Re: [PATCH] mm/page-writeback: Raise wb_thresh to prevent write blocking with strictlimit

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 24 Oct 2024 14:09:54 +0800 Jim Zhao <jimzhao.ai@gmail.com> wrote:

> > On Wed, 23 Oct 2024 18:00:32 +0800 Jim Zhao <jimzhao.ai@gmail.com> 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.
> 
> > Please tell us more about the userspace-visible effects of this.  It
> > *sounds* like a serious (but occasional) problem, but that is unclear.
> 
> > And, very much relatedly, do you feel this fix is needed in earlier
> > (-stable) kernels?
> 
> The problem exists in two scenarios:
> 1. FUSE Write Transition from Inactive to Active
> 
> sometimes, active writes require several pauses to ramp up to the appropriate wb_thresh.
> As shown in the trace below, both bdi_setpoint and task_ratelimit are 0, means wb_thresh is 0. 
> The dd process pauses multiple times before reaching a normal state.
> 
> dd-1206590 [003] .... 62988.324049: balance_dirty_pages: bdi 0:51: limit=295073 setpoint=259360 dirty=454 bdi_setpoint=0 bdi_dirty=32 dirty_ratelimit=18716 task_ratelimit=0 dirtied=32 dirtied_pause=32 paused=0 pause=4 period=4 think=0 cgroup_ino=1
> dd-1206590 [003] .... 62988.332063: balance_dirty_pages: bdi 0:51: limit=295073 setpoint=259453 dirty=454 bdi_setpoint=0 bdi_dirty=33 dirty_ratelimit=18716 task_ratelimit=0 dirtied=1 dirtied_pause=0 paused=0 pause=4 period=4 think=4 cgroup_ino=1
> dd-1206590 [003] .... 62988.340064: balance_dirty_pages: bdi 0:51: limit=295073 setpoint=259526 dirty=454 bdi_setpoint=0 bdi_dirty=34 dirty_ratelimit=18716 task_ratelimit=0 dirtied=1 dirtied_pause=0 paused=0 pause=4 period=4 think=4 cgroup_ino=1
> dd-1206590 [003] .... 62988.348061: balance_dirty_pages: bdi 0:51: limit=295073 setpoint=259531 dirty=489 bdi_setpoint=0 bdi_dirty=35 dirty_ratelimit=18716 task_ratelimit=0 dirtied=1 dirtied_pause=0 paused=0 pause=4 period=4 think=4 cgroup_ino=1
> dd-1206590 [003] .... 62988.356063: balance_dirty_pages: bdi 0:51: limit=295073 setpoint=259531 dirty=490 bdi_setpoint=0 bdi_dirty=36 dirty_ratelimit=18716 task_ratelimit=0 dirtied=1 dirtied_pause=0 paused=0 pause=4 period=4 think=4 cgroup_ino=1
> ...
> 
> 2. FUSE with Unstable Network Backends and Occasional Writes
> Not easy to reproduce, but when it occurs in this scenario, 
> it causes the write thread to experience more pauses and longer durations.

Thanks, but it's still unclear how this impacts our users.  How lenghty
are these pauses?