From: Andrew Morton <akpm@linux-foundation.org>
To: Roman Gushchin <guro@fb.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
kernel-team@fb.com, Shakeel Butt <shakeelb@google.com>,
Michal Hocko <mhocko@kernel.org>,
Johannes Weiner <hannes@cmpxchg.org>, Tejun Heo <tj@kernel.org>,
Rik van Riel <riel@surriel.com>,
Konstantin Khlebnikov <koct9i@gmail.com>,
Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v3 3/3] mm: don't miss the last page because of round-off error
Date: Mon, 27 Aug 2018 14:04:32 -0700 [thread overview]
Message-ID: <20180827140432.b3c792f60235a13739038808@linux-foundation.org> (raw)
In-Reply-To: <20180827162621.30187-3-guro@fb.com>
On Mon, 27 Aug 2018 09:26:21 -0700 Roman Gushchin <guro@fb.com> wrote:
> I've noticed, that dying memory cgroups are often pinned
> in memory by a single pagecache page. Even under moderate
> memory pressure they sometimes stayed in such state
> for a long time. That looked strange.
>
> My investigation showed that the problem is caused by
> applying the LRU pressure balancing math:
>
> scan = div64_u64(scan * fraction[lru], denominator),
>
> where
>
> denominator = fraction[anon] + fraction[file] + 1.
>
> Because fraction[lru] is always less than denominator,
> if the initial scan size is 1, the result is always 0.
>
> This means the last page is not scanned and has
> no chances to be reclaimed.
>
> Fix this by rounding up the result of the division.
>
> In practice this change significantly improves the speed
> of dying cgroups reclaim.
>
> ...
>
> --- a/include/linux/math64.h
> +++ b/include/linux/math64.h
> @@ -281,4 +281,6 @@ static inline u64 mul_u64_u32_div(u64 a, u32 mul, u32 divisor)
> }
> #endif /* mul_u64_u32_div */
>
> +#define DIV64_U64_ROUND_UP(ll, d) div64_u64((ll) + (d) - 1, (d))
This macro references arg `d' more than once. That can cause problems
if the passed expression has side-effects and is poor practice. Can
we please redo this with a temporary?
> #endif /* _LINUX_MATH64_H */
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d649b242b989..2c67a0121c6d 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2446,9 +2446,11 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
> /*
> * Scan types proportional to swappiness and
> * their relative recent reclaim efficiency.
> + * Make sure we don't miss the last page
> + * because of a round-off error.
> */
> - scan = div64_u64(scan * fraction[file],
> - denominator);
> + scan = DIV64_U64_ROUND_UP(scan * fraction[file],
> + denominator);
> break;
> case SCAN_FILE:
> case SCAN_ANON:
next prev parent reply other threads:[~2018-08-27 21:04 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-27 16:26 [PATCH v3 1/3] mm: rework memcg kernel stack accounting Roman Gushchin
2018-08-27 16:26 ` [PATCH v3 2/3] mm: drain memcg stocks on css offlining Roman Gushchin
2018-08-27 16:26 ` [PATCH v3 3/3] mm: don't miss the last page because of round-off error Roman Gushchin
2018-08-27 21:04 ` Andrew Morton [this message]
2018-08-27 23:24 ` Roman Gushchin
2018-08-29 21:33 ` Roman Gushchin
2018-09-05 21:08 ` Andrew Morton
2018-08-27 21:01 ` [PATCH v3 1/3] mm: rework memcg kernel stack accounting Andrew Morton
2018-08-27 23:19 ` Roman Gushchin
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20180827140432.b3c792f60235a13739038808@linux-foundation.org \
--to=akpm@linux-foundation.org \
--cc=guro@fb.com \
--cc=hannes@cmpxchg.org \
--cc=kernel-team@fb.com \
--cc=koct9i@gmail.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mhocko@kernel.org \
--cc=riel@surriel.com \
--cc=shakeelb@google.com \
--cc=tj@kernel.org \
--cc=willy@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).