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Message-ID: <20241120115731.gzxozbnb6eazhil7@quack3>
Date: Wed, 20 Nov 2024 12:57:31 +0100
From: Jan Kara <jack@...e.cz>
To: Jim Zhao <jimzhao.ai@...il.com>
Cc: jack@...e.cz, shikemeng@...weicloud.com, 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
Hello!
On Tue 19-11-24 20:29:22, Jim Zhao wrote:
> Thanks, Jan, I just sent patch v2, could you please review it ?
Yes, the patch looks good to me.
>
> And I found the debug info in the bdi stats.
> The BdiDirtyThresh value may be greater than DirtyThresh, and after
> applying this patch, the value of BdiDirtyThresh could become even
> larger.
>
> without patch:
> ---
> root@...ntu:/sys/kernel/debug/bdi/8:0# cat stats
> BdiWriteback: 0 kB
> BdiReclaimable: 96 kB
> BdiDirtyThresh: 1346824 kB
But this is odd. The machine appears to have around 3GB of memory, doesn't
it? I suspect this is caused by multiple cgroup-writeback contexts
contributing to BdiDirtyThresh - in fact I think the math in
bdi_collect_stats() is wrong as it is adding wb_thresh() calculated based
on global dirty_thresh for each cgwb whereas it should be adding
wb_thresh() calculated based on per-memcg dirty_thresh... You can have a
look at /sys/kernel/debug/bdi/8:0/wb_stats file which should have correct
limits as far as I'm reading the code.
Honza
> DirtyThresh: 673412 kB
> BackgroundThresh: 336292 kB
> BdiDirtied: 19872 kB
> BdiWritten: 19776 kB
> BdiWriteBandwidth: 0 kBps
> b_dirty: 0
> b_io: 0
> b_more_io: 0
> b_dirty_time: 0
> bdi_list: 1
> state: 1
>
> with patch:
> ---
> root@...ntu:/sys/kernel/debug/bdi/8:0# cat stats
> BdiWriteback: 96 kB
> BdiReclaimable: 192 kB
> BdiDirtyThresh: 3090736 kB
> DirtyThresh: 650716 kB
> BackgroundThresh: 324960 kB
> BdiDirtied: 472512 kB
> BdiWritten: 470592 kB
> BdiWriteBandwidth: 106268 kBps
> b_dirty: 2
> b_io: 0
> b_more_io: 0
> b_dirty_time: 0
> bdi_list: 1
> state: 1
>
>
> @kemeng, is this a normal behavior or an issue ?
>
> Thanks,
> Jim Zhao
>
>
> > 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.
> > ---
> > 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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