Messages in this thread | | | Date | Fri, 15 May 2009 10:28:50 +0900 | Subject | Re: [PATCH] mmtom: Prevent shrinking of active anon lru list in case of no swap space V3 | From | Minchan Kim <> |
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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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