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Date: Thu, 27 Jun 2013 11:37:21 +0200
From: Michal Hocko <mhocko@suse.cz>
Subject: Re: [PATCH v2] vmpressure: consider "scanned < reclaimed" case when
 calculating  a pressure level.
Message-ID: <20130627093721.GC17647@dhcp22.suse.cz>
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To: Hyunhee Kim <hyunhee.kim@samsung.com>
Cc: 'Anton Vorontsov' <anton@enomsg.org>, linux-mm@kvack.org, akpm@linux-foundation.org, rob@landley.net, kamezawa.hiroyu@jp.fujitsu.com, hannes@cmpxchg.org, rientjes@google.com, kirill@shutemov.name, 'Kyungmin Park' <kyungmin.park@samsung.com>, 'Minchan Kim' <minchan@kernel.org>

On Thu 27-06-13 15:12:10, Hyunhee Kim wrote:
> In vmpressure, the pressure level is calculated based on the ratio
> of how many pages were scanned vs. reclaimed in a given time window.
> However, there is a possibility that "scanned < reclaimed" in such a
> case, when reclaiming ends by fatal signal in shrink_inactive_list.
> So, with this patch, we just return "low" level when "scanned < reclaimed"
> happens not to have userland miss reclaim activity.

Hmm, fatal signal pending on kswapd doesn't make sense to me so it has
to be a direct reclaim path. Does it really make sense to signal LOW
when there is probably a big memory pressure and somebody is killing the
current allocator?

The THP case made sense because nr_scanned is in LRU elements units
while nr_reclaimed is in page units which are different so nr_reclaim
might be higher than nr_scanned (so nr_taken would be more approapriate
for vmpressure).
 
> Signed-off-by: Hyunhee Kim <hyunhee.kim@samsung.com>
> Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
> ---
>  mm/vmpressure.c |    8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/mm/vmpressure.c b/mm/vmpressure.c
> index 736a601..8c60cad 100644
> --- a/mm/vmpressure.c
> +++ b/mm/vmpressure.c
> @@ -119,6 +119,14 @@ static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
>  	unsigned long pressure;
>  
>  	/*
> +	 * This could happen, in such a case, when reclaiming ends by fatal
> +	 * signal in shrink_inactive_list(). In this case, return
> +	 * VMPRESSURE_LOW not to have userland miss reclaim activity.
> +	 */
> +	if (reclaimed > scanned)
> +		return VMPRESSURE_LOW;
> +
> +	/*
>  	 * We calculate the ratio (in percents) of how many pages were
>  	 * scanned vs. reclaimed in a given time frame (window). Note that
>  	 * time is in VM reclaimer's "ticks", i.e. number of pages
> -- 
> 1.7.9.5
> 
> 
> --
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-- 
Michal Hocko
SUSE Labs

--
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