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Message-ID: <20241121114950.5ie64l3lmi3dkoz5@quack3>
Date: Thu, 21 Nov 2024 12:49:50 +0100
From: Jan Kara <jack@...e.cz>
To: Jim Zhao <jimzhao.ai@...il.com>
Cc: jack@...e.cz, akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
willy@...radead.org
Subject: Re: [PATCH v2] mm/page-writeback: raise wb_thresh to prevent write
blocking with strictlimit
On Tue 19-11-24 19:44:42, Jim Zhao wrote:
> With the strictlimit flag, wb_thresh acts as a hard limit in
> balance_dirty_pages() and wb_position_ratio(). When device write
> operations are inactive, wb_thresh can drop to 0, causing writes to be
> blocked. The issue occasionally occurs in fuse fs, particularly with
> network backends, the write thread is blocked frequently during a period.
> To address it, this patch raises the minimum wb_thresh to a controllable
> level, similar to the non-strictlimit case.
>
> Signed-off-by: Jim Zhao <jimzhao.ai@...il.com>
> ---
> Changes in v2:
> 1. Consolidate all wb_thresh bumping logic in __wb_calc_thresh for consistency;
> 2. Replace the limit variable with thresh for calculating the bump value,
> as __wb_calc_thresh is also used to calculate the background threshold;
> 3. Add domain_dirty_avail in wb_calc_thresh to get dtc->dirty.
Since the odd value of BdiDirryThresh got explained (independent cosmetic
bug), feel free to add:
Reviewed-by: Jan Kara <jack@...e.cz>
Honza
> ---
> mm/page-writeback.c | 48 ++++++++++++++++++++++-----------------------
> 1 file changed, 23 insertions(+), 25 deletions(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index e5a9eb795f99..8b13bcb42de3 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -917,7 +917,9 @@ static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc,
> unsigned long thresh)
> {
> struct wb_domain *dom = dtc_dom(dtc);
> + struct bdi_writeback *wb = dtc->wb;
> u64 wb_thresh;
> + u64 wb_max_thresh;
> unsigned long numerator, denominator;
> unsigned long wb_min_ratio, wb_max_ratio;
>
> @@ -931,11 +933,27 @@ static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc,
> wb_thresh *= numerator;
> wb_thresh = div64_ul(wb_thresh, denominator);
>
> - wb_min_max_ratio(dtc->wb, &wb_min_ratio, &wb_max_ratio);
> + wb_min_max_ratio(wb, &wb_min_ratio, &wb_max_ratio);
>
> wb_thresh += (thresh * wb_min_ratio) / (100 * BDI_RATIO_SCALE);
> - if (wb_thresh > (thresh * wb_max_ratio) / (100 * BDI_RATIO_SCALE))
> - wb_thresh = thresh * wb_max_ratio / (100 * BDI_RATIO_SCALE);
> +
> + /*
> + * It's very possible that wb_thresh is close to 0 not because the
> + * device is slow, but that it has remained inactive for long time.
> + * Honour such devices a reasonable good (hopefully IO efficient)
> + * threshold, so that the occasional writes won't be blocked and active
> + * writes can rampup the threshold quickly.
> + */
> + if (thresh > dtc->dirty) {
> + if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT))
> + wb_thresh = max(wb_thresh, (thresh - dtc->dirty) / 100);
> + else
> + wb_thresh = max(wb_thresh, (thresh - dtc->dirty) / 8);
> + }
> +
> + wb_max_thresh = thresh * wb_max_ratio / (100 * BDI_RATIO_SCALE);
> + if (wb_thresh > wb_max_thresh)
> + wb_thresh = wb_max_thresh;
>
> return wb_thresh;
> }
> @@ -944,6 +962,7 @@ unsigned long wb_calc_thresh(struct bdi_writeback *wb, unsigned long thresh)
> {
> struct dirty_throttle_control gdtc = { GDTC_INIT(wb) };
>
> + domain_dirty_avail(&gdtc, true);
> return __wb_calc_thresh(&gdtc, thresh);
> }
>
> @@ -1120,12 +1139,6 @@ static void wb_position_ratio(struct dirty_throttle_control *dtc)
> if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
> long long wb_pos_ratio;
>
> - if (dtc->wb_dirty < 8) {
> - dtc->pos_ratio = min_t(long long, pos_ratio * 2,
> - 2 << RATELIMIT_CALC_SHIFT);
> - return;
> - }
> -
> if (dtc->wb_dirty >= wb_thresh)
> return;
>
> @@ -1196,14 +1209,6 @@ static void wb_position_ratio(struct dirty_throttle_control *dtc)
> */
> if (unlikely(wb_thresh > dtc->thresh))
> wb_thresh = dtc->thresh;
> - /*
> - * It's very possible that wb_thresh is close to 0 not because the
> - * device is slow, but that it has remained inactive for long time.
> - * Honour such devices a reasonable good (hopefully IO efficient)
> - * threshold, so that the occasional writes won't be blocked and active
> - * writes can rampup the threshold quickly.
> - */
> - wb_thresh = max(wb_thresh, (limit - dtc->dirty) / 8);
> /*
> * scale global setpoint to wb's:
> * wb_setpoint = setpoint * wb_thresh / thresh
> @@ -1459,17 +1464,10 @@ static void wb_update_dirty_ratelimit(struct dirty_throttle_control *dtc,
> * balanced_dirty_ratelimit = task_ratelimit * write_bw / dirty_rate).
> * Hence, to calculate "step" properly, we have to use wb_dirty as
> * "dirty" and wb_setpoint as "setpoint".
> - *
> - * We rampup dirty_ratelimit forcibly if wb_dirty is low because
> - * it's possible that wb_thresh is close to zero due to inactivity
> - * of backing device.
> */
> if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
> dirty = dtc->wb_dirty;
> - if (dtc->wb_dirty < 8)
> - setpoint = dtc->wb_dirty + 1;
> - else
> - setpoint = (dtc->wb_thresh + dtc->wb_bg_thresh) / 2;
> + setpoint = (dtc->wb_thresh + dtc->wb_bg_thresh) / 2;
> }
>
> if (dirty < setpoint) {
> --
> 2.20.1
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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