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mm/mglru: try to stop at high watermarks
The initial MGLRU patchset didn't include the memcg LRU support, and it relied on should_abort_scan(), added by commit f76c833 ("mm: multi-gen LRU: optimize multiple memcgs"), to "backoff to avoid overshooting their aggregate reclaim target by too much". Later on when the memcg LRU was added, should_abort_scan() was deemed unnecessary, and the test results [1] showed no side effects after it was removed by commit a579086 ("mm: multi-gen LRU: remove eviction fairness safeguard"). However, that test used memory.reclaim, which sets nr_to_reclaim to SWAP_CLUSTER_MAX. So it can overshoot only by SWAP_CLUSTER_MAX-1 pages, i.e., from nr_reclaimed=nr_to_reclaim-1 to nr_reclaimed=nr_to_reclaim+SWAP_CLUSTER_MAX-1. Compared with the batch size kswapd sets to nr_to_reclaim, SWAP_CLUSTER_MAX is tiny. Therefore that test isn't able to reproduce the worst case scenario, i.e., kswapd overshooting GBs on large systems and "consuming 100% CPU" (see the Closes tag). Bring back a simplified version of should_abort_scan() on top of the memcg LRU, so that kswapd stops when all eligible zones are above their respective high watermarks plus a small delta to lower the chance of KSWAPD_HIGH_WMARK_HIT_QUICKLY. Note that this only applies to order-0 reclaim, meaning compaction-induced reclaim can still run wild (which is a different problem). On Android, launching 55 apps sequentially: Before After Change pgpgin 838377172 802955040 -4% pgpgout 38037080 34336300 -10% [1] https://lore.kernel.org/[email protected]/ Link: https://lkml.kernel.org/r/[email protected] Fixes: a579086 ("mm: multi-gen LRU: remove eviction fairness safeguard") Signed-off-by: Yu Zhao <[email protected]> Reported-by: Charan Teja Kalla <[email protected]> Reported-by: Jaroslav Pulchart <[email protected]> Closes: https://lore.kernel.org/CAK8fFZ4DY+GtBA40Pm7Nn5xCHy+51w3sfxPqkqpqakSXYyX+Wg@mail.gmail.com/ Tested-by: Jaroslav Pulchart <[email protected]> Tested-by: Kalesh Singh <[email protected]> Cc: Hillf Danton <[email protected]> Cc: Kairui Song <[email protected]> Cc: T.J. Mercier <[email protected]> Cc: <[email protected]> Signed-off-by: Andrew Morton <[email protected]>
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