From: Konstantin Khlebnikov <koct9i@gmail.com>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@infradead.org>,
Roman Gushchin <guro@fb.com>,
linux-mm@kvack.org,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
kernel-team@fb.com, Andrew Morton <akpm@linux-foundation.org>,
Michal Hocko <mhocko@kernel.org>, Tejun Heo <tj@kernel.org>,
Rik van Riel <riel@surriel.com>
Subject: Re: [PATCH RFC] mm: don't miss the last page because of round-off error
Date: Wed, 22 Aug 2018 09:01:19 +0300 [thread overview]
Message-ID: <CALYGNiMO973zcp85i5yd=Fa-Jo7SdObXH6sPuogoO3hnB-GgRA@mail.gmail.com> (raw)
In-Reply-To: <20180821171555.GA16545@cmpxchg.org>
On Tue, Aug 21, 2018 at 8:15 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> On Tue, Aug 21, 2018 at 08:11:44AM +0300, Konstantin Khlebnikov wrote:
>> On Sat, Aug 18, 2018 at 4:22 AM, Matthew Wilcox <willy@infradead.org> wrote:
>> > On Fri, Aug 17, 2018 at 04:18:34PM -0700, Roman Gushchin wrote:
>> >> - scan = div64_u64(scan * fraction[file],
>> >> - denominator);
>> >> + if (scan > 1)
>> >> + scan = div64_u64(scan * fraction[file],
>> >> + denominator);
>> >
>> > Wouldn't we be better off doing a div_round_up? ie:
>> >
>> > scan = div64_u64(scan * fraction[file] + denominator - 1, denominator);
>> >
>> > although i'd rather hide that in a new macro in math64.h than opencode it
>> > here.
>>
>> All numbers here should be up to nr_pages * 200 and fit into unsigned long.
>> I see no reason for u64. If they overflow then u64 wouldn't help either.
>
> It is nr_pages * 200 * recent_scanned, where recent_scanned can be up
> to four times of what's on the LRUs. That can overflow a u32 with even
> small amounts of memory.
Ah, this thing is inverted because it aims to proportional reactivation rate
rather than the proportional pressure to reclaimable pages.
That's not obvious. I suppose this should be in comment above it.
Well, at least denominator should fit into unsigned long. So full
64/64 division is redundant.
next prev parent reply other threads:[~2018-08-22 6:01 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-17 23:18 [PATCH RFC] mm: don't miss the last page because of round-off error Roman Gushchin
2018-08-18 1:22 ` Matthew Wilcox
2018-08-20 17:19 ` Roman Gushchin
2018-08-21 5:11 ` Konstantin Khlebnikov
2018-08-21 13:35 ` Matthew Wilcox
2018-08-21 17:15 ` Johannes Weiner
2018-08-22 6:01 ` Konstantin Khlebnikov [this message]
2018-08-22 17:50 ` Roman Gushchin
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