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

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Message-ID: <20210510104453.GE11100@quack2.suse.cz>
Date:   Mon, 10 May 2021 12:44:53 +0200
From:   Jan Kara <jack@...e.cz>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Chi Wu <wuchi.zero@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, tj@...nel.org,
        Howard Cochran <hcochran@...nelspring.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Jens Axboe <axboe@...com>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] mm/page-writeback: Fix performance when BDI's share of
 ratio is 0.

On Sun 09-05-21 16:36:33, Andrew Morton wrote:
> On Thu, 29 Apr 2021 06:50:46 +0800 Chi Wu <wuchi.zero@...il.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)

Thanks for forwarding Andrew!

> > Signed-off-by: Chi Wu <wuchi.zero@...il.com>
> 
> Thanks.  I'll add it for some testing and hopefully someone will find
> the time to review this.

Thanks for the patch! It looks good to me, good catch! Feel free to add:

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

								Honza

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