RE: [PATCH 3/4] drm/ttm: handle already locked BOs during eviction and swapout.

[He, Roger]

-----Original Message-----
From: Christian KÃnig [mailto:ckoenig.leichtzumerken@xxxxxxxxx] 
Sent: Friday, February 23, 2018 8:06 PM
To: He, Roger <Hongbo.He@xxxxxxx>; amd-gfx@xxxxxxxxxxxxxxxxxxxxx; dri-devel@xxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
Subject: Re: [PATCH 3/4] drm/ttm: handle already locked BOs during eviction and swapout.

Am 23.02.2018 um 10:46 schrieb He, Roger:
>
> -----Original Message-----
> From: dri-devel [mailto:dri-devel-bounces@xxxxxxxxxxxxxxxxxxxxx] On 
> Behalf Of Christian K?nig
> Sent: Tuesday, February 20, 2018 8:58 PM
> To: amd-gfx@xxxxxxxxxxxxxxxxxxxxx; dri-devel@xxxxxxxxxxxxxxxxxxxxx; 
> linux-kernel@xxxxxxxxxxxxxxx
> Subject: [PATCH 3/4] drm/ttm: handle already locked BOs during eviction and swapout.
>
> This solves the problem that when we swapout a BO from a domain we sometimes couldn't make room for it because holding the lock blocks all other BOs with this reservation object.
>
> Signed-off-by: Christian KÃnig <christian.koenig@xxxxxxx>
> ---
>   drivers/gpu/drm/ttm/ttm_bo.c | 33 ++++++++++++++++-----------------
>   1 file changed, 16 insertions(+), 17 deletions(-)
>
> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c 
> b/drivers/gpu/drm/ttm/ttm_bo.c index d90b1cf10b27..3a44c2ee4155 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
> @@ -713,31 +713,30 @@ bool ttm_bo_eviction_valuable(struct 
> ttm_buffer_object *bo,  EXPORT_SYMBOL(ttm_bo_eviction_valuable);
>   
>   /**
> - * Check the target bo is allowable to be evicted or swapout, including cases:
> - *
> - * a. if share same reservation object with ctx->resv, have 
> assumption
> - * reservation objects should already be locked, so not lock again 
> and
> - * return true directly when either the opreation 
> allow_reserved_eviction
> - * or the target bo already is in delayed free list;
> - *
> - * b. Otherwise, trylock it.
> + * Check if the target bo is allowed to be evicted or swapedout.
>    */
>   static bool ttm_bo_evict_swapout_allowable(struct ttm_buffer_object *bo,
> -			struct ttm_operation_ctx *ctx, bool *locked)
> +					   struct ttm_operation_ctx *ctx,
> +					   bool *locked)
>   {
> -	bool ret = false;
> +	/* First check if we can lock it */
> +	*locked = reservation_object_trylock(bo->resv);
> +	if (*locked)
> +		return true;
>   
> -	*locked = false;
> +	/* Check if it's locked because it is part of the current operation 
> +*/
>   	if (bo->resv == ctx->resv) {
>   		reservation_object_assert_held(bo->resv);
> -		if (ctx->allow_reserved_eviction || !list_empty(&bo->ddestroy))
> -			ret = true;
> -	} else {
> -		*locked = reservation_object_trylock(bo->resv);
> -		ret = *locked;
> +		return ctx->allow_reserved_eviction ||
> +			!list_empty(&bo->ddestroy);
>   	}
>   
> -	return ret;
> +	/* Check if it's locked because it was already evicted */
> +	if (ww_mutex_is_owned_by(&bo->resv->lock, NULL))
> +		return true;
>
> For the special case: when command submission with Per-VM-BO enabled, 
> All BOs  a/b/c are always valid BO. After the validation of BOs a and 
> b, when validation of BO c, is it possible to return true and then evict BO a and b by mistake ?
> Because a/b/c share same task_struct.

	No, that's why I check the context as well. BOs explicitly reserved have a non NULL context while BOs trylocked for swapout have a NULL context.

When BOs have a non NULL context only when command submission and reserved by ttm_eu_re6serve_buffers  .
But for Per-VM-BO a/b/c they always are not in BO list, so they will be not reserved and have always NULL context.
So above case also can happen. Anything missing here?  

Thanks
Roger(Hongbo.He)
>
> +	/* Some other thread is using it, don't touch it */
> +	return false;
>   }
>   
>   static int ttm_mem_evict_first(struct ttm_bo_device *bdev,
> --
> 2.14.1
>
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