On Mon, Apr 03, 2023 at 10:03:33PM +0000, Yosry Ahmed <yosryahmed@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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@xxxxxxxx>
Attachment:
signature.asc
Description: PGP signature
