On Fri, Jul 31, 2015 at 09:25:13AM +0200, Vlastimil Babka wrote: > On 07/31/2015 09:11 AM, Mel Gorman wrote: > >On Fri, Jul 31, 2015 at 02:54:07PM +0900, Joonsoo Kim wrote: > >>Hello, Mel. > >> > >>On Mon, Jul 20, 2015 at 09:00:18AM +0100, Mel Gorman wrote: > >>>From: Mel Gorman <mgorman@xxxxxxx> > >>> > >>>High-order watermark checking exists for two reasons -- kswapd high-order > >>>awareness and protection for high-order atomic requests. Historically we > >>>depended on MIGRATE_RESERVE to preserve min_free_kbytes as high-order free > >>>pages for as long as possible. This patch introduces MIGRATE_HIGHATOMIC > >>>that reserves pageblocks for high-order atomic allocations. This is expected > >>>to be more reliable than MIGRATE_RESERVE was. > >> > >>I have some concerns on this patch. > >> > >>1) This patch breaks intention of __GFP_WAIT. > >>__GFP_WAIT is used when we want to succeed allocation even if we need > >>to do some reclaim/compaction work. That implies importance of > >>allocation success. But, reserved pageblock for MIGRATE_HIGHATOMIC makes > >>atomic allocation (~__GFP_WAIT) more successful than allocation with > >>__GFP_WAIT in many situation. It breaks basic assumption of gfp flags > >>and doesn't make any sense. > >> > > > >Currently allocation requests that do not specify __GFP_WAIT get the > >ALLOC_HARDER flag which allows them to dip further into watermark reserves. > >It already is the case that there are corner cases where a high atomic > >allocation can succeed when a non-atomic allocation would reclaim. > > I think (and said so before elsewhere) is that the problem is that we don't > currently distinguish allocations that can't wait (=are really atomic and > have no order-0 fallback) and allocations that just don't want to wait > (=they have fallbacks). The second ones should obviously not access the > current ALLOC_HARDER watermark-based reserves nor the proposed highatomic > reserves. > It's a separate issue though. There are a number of cases 1. can't wait because a spinlock is held or in interrupt 2. does not want to wait because a fallback option is available 3. does not want to wait or wake kswapd because a fallback option is available 4. should not fail as it would cause major difficulties 5. cannot fail because it's a functional failure 5 is never meant to occur might be the situation on embedded platforms. If this is the case then they should consider modifying MIGRATE_HIGHATOMIC in this patch series but I don't think it belongs in mainline as it has other consequences. 1-4 are a separate series. Right now, this series does not drastically alter the concept that in some cases atomic allocations will succeed without delay when a !atomic allocation would have to reclaim. There is certainly value to ironing out 1-4 on top and teaching SLUB, THP and networking the distinction. > Well we do look at __GFP_NO_KSWAPD flag to treat allocation as non-atomic, > so that covers THP allocations and two drivers. But the recent networking > commit fb05e7a89f50 didn't add the flag and nor does Joonsoo's slub patch > use it. Either we should rename the flag and employ it where appropriate, or > agree that access to reserves is orthogonal concern to waking up kswapd, and > distinguish non-atomic non-__GFP_WAIT allocations differently. > Separate problem with a separate series. This one is about removing the zonelist cache due to complexity and removing an odd anomaly where allocations can fail due to how watermarks are calculated. > >>>A MIGRATE_HIGHORDER pageblock is created when an allocation request steals > >>>a pageblock but limits the total number to 10% of the zone. > >> > >>When steals happens, pageblock already can be fragmented and we can't > >>fully utilize this pageblock without allowing order-0 allocation. This > >>is very waste. > >> > > > >If the pageblock was stolen, it implies there was at least 1 usable page > >of the correct order. As the pageblock is then reserved, any pages that > >free in that block stay free for use by high-order atomic allocations. > >Else, the number of pageblocks will increase again until the 10% limit > >is hit. > > It's however true that many of the "any pages free in that block" may be > order-0, so they both won't be useful to high-order atomic allocations, and > won't be available to other allocations, so they might remain unused. I typoed lightly and missed a letter but the same outcome applies when slightly corrected -- any pages that are *freed* in that block stay free if it merges with buddies for use by high-order atomic allocations. or else, the number of pageblocks will increase again until the 10% limit is hit. If the limit is hit and we are still failing then it's no different to what can happen today except it took a lot longer and was a lot harder to trigger. As the changelog pointed out, with this approach the allocation failure rate was massively reduced but not eliminated. -- Mel Gorman SUSE Labs -- 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>