On Thu, Aug 01, 2013 at 12:31:34AM -0400, Rik van Riel wrote: > On 07/31/2013 10:56 PM, Minchan Kim wrote: > > > Yes, it's not really slow path because it could return to normal status > > without calling significant slow functions by reset batchcount of > > prepare_slowpath. > > > > I think it's tradeoff and I am biased your approach although we would > > lose a little performance because fair aging would recover the loss by > > fastpath's overhead. But who knows? Someone has a concern. > > > > So we should mention about such problems. > > If the atomic operation in the fast path turns out to be a problem, > I suspect we may be able to fix it by using per-cpu counters, and > consolidating those every once in a while. > > However, it may be good to see whether there is a problem in the > first place, before adding complexity. prepare_slowpath is racy anyway, so I don't see the point in wanting to do atomic ops in the first place. Much better to do the increment in assembly in a single insn to reduce the window, but there's no point in using the lock prefix when atomic_set will screw it over while somebody does atomic_dec under it anyway. atomic_set cannot be atomic, so it'll screw over any perfectly accurate accounted atomic_dec anyway. I think it should be done in C without atomic_dec or by introducing an __atomic_dec that isn't using the lock prefix and just tries to do the dec in a single (or as few as possible) asm insn. And because the whole thing is racy, after prepare_slowpath (which I think is a too generic name and should be renamed reset_zonelist_alloc_batch), this last attempt before invoking reclaim: /* This is the last chance, in general, before the goto nopage. */ page = get_page_from_freelist(gfp_mask, nodemask, order, zonelist, high_zoneidx, alloc_flags & ~ALLOC_NO_WATERMARKS, preferred_zone, migratetype); which is trying again with the MIN wmark, should be also altered so that we do one pass totally disregarding alloc_batch, just before declaring MIN-wmark failure in the allocation, and invoking direct reclaim. Either we add a (alloc_flags | ALLOC_NO_ALLOC_BATCH) to the above alloc_flags in the above call, or we run a second get_page_from_freelist if the above one fails with ALLOC_NO_ALLOC_BATCH set. Otherwise because of the inherent racy behavior of the alloc_batch logic, what will happen is that we'll eventually have enough hugepage allocations in enough CPUs to hit the alloc_batch limit again in between the prepare_slowpath and the above get_page_from_freelist, that will lead us to call spurious direct_reclaim invocations, when it was just a false negative of alloc_batch. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>