linux-kernel - Re: [PATCH v3 3/3] mm: don't miss the last page because of round-off error

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Date:   Mon, 27 Aug 2018 16:24:58 -0700
From:   Roman Gushchin <guro@...com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <kernel-team@...com>, Shakeel Butt <shakeelb@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Tejun Heo <tj@...nel.org>, Rik van Riel <riel@...riel.com>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 3/3] mm: don't miss the last page because of round-off
 error

On Mon, Aug 27, 2018 at 02:04:32PM -0700, Andrew Morton wrote:
> On Mon, 27 Aug 2018 09:26:21 -0700 Roman Gushchin <guro@...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?

Sure. This was copy-pasted to match the existing DIV_ROUND_UP
(probably, not the best idea).

So let me fix them both in a separate patch.

Thanks!