linux-kernel - Re: [PATCH] mm:workingset use real time to judge activity of the file page

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Date:   Thu, 4 Apr 2019 20:24:01 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Zhaoyang Huang <huangzhaoyang@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        David Rientjes <rientjes@...gle.com>,
        Zhaoyang Huang <zhaoyang.huang@...soc.com>,
        Roman Gushchin <guro@...com>, Jeff Layton <jlayton@...hat.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm:workingset use real time to judge activity of the
 file page

On Thu, Apr 04, 2019 at 11:30:17AM +0800, Zhaoyang Huang wrote:
> +++ b/mm/workingset.c
> @@ -159,7 +159,7 @@
>  			 NODES_SHIFT +	\
>  			 MEM_CGROUP_ID_SHIFT)
>  #define EVICTION_MASK	(~0UL >> EVICTION_SHIFT)
> -
> +#define EVICTION_JIFFIES (BITS_PER_LONG >> 3)
>  /*
>   * Eviction timestamps need to be able to cover the full range of
>   * actionable refaults. However, bits are tight in the radix tree
> @@ -175,18 +175,22 @@ static void *pack_shadow(int memcgid, pg_data_t *pgdat, unsigned long eviction)
>  	eviction >>= bucket_order;
>  	eviction = (eviction << MEM_CGROUP_ID_SHIFT) | memcgid;
>  	eviction = (eviction << NODES_SHIFT) | pgdat->node_id;
> +	eviction = (eviction << EVICTION_JIFFIES) | (jiffies >> EVICTION_JIFFIES);
>  	eviction = (eviction << RADIX_TREE_EXCEPTIONAL_SHIFT);

... this isn't against current, or even 5.0.

>  	entry >>= RADIX_TREE_EXCEPTIONAL_SHIFT;
> +	entry >>= EVICTION_JIFFIES;
> +	prev_jiff = (entry & ((1UL << EVICTION_JIFFIES) - 1)) << EVICTION_JIFFIES;

These two lines are in the wrong order.  So you're getting (effectively) a
random answer in your 'prev_jiff', which means your testing isn't thorough
enough.  I suspect you're only testing cases you're expecting to improve,
and you aren't testing to make sure that other cases don't regress.