[Intel-gfx] [PATCH] drm/mm: Break long searches in fragmented address spaces

[Andi Shyti]

Hi Chris,

On Fri, Feb 07, 2020 at 03:17:20PM +0000, Chris Wilson wrote:
> We try hard to select a suitable hole in the drm_mm first time. But if
> that is unsuccessful, we then have to look at neighbouring nodes, and
> this requires traversing the rbtree. Walking the rbtree can be slow
> (much slower than a linear list for deep trees), and if the drm_mm has
> been purposefully fragmented our search can be trapped for a long, long
> time. For non-preemptible kernels, we need to break up long CPU bound
> sections by manually checking for cond_resched(); similarly we should

checking for "fatal signals" you mean?

> also bail out if we have been told to terminate. (In an ideal world, we
> would break for any signal, but we need to trade off having to perform
> the search again after ERESTARTSYS, which again may form a trap of
> making no forward progress.)
> 
> Reported-by: Zbigniew Kempczyński <zbigniew.kempczynski at intel.com>
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Zbigniew Kempczyński <zbigniew.kempczynski at intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>

nice!

Reviewed-by: Andi Shyti <andi.shyti at intel.com>

Thanks,
Andi

> ---
>  drivers/gpu/drm/drm_mm.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_mm.c b/drivers/gpu/drm/drm_mm.c
> index 2a6e34663146..47d5de9ca0a8 100644
> --- a/drivers/gpu/drm/drm_mm.c
> +++ b/drivers/gpu/drm/drm_mm.c
> @@ -45,6 +45,7 @@
>  #include <linux/export.h>
>  #include <linux/interval_tree_generic.h>
>  #include <linux/seq_file.h>
> +#include <linux/sched/signal.h>
>  #include <linux/slab.h>
>  #include <linux/stacktrace.h>
>  
> @@ -366,6 +367,11 @@ next_hole(struct drm_mm *mm,
>  	  struct drm_mm_node *node,
>  	  enum drm_mm_insert_mode mode)
>  {
> +	/* Searching is slow; check if we ran out of time/patience */
> +	cond_resched();
> +	if (fatal_signal_pending(current))
> +		return NULL;
> +
>  	switch (mode) {
>  	default:
>  	case DRM_MM_INSERT_BEST:
> @@ -557,7 +563,7 @@ int drm_mm_insert_node_in_range(struct drm_mm * const mm,
>  		return 0;
>  	}
>  
> -	return -ENOSPC;
> +	return signal_pending(current) ? -ERESTARTSYS : -ENOSPC;
>  }
>  EXPORT_SYMBOL(drm_mm_insert_node_in_range);
>  
> -- 
> 2.25.0
> 
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