[PATCH v2] mm/mglru: yield the lru_lock earlier in fill/drain_evictable()

["Hui Zhu" <hui.zhu@linux.dev>]

From: Hui Zhu <zhuhui@kylinos.cn>

lru_gen_change_state() migrates folios in batches of up to
MAX_LRU_BATCH (4096) folios under lruvec_lock_irq().  A full batch
keeps local interrupts off for up to about a millisecond, and CPUs
contending for the same lru_lock spin with interrupts disabled for
that long.

The caller already reschedules between batches, but a batch itself
only ends after 4096 folios.  Stop it early when a resched is
pending, or when the lock has waiters and the current hold has moved
at least MIN_LRU_BATCH folios; the caller restarts where it left off.
TIF_NEED_RESCHED can still be set with interrupts off: a remote
resched_curr() sets it directly in the target's thread_info.  The
minimum batch avoids lock bouncing under sustained contention, while
a waiter now waits ~15us instead of ~1ms.

This follows lru_gen_look_around() in this file, security/keys/gc.c
and jbd2_log_do_checkpoint(), and commit 6e9be217a3ce ("mm/mglru:
use a smaller batch for reclaim").

Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
---
Changlog:
v2:
Also yield on lock contention after a minimum batch (Kairui Song).

 mm/vmscan.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 6a1af9b92349..c1231925159f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -5533,7 +5533,10 @@ static bool fill_evictable(struct lruvec *lruvec)
 			success = lru_gen_add_folio(lruvec, folio, !active);
 			VM_WARN_ON_ONCE(!success);
 
-			if (!--remaining)
+			/* stop early to shorten the irq-off section */
+			if (!--remaining || need_resched() ||
+			    (MAX_LRU_BATCH - remaining >= MIN_LRU_BATCH &&
+			     spin_is_contended(&lruvec->lru_lock)))
 				return false;
 		}
 	}
@@ -5562,7 +5565,10 @@ static bool drain_evictable(struct lruvec *lruvec)
 			VM_WARN_ON_ONCE(!success);
 			lruvec_add_folio(lruvec, folio);
 
-			if (!--remaining)
+			/* stop early to shorten the irq-off section */
+			if (!--remaining || need_resched() ||
+			    (MAX_LRU_BATCH - remaining >= MIN_LRU_BATCH &&
+			     spin_is_contended(&lruvec->lru_lock)))
 				return false;
 		}
 	}
-- 
2.53.0