linux-kernel - Re: [PATCH 5/5] blkcg, writeback: dead memcgs shouldn't contribute to writeback ownership arbitration

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Date:   Wed, 19 Jun 2019 13:20:03 +0200
From:   Jan Kara <jack@...e.cz>
To:     Tejun Heo <tj@...nel.org>
Cc:     axboe@...nel.dk, jbacik@...com, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org, kernel-team@...com, dennis@...nel.org,
        jack@...e.cz
Subject: Re: [PATCH 5/5] blkcg, writeback: dead memcgs shouldn't contribute
 to writeback ownership arbitration

On Thu 13-06-19 15:30:41, Tejun Heo wrote:
> wbc_account_io() collects information on cgroup ownership of writeback
> pages to determine which cgroup should own the inode.  Pages can stay
> associated with dead memcgs but we want to avoid attributing IOs to
> dead blkcgs as much as possible as the association is likely to be
> stale.  However, currently, pages associated with dead memcgs
> contribute to the accounting delaying and/or confusing the
> arbitration.
> 
> Fix it by ignoring pages associated with dead memcgs.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Cc: Jan Kara <jack@...e.cz>

I see Jens has already pulled the changes so this is mostly informative but
the patch looks good to me.

								Honza
> ---
>  fs/fs-writeback.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index e41cbe8e81b9..9ebfb1b28430 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -715,6 +715,7 @@ void wbc_detach_inode(struct writeback_control *wbc)
>  void wbc_account_io(struct writeback_control *wbc, struct page *page,
>  		    size_t bytes)
>  {
> +	struct cgroup_subsys_state *css;
>  	int id;
>  
>  	/*
> @@ -726,7 +727,12 @@ void wbc_account_io(struct writeback_control *wbc, struct page *page,
>  	if (!wbc->wb)
>  		return;
>  
> -	id = mem_cgroup_css_from_page(page)->id;
> +	css = mem_cgroup_css_from_page(page);
> +	/* dead cgroups shouldn't contribute to inode ownership arbitration */
> +	if (!(css->flags & CSS_ONLINE))
> +		return;
> +
> +	id = css->id;
>  
>  	if (id == wbc->wb_id) {
>  		wbc->wb_bytes += bytes;
> -- 
> 2.17.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR