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

[Jim Zhao <jimzhao.ai@gmail.com>]

> On Wed 23-10-24 18:00:32, 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@gmail.com>
> 
> ...
> 
> > +	/*
> > +	 * With strictlimit flag, the wb_thresh is treated as
> > +	 * a hard limit in balance_dirty_pages() and wb_position_ratio().
> > +	 * It's possible that wb_thresh is close to zero, not because
> > +	 * the device is slow, but because it has been inactive.
> > +	 * To prevent occasional writes from being blocked, we raise wb_thresh.
> > +	 */
> > +	if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
> > +		unsigned long limit = hard_dirty_limit(dom, dtc->thresh);
> > +		u64 wb_scale_thresh = 0;
> > +
> > +		if (limit > dtc->dirty)
> > +			wb_scale_thresh = (limit - dtc->dirty) / 100;
> > +		wb_thresh = max(wb_thresh, min(wb_scale_thresh, wb_max_thresh / 4));
> > +	}
> 
> What you propose makes sense in principle although I'd say this is mostly a
> userspace setup issue - with strictlimit enabled, you're kind of expected
> to set min_ratio exactly if you want to avoid these startup issues. But I
> tend to agree that we can provide a bit of a slack for a bdi without
> min_ratio configured to ramp up.
> 
> But I'd rather pick the logic like:
> 
> 	/*
> 	 * If bdi does not have min_ratio configured and it was inactive,
> 	 * bump its min_ratio to 0.1% to provide it some room to ramp up.
> 	 */
> 	if (!wb_min_ratio && !numerator)
> 		wb_min_ratio = min(BDI_RATIO_SCALE / 10, wb_max_ratio / 2);
> 
> That would seem like a bit more systematic way than the formula you propose
> above...

Thanks for the advice.
Here's the explanation of the formula:
1. when writes are small and intermittent，wb_thresh can approach 0, not just 0, making the numerator value difficult to verify.
2. The ramp-up margin, whether 0.1% or another value, needs consideration.
I based this on the logic of wb_position_ratio in the non-strictlimit scenario:
wb_thresh = max(wb_thresh, (limit - dtc->dirty) / 8);
It seems provides more room and ensures ramping up within a controllable range.

---
Jim Zhao
Thanks