[PATCH v2] drm/xe: Defer buffer object shrinker write-backs and GPU waits

[Thomas Hellström]

On Tue, 2025-08-05 at 14:40 +0100, Matthew Auld wrote:
> On 05/08/2025 08:48, Thomas Hellström wrote:
> > When the xe buffer-object shrinker allows GPU waits and write-back,
> > (typically from kswapd), perform multiple passes, skipping
> > subsequent passes if the shrinker number of scanned objects target
> > is reached.
> > 
> > 1) Without GPU waits and write-back
> > 2) Without write-back
> > 3) With both GPU-waits and write-back
> > 
> > This is to avoid stalls and costly write- and readbacks unless they
> > are really necessary.
> > 
> > v2:
> > - Don't test for scan completion twice. (Stuart Summers)
> > - Update tags.
> > 
> > Reported-by: melvyn <melvyn2 at dnsense.pub>
> > Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/5557
> > Cc: Summers Stuart <stuart.summers at intel.com>
> > Fixes: 00c8efc3180f ("drm/xe: Add a shrinker for xe bos")
> > Cc: <stable at vger.kernel.org> # v6.15+
> > Signed-off-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> > ---
> >   drivers/gpu/drm/xe/xe_shrinker.c | 51
> > +++++++++++++++++++++++++++++---
> >   1 file changed, 47 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/xe/xe_shrinker.c
> > b/drivers/gpu/drm/xe/xe_shrinker.c
> > index 1c3c04d52f55..90244fe59b59 100644
> > --- a/drivers/gpu/drm/xe/xe_shrinker.c
> > +++ b/drivers/gpu/drm/xe/xe_shrinker.c
> > @@ -54,10 +54,10 @@ xe_shrinker_mod_pages(struct xe_shrinker
> > *shrinker, long shrinkable, long purgea
> >   	write_unlock(&shrinker->lock);
> >   }
> >   
> > -static s64 xe_shrinker_walk(struct xe_device *xe,
> > -			    struct ttm_operation_ctx *ctx,
> > -			    const struct xe_bo_shrink_flags flags,
> > -			    unsigned long to_scan, unsigned long
> > *scanned)
> > +static s64 __xe_shrinker_walk(struct xe_device *xe,
> > +			      struct ttm_operation_ctx *ctx,
> > +			      const struct xe_bo_shrink_flags
> > flags,
> > +			      unsigned long to_scan, unsigned long
> > *scanned)
> >   {
> >   	unsigned int mem_type;
> >   	s64 freed = 0, lret;
> > @@ -93,6 +93,48 @@ static s64 xe_shrinker_walk(struct xe_device
> > *xe,
> >   	return freed;
> >   }
> >   
> > +/*
> > + * Try shrinking idle objects without writeback first, then if not
> > sufficient,
> > + * try also non-idle objects and finally if that's not sufficient
> > either,
> > + * add writeback. This avoids stalls and explicit writebacks with
> > light or
> > + * moderate memory pressure.
> 
> Just one question here, with writeback=false it doesn't really
> influence 
> which objects are chosen for shrinking, unlike with no_wait_gpu,
> right? 
> Will having another pass just with writeback=true yield anything 
> different, assuming here that the previous two passes would have
> already 
> hoovered ~everything up that was a possible candidate, so this pass 
> won't really find anything in practice? If so, does that also mean we
> never really end up using the writeback=true behaviour any more?

Good point.

The assumption is that if allocating shmem backing-store fails during
shrinking, we'd see an -ENOMEM and fail our target, and the next pass
with writeback would help avoiding that.

Ofc that requires that a shmem_read_folio() from within reclaim returns
an ERR_PTR(-ENOMEM) if the kernel reserves are depleted rather than to
invoke the OOM killer. I should perhaps test that.

Other options would ofc be to include the writeback in pass 2, which
would be similar to what the i915 shrinker does.

Thoughts?

Thanks,
Thomas



> 
> > + */
> > +static s64 xe_shrinker_walk(struct xe_device *xe,
> > +			    struct ttm_operation_ctx *ctx,
> > +			    const struct xe_bo_shrink_flags flags,
> > +			    unsigned long to_scan, unsigned long
> > *scanned)
> > +{
> > +	bool no_wait_gpu = true;
> > +	struct xe_bo_shrink_flags save_flags = flags;
> > +	s64 lret, freed;
> > +
> > +	swap(no_wait_gpu, ctx->no_wait_gpu);
> > +	save_flags.writeback = false;
> > +	lret = __xe_shrinker_walk(xe, ctx, save_flags, to_scan,
> > scanned);
> > +	swap(no_wait_gpu, ctx->no_wait_gpu);
> > +	if (lret < 0 || *scanned >= to_scan)
> > +		return lret;
> > +
> > +	freed = lret;
> > +	if (!ctx->no_wait_gpu) {
> > +		lret = __xe_shrinker_walk(xe, ctx, save_flags,
> > to_scan, scanned);
> > +		if (lret < 0)
> > +			return lret;
> > +		freed += lret;
> > +		if (*scanned >= to_scan)
> > +			return freed;
> > +	}
> > +
> > +	if (flags.writeback) {
> > +		lret = __xe_shrinker_walk(xe, ctx, flags, to_scan,
> > scanned);
> > +		if (lret < 0)
> > +			return lret;
> > +		freed += lret;
> > +	}
> > +
> > +	return freed;
> > +}
> > +
> >   static unsigned long
> >   xe_shrinker_count(struct shrinker *shrink, struct shrink_control
> > *sc)
> >   {
> > @@ -199,6 +241,7 @@ static unsigned long xe_shrinker_scan(struct
> > shrinker *shrink, struct shrink_con
> >   		runtime_pm = xe_shrinker_runtime_pm_get(shrinker,
> > true, 0, can_backup);
> >   
> >   	shrink_flags.purge = false;
> > +
> >   	lret = xe_shrinker_walk(shrinker->xe, &ctx, shrink_flags,
> >   				nr_to_scan, &nr_scanned);
> >   	if (lret >= 0)
>