Re: [PATCH] mm/page-writeback: Fix performance when BDI's share of ratio is 0.

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

On Thu, 29 Apr 2021 06:50:46 +0800 Chi Wu <wuchi.zero@gmail.com> wrote:

> Fix performance when BDI's share of ratio is 0.
> 
> The issue is similar to commit 74d369443325 ("writeback: Fix
> performance regression in wb_over_bg_thresh()").
> 
> Balance_dirty_pages and the writeback worker will also disagree on
> whether writeback when a BDI uses BDI_CAP_STRICTLIMIT and BDI's share
> of the thresh ratio is zero.
> 
> For example, A thread on cpu0 writes 32 pages and then
> balance_dirty_pages, it will wake up background writeback and pauses
> because wb_dirty > wb->wb_thresh = 0 (share of thresh ratio is zero).
> A thread may runs on cpu0 again because scheduler prefers pre_cpu.
> Then writeback worker may runs on other cpus(1,2..) which causes the
> value of wb_stat(wb, WB_RECLAIMABLE) in wb_over_bg_thresh is 0 and does
> not writeback and returns.
> 
> Thus, balance_dirty_pages keeps looping, sleeping and then waking up the
> worker who will do nothing. It remains stuck in this state until the
> writeback worker hit the right dirty cpu or the dirty pages expire.
> 
> The fix that we should get the wb_stat_sum radically when thresh is low.

(optimistically Cc's various people who might remember how this code works)

> Signed-off-by: Chi Wu <wuchi.zero@gmail.com>

Thanks.  I'll add it for some testing and hopefully someone will find
the time to review this.

> ---
>  mm/page-writeback.c | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 0062d5c57d41..bd7052295246 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1945,6 +1945,8 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
>  	struct dirty_throttle_control * const gdtc = &gdtc_stor;
>  	struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
>  						     &mdtc_stor : NULL;
> +	unsigned long reclaimable;
> +	unsigned long thresh;
>  
>  	/*
>  	 * Similar to balance_dirty_pages() but ignores pages being written
> @@ -1957,8 +1959,13 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
>  	if (gdtc->dirty > gdtc->bg_thresh)
>  		return true;
>  
> -	if (wb_stat(wb, WB_RECLAIMABLE) >
> -	    wb_calc_thresh(gdtc->wb, gdtc->bg_thresh))
> +	thresh = wb_calc_thresh(gdtc->wb, gdtc->bg_thresh);
> +	if (thresh < 2 * wb_stat_error())
> +		reclaimable = wb_stat_sum(wb, WB_RECLAIMABLE);
> +	else
> +		reclaimable = wb_stat(wb, WB_RECLAIMABLE);
> +
> +	if (reclaimable > thresh)
>  		return true;
>  
>  	if (mdtc) {
> @@ -1972,8 +1979,13 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
>  		if (mdtc->dirty > mdtc->bg_thresh)
>  			return true;
>  
> -		if (wb_stat(wb, WB_RECLAIMABLE) >
> -		    wb_calc_thresh(mdtc->wb, mdtc->bg_thresh))
> +		thresh = wb_calc_thresh(mdtc->wb, mdtc->bg_thresh);
> +		if (thresh < 2 * wb_stat_error())
> +			reclaimable = wb_stat_sum(wb, WB_RECLAIMABLE);
> +		else
> +			reclaimable = wb_stat(wb, WB_RECLAIMABLE);
> +
> +		if (reclaimable > thresh)
>  			return true;
>  	}
>  
> -- 
> 2.17.1