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

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

[Chi Wu]

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.

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

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

[chi wu]

Ping...

Chi Wu <wuchi.zero@gmail.com> 于2021年4月29日周四 上午6:51写道：
>
> 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.
>
> Signed-off-by: Chi Wu <wuchi.zero@gmail.com>
> ---
>  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
>

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

[Andrew Morton]

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

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

[chi wu]

Andrew Morton <akpm@linux-foundation.org> 于2021年5月10日周一 上午7:36写道：
>
> 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)

Thanks, I'll correct it in the future.

>
> > 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

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

[Jan Kara]

On Sun 09-05-21 16:36:33, Andrew Morton wrote:
> 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)

Thanks for forwarding Andrew!

> > 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.

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

Reviewed-by: Jan Kara <jack@suse.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@suse.com>
SUSE Labs, CR