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
