Re: [PATCH] mmtom: Prevent shrinking of active anon lru list in case of no swap space V3

[Minchan Kim <minchan.kim@gmail.com>]

On Fri, May 15, 2009 at 1:22 AM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> On Thu, May 14, 2009 at 11:39:49PM +0900, Minchan Kim wrote:
>> On Thu, May 14, 2009 at 11:27 PM, KOSAKI Motohiro
>> <kosaki.motohiro@jp.fujitsu.com> wrote:
>> >>  mm/vmscan.c |    2 +-
>> >>  1 files changed, 1 insertions(+), 1 deletions(-)
>> >>
>> >> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> >> index 2f9d555..621708f 100644
>> >> --- a/mm/vmscan.c
>> >> +++ b/mm/vmscan.c
>> >> @@ -1577,7 +1577,7 @@ static void shrink_zone(int priority, struct zone *zone,
>> >>         * Even if we did not try to evict anon pages at all, we want to
>> >>         * rebalance the anon lru active/inactive ratio.
>> >>         */
>> >> -       if (inactive_anon_is_low(zone, sc))
>> >> +       if (inactive_anon_is_low(zone, sc) && nr_swap_pages > 0)
>> >>                shrink_active_list(SWAP_CLUSTER_MAX, zone, sc, priority, 0);
>> >
>> >
>> >       if (nr_swap_pages > 0 && inactive_anon_is_low(zone, sc))
>> >
>> > is better?
>> > compiler can't swap evaluate order around &&.
>>
>> If GCC optimizes away that branch with CONFIG_SWAP=n as Rik mentioned,
>> we don't have a concern.
>
> It can only optimize it away when the condition is a compile time
> constant.
>
> But inactive_anon_is_low() contains atomic operations which the
> compiler is not allowed to drop and so the && semantics lead to
>
>        atomic_read() && 0
>
> emitting the read while still knowing the whole expression is 0 at
> compile-time, optimizing away only the branch itself but leaving the
> read in place!
>
> Compared to
>
>        0 && atomic_read()
>
> where the && short-circuitry leads to atomic_read() not being
> executed.  And since the 0 is a compile time constant, no code has to
> be emitted for the read.
>
> So KOSAKI-san's is right.  Your version results in bigger object code.

You're right.  I realized it from you.
I will repost this.
Thanks for great review, Hannes :)

>        Hannes
>



-- 
Kinds regards,
Minchan Kim

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