Hi, Mel, Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> writes: > On Tue, Jun 13, 2023 at 04:55:04PM +0800, Baolin Wang wrote: >> On some machines, the normal zone can have a large memory hole like >> below memory layout, and we can see the range from 0x100000000 to >> 0x1800000000 is a hole. So when isolating some migratable pages, the >> scanner can meet the hole and it will take more time to skip the large >> hole. From my measurement, I can see the isolation scanner will take >> 80us ~ 100us to skip the large hole [0x100000000 - 0x1800000000]. >> >> So adding a new helper to fast search next online memory section >> to skip the large hole can help to find next suitable pageblock >> efficiently. With this patch, I can see the large hole scanning only >> takes < 1us. >> >> [ 0.000000] Zone ranges: >> [ 0.000000] DMA [mem 0x0000000040000000-0x00000000ffffffff] >> [ 0.000000] DMA32 empty >> [ 0.000000] Normal [mem 0x0000000100000000-0x0000001fa7ffffff] >> [ 0.000000] Movable zone start for each node >> [ 0.000000] Early memory node ranges >> [ 0.000000] node 0: [mem 0x0000000040000000-0x0000000fffffffff] >> [ 0.000000] node 0: [mem 0x0000001800000000-0x0000001fa3c7ffff] >> [ 0.000000] node 0: [mem 0x0000001fa3c80000-0x0000001fa3ffffff] >> [ 0.000000] node 0: [mem 0x0000001fa4000000-0x0000001fa402ffff] >> [ 0.000000] node 0: [mem 0x0000001fa4030000-0x0000001fa40effff] >> [ 0.000000] node 0: [mem 0x0000001fa40f0000-0x0000001fa73cffff] >> [ 0.000000] node 0: [mem 0x0000001fa73d0000-0x0000001fa745ffff] >> [ 0.000000] node 0: [mem 0x0000001fa7460000-0x0000001fa746ffff] >> [ 0.000000] node 0: [mem 0x0000001fa7470000-0x0000001fa758ffff] >> [ 0.000000] node 0: [mem 0x0000001fa7590000-0x0000001fa7ffffff] >> >> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> > > This may only be necessary for non-contiguous zones so a check for > zone_contiguous could be made but I suspect the saving, if any, would be > marginal. > > However, it's subtle that block_end_pfn can end up in an arbirary location > past the end of the zone or past cc->free_pfn. As the "continue" will update > cc->migrate_pfn, that might lead to errors in the future. It would be a > lot safer to pass in cc->free_pfn and do two things with the value. First, > there is no point scanning for a valid online section past cc->free_pfn so > terminating after cc->free_pfn may save some cycles. Second, cc->migrate_pfn > does not end up with an arbitrary value which is a more defensive approach > to any future programming errors. I have thought about this before. Originally, I had thought that we were safe because cc->free_pfn should be in a online section and block_end_pfn should reach cc->free_pfn before the end of zone. But after checking more code and thinking about it again, I found that the underlying sections may go offline under us during compaction. So that, cc->free_pfn may be in a offline section or after the end of zone. So, you are right, we need to consider the range of block_end_pfn. But, if we thought in this way (memory online/offline at any time), it appears that we need to check whether the underlying section was offlined. For example, is it safe to use "pfn_to_page()" in "isolate_migratepages_block()"? Is it possible for the underlying section to be offlined under us? Hi, David, can you teach me on this too? Best Regards, Huang, Ying
