On Tue, May 24, 2011 at 12:07:36AM +0400, James Bottomley wrote: > On Mon, 2011-05-23 at 13:03 -0700, Andrew Morton wrote: > > On Mon, 23 May 2011 10:53:55 +0100 > > Mel Gorman <mgorman@xxxxxxx> wrote: > > > > > It has been reported on some laptops that kswapd is consuming large > > > amounts of CPU and not being scheduled when SLUB is enabled during > > > large amounts of file copying. It is expected that this is due to > > > kswapd missing every cond_resched() point because; > > > > > > shrink_page_list() calls cond_resched() if inactive pages were isolated > > > which in turn may not happen if all_unreclaimable is set in > > > shrink_zones(). If for whatver reason, all_unreclaimable is > > > set on all zones, we can miss calling cond_resched(). > > > > > > balance_pgdat() only calls cond_resched if the zones are not > > > balanced. For a high-order allocation that is balanced, it > > > checks order-0 again. During that window, order-0 might have > > > become unbalanced so it loops again for order-0 and returns > > > that it was reclaiming for order-0 to kswapd(). It can then > > > find that a caller has rewoken kswapd for a high-order and > > > re-enters balance_pgdat() without ever calling cond_resched(). > > > > > > shrink_slab only calls cond_resched() if we are reclaiming slab > > > pages. If there are a large number of direct reclaimers, the > > > shrinker_rwsem can be contended and prevent kswapd calling > > > cond_resched(). > > > > > > This patch modifies the shrink_slab() case. If the semaphore is > > > contended, the caller will still check cond_resched(). After each > > > successful call into a shrinker, the check for cond_resched() remains > > > in case one shrinker is particularly slow. > > > > So CONFIG_PREEMPT=y kernels don't exhibit this problem? > > Yes, they do. They just don't hang on my sandybridge system in the same > way than non-PREEMPT kernels do. I'm still sure it's got something to > do with rescheduling kswapd onto a different CPU ... > > > I'm still unconvinced that we know what's going on here. What's kswapd > > *doing* with all those cycles? And if kswapd is now scheduling away, > > who is doing that work instead? Direct reclaim? > > Still in the dark about this one, too. > I still very strongly suspect that what gets us into this situation is all_unreclaiable being set when there are a large bunch of dirty pages together in the LRU pushing up the scanning rates high enough after slab is shrunk as far as they can be at this time. Without a local reproduction case, I'm undecided as to how this should be investigated other than sticking in printks when all_unreclaimable is set that outputs the number of LRU pages - anon, file and dirty (even though this information in itself will be incomplete) and see what falls out. I'm trying to borrow a similar laptop but haven't found someone with a similar model yet in the locality. -- Mel Gorman SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
