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

[Baolin Wang]

On 11/13/26 10:01 AM, Hui Zhu wrote:
> From: Hui Zhu <zhuhui@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> ---
> 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;

IIUC, Kairui's point[1] is to just drop the 'MAX_LRU_BATCH - remaining 
>= MIN_LRU_BATCH' condition, no?

if (!--remaining || need_resched() || spin_is_contended(&lruvec->lru_lock))

[1] 
https://lore.kernel.org/all/CAMgjq7CUMuNSdCXZ7_kDdo9+0BFvPNHuO=mq=ct_Q5YUyyckvQ@xxxxxxxxxxxxxx/