Re: [PATCH 1/2] mm: Track actual nr_scanned during shrink_slab()

[Minchan Kim <minchan@kernel.org>]

Hello Chris,

On Tue, Aug 22, 2017 at 02:53:24PM +0100, Chris Wilson wrote:
> Some shrinkers may only be able to free a bunch of objects at a time, and
> so free more than the requested nr_to_scan in one pass. Whilst other
> shrinkers may find themselves even unable to scan as many objects as
> they counted, and so underreport. Account for the extra freed/scanned
> objects against the total number of objects we intend to scan, otherwise
> we may end up penalising the slab far more than intended. Similarly,
> we want to add the underperforming scan to the deferred pass so that we
> try harder and harder in future passes.
> 
> v2: Andrew's shrinkctl->nr_scanned
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Shaohua Li <shli@fb.com>
> Cc: linux-mm@kvack.org
> ---
>  include/linux/shrinker.h | 7 +++++++
>  mm/vmscan.c              | 7 ++++---
>  2 files changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> index 4fcacd915d45..51d189615bda 100644
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -18,6 +18,13 @@ struct shrink_control {
>  	 */
>  	unsigned long nr_to_scan;
>  
> +	/*
> +	 * How many objects did scan_objects process?
> +	 * This defaults to nr_to_scan before every call, but the callee
> +	 * should track its actual progress.

So, if shrinker scans object more than requested, it shoud add up
top nr_scanned?

opposite case, if shrinker scans less than requested, it should reduce
nr_scanned to the value scanned real?

To track the progress is burden for the shrinker users. Even if a
shrinker has a mistake, VM will have big trouble like infinite loop.

IMHO, we need concrete reason to do it but fail to see it at this moment.

Could we just add up more freed object than requested to total_scan
like you did in first version[1]?
[1] lkml.kernel.org/r/<20170812113437.7397-1-chris@chris-wilson.co.uk>

> +	 */
> +	unsigned long nr_scanned;
> +
>  	/* current node being shrunk (for NUMA aware shrinkers) */
>  	int nid;
>  
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a1af041930a6..339b8fc95fc9 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -393,14 +393,15 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  		unsigned long nr_to_scan = min(batch_size, total_scan);
>  
>  		shrinkctl->nr_to_scan = nr_to_scan;
> +		shrinkctl->nr_scanned = nr_to_scan;
>  		ret = shrinker->scan_objects(shrinker, shrinkctl);
>  		if (ret == SHRINK_STOP)
>  			break;
>  		freed += ret;
>  
> -		count_vm_events(SLABS_SCANNED, nr_to_scan);
> -		total_scan -= nr_to_scan;
> -		scanned += nr_to_scan;
> +		count_vm_events(SLABS_SCANNED, shrinkctl->nr_scanned);
> +		total_scan -= shrinkctl->nr_scanned;
> +		scanned += shrinkctl->nr_scanned;

If we really want to go this way, at least, We need some defense code
to prevent infinite loop when shrinker doesn't have object any more.
However, I really want to go with your first version.

Andrew?

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