Re: [RFC 10/17] drm/amdgpu: s/GFP_KERNEL/GFP_ATOMIC in scheduler code

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On Tue, May 12, 2020 at 5:56 PM Christian König
<christian.koenig@amd.com> wrote:
>
> Hui what? Of hand that doesn't looks correct to me.

It's not GFP_ATOMIC, it's just that GFP_ATOMIC is the only shotgun we
have to avoid direct reclaim. And direct reclaim might need to call
into your mmu notifier, which might need to wait on a fence, which is
never going to happen because your scheduler is stuck.

Note that all the explanations for the deadlocks and stuff I'm trying
to hunt here are in the other patches, the driver ones are more
informational, so I left these here rather bare-bones to shut up
lockdep so I can get through the entire driver and all major areas
(scheduler, reset, modeset code).

Now you can do something like GFP_NOFS, but the only reasons that
works is because the direct reclaim annotations
(fs_reclaim_acquire/release) only validates against __GFP_FS, and not
against any of the other flags. We should probably add some lockdep
annotations so that __GFP_RECLAIM is annotated against the
__mmu_notifier_invalidate_range_start_map lockdep map I've recently
added for mmu notifiers. End result (assuming I'm not mixing anything
up here, this is all rather tricky stuff): GFP_ATOMIC is the only kind
of memory allocation you can do.

> Why the heck should this be an atomic context? If that's correct
> allocating memory is the least of the problems we have.

It's not about atomic, it's !__GFP_RECLAIM. Which more or less is
GFP_ATOMIC. Correct fix is probably GFP_ATOMIC + a mempool for the
scheduler fixes so that if you can't allocate them for some reason,
you at least know that your scheduler should eventually retire retire
some of them, which you can then pick up from the mempool to guarantee
forward progress.

But I really didn't dig into details of the code, this was just a quick hack.

So sleeping and taking all kinds of locks (but not all, e.g.
dma_resv_lock and drm_modeset_lock are no-go) is still totally ok.
Just think

#define GFP_NO_DIRECT_RECLAIM GFP_ATOMIC

Cheers, Daniel

>
> Regards,
> Christian.
>
> Am 12.05.20 um 10:59 schrieb Daniel Vetter:
> > My dma-fence lockdep annotations caught an inversion because we
> > allocate memory where we really shouldn't:
> >
> >       kmem_cache_alloc+0x2b/0x6d0
> >       amdgpu_fence_emit+0x30/0x330 [amdgpu]
> >       amdgpu_ib_schedule+0x306/0x550 [amdgpu]
> >       amdgpu_job_run+0x10f/0x260 [amdgpu]
> >       drm_sched_main+0x1b9/0x490 [gpu_sched]
> >       kthread+0x12e/0x150
> >
> > Trouble right now is that lockdep only validates against GFP_FS, which
> > would be good enough for shrinkers. But for mmu_notifiers we actually
> > need !GFP_ATOMIC, since they can be called from any page laundering,
> > even if GFP_NOFS or GFP_NOIO are set.
> >
> > I guess we should improve the lockdep annotations for
> > fs_reclaim_acquire/release.
> >
> > Ofc real fix is to properly preallocate this fence and stuff it into
> > the amdgpu job structure. But GFP_ATOMIC gets the lockdep splat out of
> > the way.
> >
> > v2: Two more allocations in scheduler paths.
> >
> > Frist one:
> >
> >       __kmalloc+0x58/0x720
> >       amdgpu_vmid_grab+0x100/0xca0 [amdgpu]
> >       amdgpu_job_dependency+0xf9/0x120 [amdgpu]
> >       drm_sched_entity_pop_job+0x3f/0x440 [gpu_sched]
> >       drm_sched_main+0xf9/0x490 [gpu_sched]
> >
> > Second one:
> >
> >       kmem_cache_alloc+0x2b/0x6d0
> >       amdgpu_sync_fence+0x7e/0x110 [amdgpu]
> >       amdgpu_vmid_grab+0x86b/0xca0 [amdgpu]
> >       amdgpu_job_dependency+0xf9/0x120 [amdgpu]
> >       drm_sched_entity_pop_job+0x3f/0x440 [gpu_sched]
> >       drm_sched_main+0xf9/0x490 [gpu_sched]
> >
> > Cc: linux-media@vger.kernel.org
> > Cc: linaro-mm-sig@lists.linaro.org
> > Cc: linux-rdma@vger.kernel.org
> > Cc: amd-gfx@lists.freedesktop.org
> > Cc: intel-gfx@lists.freedesktop.org
> > Cc: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Cc: Christian König <christian.koenig@amd.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > ---
> >   drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c | 2 +-
> >   drivers/gpu/drm/amd/amdgpu/amdgpu_ids.c   | 2 +-
> >   drivers/gpu/drm/amd/amdgpu/amdgpu_sync.c  | 2 +-
> >   3 files changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
> > index d878fe7fee51..055b47241bb1 100644
> > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
> > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
> > @@ -143,7 +143,7 @@ int amdgpu_fence_emit(struct amdgpu_ring *ring, struct dma_fence **f,
> >       uint32_t seq;
> >       int r;
> >
> > -     fence = kmem_cache_alloc(amdgpu_fence_slab, GFP_KERNEL);
> > +     fence = kmem_cache_alloc(amdgpu_fence_slab, GFP_ATOMIC);
> >       if (fence == NULL)
> >               return -ENOMEM;
> >
> > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ids.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ids.c
> > index fe92dcd94d4a..fdcd6659f5ad 100644
> > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ids.c
> > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ids.c
> > @@ -208,7 +208,7 @@ static int amdgpu_vmid_grab_idle(struct amdgpu_vm *vm,
> >       if (ring->vmid_wait && !dma_fence_is_signaled(ring->vmid_wait))
> >               return amdgpu_sync_fence(sync, ring->vmid_wait, false);
> >
> > -     fences = kmalloc_array(sizeof(void *), id_mgr->num_ids, GFP_KERNEL);
> > +     fences = kmalloc_array(sizeof(void *), id_mgr->num_ids, GFP_ATOMIC);
> >       if (!fences)
> >               return -ENOMEM;
> >
> > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_sync.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_sync.c
> > index b87ca171986a..330476cc0c86 100644
> > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_sync.c
> > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_sync.c
> > @@ -168,7 +168,7 @@ int amdgpu_sync_fence(struct amdgpu_sync *sync, struct dma_fence *f,
> >       if (amdgpu_sync_add_later(sync, f, explicit))
> >               return 0;
> >
> > -     e = kmem_cache_alloc(amdgpu_sync_slab, GFP_KERNEL);
> > +     e = kmem_cache_alloc(amdgpu_sync_slab, GFP_ATOMIC);
> >       if (!e)
> >               return -ENOMEM;
> >
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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