Messages in this thread | | | From | KOSAKI Motohiro <> | Subject | Re: [PATCH 1/3] Reintroduce zone_reclaim_interval for when zone_reclaim() scans and fails to avoid CPU spinning at 100% on NUMA | Date | Tue, 9 Jun 2009 19:50:30 +0900 (JST) |
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> 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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