| Message ID | 20240813163759.742675-1-yuanchu@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm: multi-gen LRU: ignore non-leaf pmd_young for force_scan=true | expand |
On Tue, Aug 13, 2024 at 10:38 AM Yuanchu Xie <yuanchu@google.com> wrote: > > When non-leaf pmd accessed bits are available, MGLRU page table walks > can clear the non-leaf pmd accessed bit and ignore the accessed bit on > the pte if it's on a different node, skipping a generation update as > well. If another scan occurs on the same node as said skipped pte. > the non-leaf pmd accessed bit might remain cleared and the pte accessed > bits won't be checked. While this is sufficient for reclaim-driven > aging, where the goal is to select a reasonably cold page, the access > can be missed when aging proactively for workingset estimation of a > node/memcg. > > In more detail, get_pfn_folio returns NULL if the folio's nid != node > under scanning, so the page table walk skips processing of said pte. Now > the pmd_young flag on this pmd is cleared, and if none of the pte's are > accessed before another scan occurs on the folio's node, the pmd_young > check fails and the pte accessed bit is skipped. > > Since force_scan disables various other optimizations, we check > force_scan to ignore the non-leaf pmd accessed bit. > > Signed-off-by: Yuanchu Xie <yuanchu@google.com> Acked-by: Yu Zhao <yuzhao@google.com>
diff --git a/mm/vmscan.c b/mm/vmscan.c index cfa839284b92..4a112c2d1a64 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -3476,7 +3476,7 @@ static void walk_pmd_range_locked(pud_t *pud, unsigned long addr, struct vm_area goto next; if (!pmd_trans_huge(pmd[i])) { - if (should_clear_pmd_young()) + if (!walk->force_scan && should_clear_pmd_young()) pmdp_test_and_clear_young(vma, addr, pmd + i); goto next; } @@ -3563,7 +3563,7 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end, walk->mm_stats[MM_NONLEAF_TOTAL]++; - if (should_clear_pmd_young()) { + if (!walk->force_scan && should_clear_pmd_young()) { if (!pmd_young(val)) continue;
When non-leaf pmd accessed bits are available, MGLRU page table walks can clear the non-leaf pmd accessed bit and ignore the accessed bit on the pte if it's on a different node, skipping a generation update as well. If another scan occurs on the same node as said skipped pte. the non-leaf pmd accessed bit might remain cleared and the pte accessed bits won't be checked. While this is sufficient for reclaim-driven aging, where the goal is to select a reasonably cold page, the access can be missed when aging proactively for workingset estimation of a node/memcg. In more detail, get_pfn_folio returns NULL if the folio's nid != node under scanning, so the page table walk skips processing of said pte. Now the pmd_young flag on this pmd is cleared, and if none of the pte's are accessed before another scan occurs on the folio's node, the pmd_young check fails and the pte accessed bit is skipped. Since force_scan disables various other optimizations, we check force_scan to ignore the non-leaf pmd accessed bit. Signed-off-by: Yuanchu Xie <yuanchu@google.com> --- mm/vmscan.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)