On Fri, 17 Apr 2020, js1304@xxxxxxxxx wrote: > From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> > > Currently, we use classzone_idx to calculate lowmem reserve proetection > for an allocation request. This classzone_idx causes a problem > on NUMA systems when the lowmem reserve protection exists for some zones > on a node that do not exist on other nodes. > > Before further explanation, I should first clarify how to compute > the classzone_idx and the high_zoneidx. > > - ac->high_zoneidx is computed via the arcane gfp_zone(gfp_mask) and > represents the index of the highest zone the allocation can use > - classzone_idx was supposed to be the index of the highest zone on > the local node that the allocation can use, that is actually available > in the system > > Think about following example. Node 0 has 4 populated zone, > DMA/DMA32/NORMAL/MOVABLE. Node 1 has 1 populated zone, NORMAL. Some zones, > such as MOVABLE, doesn't exist on node 1 and this makes following > difference. > > Assume that there is an allocation request whose gfp_zone(gfp_mask) is > the zone, MOVABLE. Then, it's high_zoneidx is 3. If this allocation is > initiated on node 0, it's classzone_idx is 3 since actually > available/usable zone on local (node 0) is MOVABLE. If this allocation > is initiated on node 1, it's classzone_idx is 2 since actually > available/usable zone on local (node 1) is NORMAL. > > You can see that classzone_idx of the allocation request are different > according to their starting node, even if their high_zoneidx is the same. > > Think more about these two allocation requests. If they are processed > on local, there is no problem. However, if allocation is initiated > on node 1 are processed on remote, in this example, at the NORMAL zone > on node 0, due to memory shortage, problem occurs. Their different > classzone_idx leads to different lowmem reserve and then different > min watermark. See the following example. > > root@ubuntu:/sys/devices/system/memory# cat /proc/zoneinfo > Node 0, zone DMA > per-node stats > ... > pages free 3965 > min 5 > low 8 > high 11 > spanned 4095 > present 3998 > managed 3977 > protection: (0, 2961, 4928, 5440) > ... > Node 0, zone DMA32 > pages free 757955 > min 1129 > low 1887 > high 2645 > spanned 1044480 > present 782303 > managed 758116 > protection: (0, 0, 1967, 2479) > ... > Node 0, zone Normal > pages free 459806 > min 750 > low 1253 > high 1756 > spanned 524288 > present 524288 > managed 503620 > protection: (0, 0, 0, 4096) > ... > Node 0, zone Movable > pages free 130759 > min 195 > low 326 > high 457 > spanned 1966079 > present 131072 > managed 131072 > protection: (0, 0, 0, 0) > ... > Node 1, zone DMA > pages free 0 > min 0 > low 0 > high 0 > spanned 0 > present 0 > managed 0 > protection: (0, 0, 1006, 1006) > Node 1, zone DMA32 > pages free 0 > min 0 > low 0 > high 0 > spanned 0 > present 0 > managed 0 > protection: (0, 0, 1006, 1006) > Node 1, zone Normal > per-node stats > ... > pages free 233277 > min 383 > low 640 > high 897 > spanned 262144 > present 262144 > managed 257744 > protection: (0, 0, 0, 0) > ... > Node 1, zone Movable > pages free 0 > min 0 > low 0 > high 0 > spanned 262144 > present 0 > managed 0 > protection: (0, 0, 0, 0) > > - static min watermark for the NORMAL zone on node 0 is 750. > - lowmem reserve for the request with classzone idx 3 at the NORMAL > on node 0 is 4096. > - lowmem reserve for the request with classzone idx 2 at the NORMAL > on node 0 is 0. > > So, overall min watermark is: > allocation initiated on node 0 (classzone_idx 3): 750 + 4096 = 4846 > allocation initiated on node 1 (classzone_idx 2): 750 + 0 = 750 > > allocation initiated on node 1 will have some precedence than allocation > initiated on node 0 because min watermark of the former allocation is > lower than the other. So, allocation initiated on node 1 could succeed > on node 0 when allocation initiated on node 0 could not, and, this could > cause too many numa_miss allocation. 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. > With this patch, both classzone_idx on above example will be 3 so will > have the same min watermark. > > allocation initiated on node 0: 750 + 4096 = 4846 > allocation initiated on node 1: 750 + 4096 = 4846 > > One could wonder if there is a side effect that allocation initiated on > node 1 will use higher bar when allocation is handled on local since > classzone_idx could be higher than before. It will not happen because > the zone without managed page doesn't contributes lowmem_reserve at all. > > Reported-by: Ye Xiaolong <xiaolong.ye@xxxxxxxxx> > Tested-by: Ye Xiaolong <xiaolong.ye@xxxxxxxxx> > Reviewed-by: Baoquan He <bhe@xxxxxxxxxx> > Acked-by: Vlastimil Babka <vbabka@xxxxxxx> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
