On Fri, 4 May 2018 11:33:22 +0100 Mel Gorman <mgorman@xxxxxxx> wrote: > On Fri, May 04, 2018 at 09:03:02AM +0200, Vlastimil Babka wrote: > > > min watermark for NORMAL zone on node 0 > > > allocation initiated on node 0: 750 + 4096 = 4846 > > > allocation initiated on node 1: 750 + 0 = 750 > > > > > > This watermark difference could cause too many numa_miss allocation > > > in some situation and then performance could be downgraded. > > > > > > Recently, there was a regression report about this problem on CMA patches > > > since CMA memory are placed in ZONE_MOVABLE by those patches. I checked > > > that problem is disappeared with this fix that uses high_zoneidx > > > for classzone_idx. > > > > > > http://lkml.kernel.org/r/20180102063528.GG30397@yexl-desktop > > > > > > Using high_zoneidx for classzone_idx is more consistent way than previous > > > approach because system's memory layout doesn't affect anything to it. > > > > So to summarize; > > - ac->high_zoneidx is computed via the arcane gfp_zone(gfp_mask) and > > represents the highest zone the allocation can use > > It's arcane but it was simply a fast-path calculation. A much older > definition would be easier to understand but it was slower. > > > - classzone_idx was supposed to be the highest zone that the allocation > > can use, that is actually available in the system. Somehow that became > > the highest zone that is available on the preferred node (in the default > > node-order zonelist), which causes the watermark inconsistencies you > > mention. > > > > I think it *always* was the index of the first preferred zone of a > zonelist. The treatment of classzone has changed a lot over the years and > I didn't do a historical check but the general intent was always "protect > some pages in lower zones". This was particularly important for 32-bit > and highmem albeit that is less of a concern today. When it transferred to > NUMA, I don't think it ever was seriously considered if it should change > as the critical node was likely to be node 0 with all the zones and the > remote nodes all used the highest zone. CMA/MOVABLE changed that slightly > by allowing the possibility of node0 having a "higher" zone than every > other node. When MOVABLE was introduced, it wasn't much of a problem as > the purpose of MOVABLE was for systems that dynamically needed to allocate > hugetlbfs later in the runtime but for CMA, it was a lot more critical > for ordinary usage so this is primarily a CMA thing. > > > I don't see a problem with your change. I would be worried about > > inflated reserves when e.g. ZONE_MOVABLE doesn't exist, but that doesn't > > seem to be the case. My laptop has empty ZONE_MOVABLE and the > > ZONE_NORMAL protection for movable is 0. > > > > But there had to be some reason for classzone_idx to be like this and > > not simple high_zoneidx. Maybe Mel remembers? Maybe it was important > > then, but is not anymore? Sigh, it seems to be pre-git. > > > > classzone predates my involvement with Linux but I would be less concerneed > about what the original intent was and instead ensure that classzone index > is consistent, sane and potentially renamed while preserving the intent of > "reserve pages in lower zones when an allocation request can use higher > zones". While historically the critical intent was to preserve Normal and > to a lesser extent DMA on 32-bit systems, there still should be some care > of DMA32 so we should not lose that. > > With the patch, the allocator looks like it would be fine as just > reservations change. I think it's unlikely that CMA usage will result > in lowmem starvation. Compaction becomes a bit weird as classzone index > has no special meaning versis highmem and I think it'll be very easy to > forget. Similarly, vmscan can reclaim pages from remote nodes and zones > that are higher than the original request. That is not likely to be a > problem but it's a change in behaviour and easy to miss. > > Fundamentally, I find it extremely weird we now have two variables that are > essentially the same thing. They should be collapsed into one variable, > renamed and documented on what the index means for page allocator, > compaction, vmscan and the special casing around CMA. You're all so young ;) classzone was Andrea. Perhaps he can shed some light upon the questions which have been raised?
