Re: [PATCH mm-unstable RFC 1/5] writeback: move wb_over_bg_thresh() call outside lock section

["Michal Koutný" <mkoutny@suse.com>]

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On Mon, Apr 03, 2023 at 10:03:33PM +0000, Yosry Ahmed <yosryahmed@google.com> wrote:
> wb_over_bg_thresh() calls mem_cgroup_wb_stats() which invokes an rstat
> flush, which can be expensive on large systems. Currently,
> wb_writeback() calls wb_over_bg_thresh() within a lock section, so we
> have to make the rstat flush atomically. On systems with a lot of
> cpus/cgroups, this can cause us to disable irqs for a long time,
> potentially causing problems.
> 
> Move the call to wb_over_bg_thresh() outside the lock section in
> preparation to make the rstat flush in mem_cgroup_wb_stats() non-atomic.
> The list_empty(&wb->work_list) should be okay outside the lock section
> of wb->list_lock as it is protected by a separate lock (wb->work_lock),
> and wb_over_bg_thresh() doesn't seem like it is modifying any of the b_*
> lists the wb->list_lock is protecting. Also, the loop seems to be
> already releasing and reacquring the lock, so this refactoring looks
> safe.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> ---
>  fs/fs-writeback.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 195dc23e0d831..012357bc8daa3 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -2021,7 +2021,6 @@ static long wb_writeback(struct bdi_writeback *wb,
>  	struct blk_plug plug;
>  
>  	blk_start_plug(&plug);
> -	spin_lock(&wb->list_lock);
>  	for (;;) {
>  		/*
>  		 * Stop writeback when nr_pages has been consumed
> @@ -2046,6 +2045,9 @@ static long wb_writeback(struct bdi_writeback *wb,
>  		if (work->for_background && !wb_over_bg_thresh(wb))
>  			break;
>  
> +
> +		spin_lock(&wb->list_lock);
> +
>  		/*
>  		 * Kupdate and background works are special and we want to
>  		 * include all inodes that need writing. Livelock avoidance is
> @@ -2075,13 +2077,19 @@ static long wb_writeback(struct bdi_writeback *wb,
>  		 * mean the overall work is done. So we keep looping as long
>  		 * as made some progress on cleaning pages or inodes.
>  		 */
> -		if (progress)
> +		if (progress) {
> +			spin_unlock(&wb->list_lock);
>  			continue;
> +		}
> +

This would release wb->list_lock temporarily with progress but that's
already not held continuously due to writeback_sb_inodes().
Holding the lock could even be shortened by taking it later after
trace_writeback_start().

Altogether, the change looks OK,
Reviewed-by: Michal Koutný <mkoutny@suse.com>


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