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Date: Fri, 3 May 2024 11:11:13 +0200
From: Jan Kara <jack@suse.cz>
To: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: willy@infradead.org, akpm@linux-foundation.org, tj@kernel.org,
	jack@suse.cz, hcochran@kernelspring.com, axboe@kernel.dk,
	mszeredi@redhat.com, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/4] mm: enable __wb_calc_thresh to calculate dirty
 background threshold
Message-ID: <20240503091113.bkrw3ihx7gggtlxo@quack3>
References: <20240425131724.36778-1-shikemeng@huaweicloud.com>
 <20240425131724.36778-2-shikemeng@huaweicloud.com>
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In-Reply-To: <20240425131724.36778-2-shikemeng@huaweicloud.com>

On Thu 25-04-24 21:17:21, Kemeng Shi wrote:
> Originally, __wb_calc_thresh always calculate wb's share of dirty
> throttling threshold. By getting thresh of wb_domain from caller,
> __wb_calc_thresh could be used for both dirty throttling and dirty
> background threshold.
> 
> This is a preparation to correct threshold calculation of wb in cgroup.
> 
> Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  mm/page-writeback.c | 33 ++++++++++++++++++---------------
>  1 file changed, 18 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index db5769cb12fd..2a3b68aae336 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -838,13 +838,15 @@ static void mdtc_calc_avail(struct dirty_throttle_control *mdtc,
>  }
>  
>  /**
> - * __wb_calc_thresh - @wb's share of dirty throttling threshold
> + * __wb_calc_thresh - @wb's share of dirty threshold
>   * @dtc: dirty_throttle_context of interest
> + * @thresh: dirty throttling or dirty background threshold of wb_domain in @dtc
>   *
> - * Note that balance_dirty_pages() will only seriously take it as a hard limit
> - * when sleeping max_pause per page is not enough to keep the dirty pages under
> - * control. For example, when the device is completely stalled due to some error
> - * conditions, or when there are 1000 dd tasks writing to a slow 10MB/s USB key.
> + * Note that balance_dirty_pages() will only seriously take dirty throttling
> + * threshold as a hard limit when sleeping max_pause per page is not enough
> + * to keep the dirty pages under control. For example, when the device is
> + * completely stalled due to some error conditions, or when there are 1000
> + * dd tasks writing to a slow 10MB/s USB key.
>   * In the other normal situations, it acts more gently by throttling the tasks
>   * more (rather than completely block them) when the wb dirty pages go high.
>   *
> @@ -855,19 +857,20 @@ static void mdtc_calc_avail(struct dirty_throttle_control *mdtc,
>   * The wb's share of dirty limit will be adapting to its throughput and
>   * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
>   *
> - * Return: @wb's dirty limit in pages. The term "dirty" in the context of
> - * dirty balancing includes all PG_dirty and PG_writeback pages.
> + * Return: @wb's dirty limit in pages. For dirty throttling limit, the term
> + * "dirty" in the context of dirty balancing includes all PG_dirty and
> + * PG_writeback pages.
>   */
> -static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
> +static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc,
> +				      unsigned long thresh)
>  {
>  	struct wb_domain *dom = dtc_dom(dtc);
> -	unsigned long thresh = dtc->thresh;
>  	u64 wb_thresh;
>  	unsigned long numerator, denominator;
>  	unsigned long wb_min_ratio, wb_max_ratio;
>  
>  	/*
> -	 * Calculate this BDI's share of the thresh ratio.
> +	 * Calculate this wb's share of the thresh ratio.
>  	 */
>  	fprop_fraction_percpu(&dom->completions, dtc->wb_completions,
>  			      &numerator, &denominator);
> @@ -887,9 +890,9 @@ static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
>  
>  unsigned long wb_calc_thresh(struct bdi_writeback *wb, unsigned long thresh)
>  {
> -	struct dirty_throttle_control gdtc = { GDTC_INIT(wb),
> -					       .thresh = thresh };
> -	return __wb_calc_thresh(&gdtc);
> +	struct dirty_throttle_control gdtc = { GDTC_INIT(wb) };
> +
> +	return __wb_calc_thresh(&gdtc, thresh);
>  }
>  
>  unsigned long cgwb_calc_thresh(struct bdi_writeback *wb)
> @@ -908,7 +911,7 @@ unsigned long cgwb_calc_thresh(struct bdi_writeback *wb)
>  	mdtc_calc_avail(&mdtc, filepages, headroom);
>  	domain_dirty_limits(&mdtc);
>  
> -	return __wb_calc_thresh(&mdtc);
> +	return __wb_calc_thresh(&mdtc, mdtc.thresh);
>  }
>  
>  /*
> @@ -1655,7 +1658,7 @@ static inline void wb_dirty_limits(struct dirty_throttle_control *dtc)
>  	 *   wb_position_ratio() will let the dirtier task progress
>  	 *   at some rate <= (write_bw / 2) for bringing down wb_dirty.
>  	 */
> -	dtc->wb_thresh = __wb_calc_thresh(dtc);
> +	dtc->wb_thresh = __wb_calc_thresh(dtc, dtc->thresh);
>  	dtc->wb_bg_thresh = dtc->thresh ?
>  		div64_u64(dtc->wb_thresh * dtc->bg_thresh, dtc->thresh) : 0;
>  
> -- 
> 2.30.0
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR