[PATCH] ttm: wait mem space if user allow while gpu busy

[Christian König]

Well that's not so easy of hand.

The basic problem here is that when you busy wait at this place you can 
easily run into situations where application A busy waits for B while B 
busy waits for A -> deadlock.

So what we need here is the deadlock detection logic of the ww_mutex. To 
use this we at least need to do the following steps:

1. Reserve the BO in DC using a ww_mutex ticket (trivial).

2. If we then run into this EBUSY condition in TTM check if the BO we 
need memory for (or rather the ww_mutex of its reservation object) has a 
ticket assigned.

3. If we have a ticket we grab a reference to the first BO on the LRU, 
drop the LRU lock and try to grab the reservation lock with the ticket.

4. If getting the reservation lock with the ticket succeeded we check if 
the BO is still the first one on the LRU in question (the BO could have 
moved).

5. If the BO is still the first one on the LRU in question we try to 
evict it as we would evict any other BO.

6. If any of the "If's" above fail we just back off and return -EBUSY.

Steps 2-5 are certainly not trivial, but doable as far as I can see.

Have fun :)
Christian.

Am 23.04.19 um 15:19 schrieb Zhou, David(ChunMing):
> How about adding more condition ctx->resv inline to address your 
> concern? As well as don't wait from same user, shouldn't lead to deadlock.
>
> Otherwise, any other idea?
>
> -------- Original Message --------
> Subject: Re: [PATCH] ttm: wait mem space if user allow while gpu busy
> From: Christian König
> To: "Liang, Prike" ,"Zhou, David(ChunMing)" 
> ,dri-devel at lists.freedesktop.org
> CC:
>
> Well that is certainly a NAK because it can lead to deadlock in the
> memory management.
>
> You can't just busy wait with all those locks held.
>
> Regards,
> Christian.
>
> Am 23.04.19 um 03:45 schrieb Liang, Prike:
> > Acked-by: Prike Liang <Prike.Liang at amd.com>
> >
> > Thanks,
> > Prike
> > -----Original Message-----
> > From: Chunming Zhou <david1.zhou at amd.com>
> > Sent: Monday, April 22, 2019 6:39 PM
> > To: dri-devel at lists.freedesktop.org
> > Cc: Liang, Prike <Prike.Liang at amd.com>; Zhou, David(ChunMing) 
> <David1.Zhou at amd.com>
> > Subject: [PATCH] ttm: wait mem space if user allow while gpu busy
> >
> > heavy gpu job could occupy memory long time, which could lead to 
> other user fail to get memory.
> >
> > Change-Id: I0b322d98cd76e5ac32b00462bbae8008d76c5e11
> > Signed-off-by: Chunming Zhou <david1.zhou at amd.com>
> > ---
> >   drivers/gpu/drm/ttm/ttm_bo.c | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/ttm/ttm_bo.c 
> b/drivers/gpu/drm/ttm/ttm_bo.c index 7c484729f9b2..6c596cc24bec 100644
> > --- a/drivers/gpu/drm/ttm/ttm_bo.c
> > +++ b/drivers/gpu/drm/ttm/ttm_bo.c
> > @@ -830,8 +830,10 @@ static int ttm_bo_mem_force_space(struct 
> ttm_buffer_object *bo,
> >                if (mem->mm_node)
> >                        break;
> >                ret = ttm_mem_evict_first(bdev, mem_type, place, ctx);
> > -             if (unlikely(ret != 0))
> > -                     return ret;
> > +             if (unlikely(ret != 0)) {
> > +                     if (!ctx || ctx->no_wait_gpu || ret != -EBUSY)
> > +                             return ret;
> > +             }
> >        } while (1);
> >        mem->mem_type = mem_type;
> >        return ttm_bo_add_move_fence(bo, man, mem);
> > --
> > 2.17.1
> >
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>
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