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

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Message-ID: <20241107153217.j6kwfgihzhj33dia@quack3>
Date: Thu, 7 Nov 2024 16:32:17 +0100
From: Jan Kara <jack@...e.cz>
To: Jim Zhao <jimzhao.ai@...il.com>
Cc: willy@...radead.org, akpm@...ux-foundation.org,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page-writeback: Raise wb_thresh to prevent write
 blocking with strictlimit

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@...il.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...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR