> > > > > > > > > > > > > > spin_unlock(&sched->job_list_lock);
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > status = job->sched->ops->timedout_job(job);
> > > > > > > > > > > > > > @@ -342,6 +342,7 @@ static void drm_sched_job_timedout(struct work_struct *work)
> > > > > > > > > > > > > > job->sched->ops->free_job(job);
> > > > > > > > > > > > > > sched->free_guilty = false;
> > > > > > > > > > > > > > }
> > > > > > > > > > > > > > + dma_fence_put(fence);
> > > > > > > > > > > > > > } else {
> > > > > > > > > > > > > > spin_unlock(&sched->job_list_lock);
> > > > > > > > > > > > > > }
> > > > > > > > > > > > > > @@ -392,20 +393,6 @@ 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
> > > > > > > > > > > > > > --
> > > > > > > > > > > > > > 2.25.1
> > > > > > > > > > > > > >
> > > > > --
> > > > > Daniel Vetter
> > > > > Software Engineer, Intel Corporation
> > > > > https://nam11.safelinks.protection.outlook.com/?url=http%3A%2F%2Fblog.ffwll.ch%2F&data=04%7C01%7Candrey.grodzovsky%40amd.com%7Ca5d9bacd4415453ba6c308d96e1de455%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637661896953391179%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=4V%2Fri%2B3gnISZfC6HOUxR1Z8dIkseE9dT1EqiXsTuVi8%3D&reserved=0
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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