Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes: > On 6/15/2023 11:22 AM, Huang, Ying wrote: >> 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? > > It is possible. There is a previous discussion[1] about the race > between pfn_to_online_page() and memory offline. > > [1] > https://lore.kernel.org/lkml/87zgc6buoq.fsf@xxxxxxxxxx/T/#m642d91bcc726437e1848b295bc57ce249c7ca399 Thank you very much for sharing! That answers my questions directly! Best Regards, Huang, Ying
