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Date:   Fri, 11 Aug 2023 11:35:37 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Huang Ying <ying.huang@intel.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Christoph Lameter <cl@linux.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH] mm: fix draining remote pageset
Message-ID: <ZNYA6YWLqtDOdQne@dhcp22.suse.cz>
References: <20230811090819.60845-1-ying.huang@intel.com>
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On Fri 11-08-23 17:08:19, Huang Ying wrote:
> If there is no memory allocation/freeing in the remote pageset after
> some time (3 seconds for now), the remote pageset will be drained to
> avoid memory wastage.
> 
> But in the current implementation, vmstat updater worker may not be
> re-queued when we are waiting for the timeout (pcp->expire != 0) if
> there are no vmstat changes, for example, when CPU goes idle.

Why is that a problem?

> This is fixed via guaranteeing that the vmstat updater worker will
> always be re-queued when we are waiting for the timeout.
> 
> We can reproduce the bug via allocating/freeing pages from remote
> node, then go idle.  And the patch can fix it.
> 
> Fixes: 7cc36bbddde5 ("vmstat: on-demand vmstat workers V8")
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Michal Hocko <mhocko@kernel.org>
> ---
>  mm/vmstat.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index b731d57996c5..111118741abf 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -856,8 +856,10 @@ static int refresh_cpu_vm_stats(bool do_pagesets)
>  				continue;
>  			}
>  
> -			if (__this_cpu_dec_return(pcp->expire))
> +			if (__this_cpu_dec_return(pcp->expire)) {
> +				changes++;
>  				continue;
> +			}
>  
>  			if (__this_cpu_read(pcp->count)) {
>  				drain_zone_pages(zone, this_cpu_ptr(pcp));
> -- 
> 2.39.2

-- 
Michal Hocko
SUSE Labs