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    SubjectRe: [PATCH 1/3] Reintroduce zone_reclaim_interval for when zone_reclaim() scans and fails to avoid CPU spinning at 100% on NUMA
    Date
    > On Tue, Jun 09, 2009 at 06:59:03PM +0900, KOSAKI Motohiro wrote:
    > > > > > > Here it is just recording the jiffies value. The real smarts with the counter
    > > > > > > use time_before() which I assumed could handle jiffie wrap-arounds. Even
    > > > > > > if it doesn't, the consequence is that one scan will occur that could have
    > > > > > > been avoided around the time of the jiffie wraparound. The value will then
    > > > > > > be reset and it will be fine.
    > > > > >
    > > > > > time_before() assume two argument are enough nearly time.
    > > > > > if we use 32bit cpu and HZ=1000, about jiffies wraparound about one month.
    > > > > >
    > > > > > Then,
    > > > > >
    > > > > > 1. zone reclaim failure occur
    > > > > > 2. system works fine for one month
    > > > > > 3. jiffies wrap and time_before() makes mis-calculation.
    > > > > >
    > > > >
    > > > > And the scan occurs uselessly and zone_reclaim_failure gets set again.
    > > > > I believe the one useless scan is not significant enough to warrent dealing
    > > > > with jiffie wraparound.
    > > >
    > > > Thank you for kindful explanation.
    > > > I fully agreed.
    > >
    > > Bah, no, not agreed.
    > > simple last failure recording makes following scenario.
    > >
    > >
    > > 1. zone reclaim failure occur. update zone_reclaim_failure.
    > > ^
    > > | time_before() return 1, and zone_reclaim() return immediately.
    > > v
    > > 2. after 32 second.
    > > ^
    > > | time_before() return 0, and zone_reclaim() works normally
    > > v
    > > 3. after one month
    > > ^
    > > | time_before() return 1, and zone_reclaim() return immediately.
    > > | although recent zone_reclaim() never failed.
    > > v
    > > 4. after more one month
    > >
    >
    > Pants.
    >
    > /me slaps self
    >
    > + /* Watch for jiffie wraparound */
    > + if (unlikely(jiffies < zone->zone_reclaim_failure))
    > + zone->zone_reclaim_failure = jiffies;
    > +
    > + /* Do not attempt a scan if scanning failed recently */
    > + if (time_before(jiffies,
    > + zone->zone_reclaim_failure + zone_reclaim_interval))
    > + return 0;
    > +
    >
    > ?

    looks good.






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