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Date:   Wed, 15 Sep 2021 13:56:38 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     NeilBrown <neilb@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.com>, linux-xfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] MM: annotate congestion_wait() and
 wait_iff_congested() as ineffective.

On Tue 14-09-21 10:13:04, Neil Brown wrote:
> Only 4 subsystems call set_bdi_congested() or clear_bdi_congested():
>  block/pktcdvd, fs/ceph fs/fuse fs/nfs
> 
> It may make sense to use congestion_wait() or wait_iff_congested()
> within these subsystems, but they have no value outside of these.
> 
> Add documentation comments to these functions to discourage further use.

This is an unfortunate state. The MM layer still relies on the API.
While adding a documentation to clarify the current status can stop more
usage I am wondering what is a real alternative. My experience tells me
that a lack of real alternative will lead to new creative ways of doing
things instead.
 
> Signed-off-by: NeilBrown <neilb@...e.de>
> ---
>  include/linux/backing-dev.h |    7 +++++++
>  mm/backing-dev.c            |    9 +++++++++
>  2 files changed, 16 insertions(+)
> 
> diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> index ac7f231b8825..cc9513840351 100644
> --- a/include/linux/backing-dev.h
> +++ b/include/linux/backing-dev.h
> @@ -153,6 +153,13 @@ static inline int wb_congested(struct bdi_writeback *wb, int cong_bits)
>  	return wb->congested & cong_bits;
>  }
>  
> +/* NOTE congestion_wait() and wait_iff_congested() are
> + * largely useless except as documentation.
> + * congestion_wait() will (almost) always wait for the given timeout.
> + * wait_iff_congested() will (almost) never wait, but will call
> + * cond_resched().
> + * Were possible an alternative waiting strategy should be found.
> + */
>  long congestion_wait(int sync, long timeout);
>  long wait_iff_congested(int sync, long timeout);
>  
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 4a9d4e27d0d9..53472ab38796 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -1023,6 +1023,11 @@ EXPORT_SYMBOL(set_bdi_congested);
>   * Waits for up to @timeout jiffies for a backing_dev (any backing_dev) to exit
>   * write congestion.  If no backing_devs are congested then just wait for the
>   * next write to be completed.
> + *
> + * NOTE: in the current implementation, hardly any backing_devs are ever
> + * marked as congested, and write-completion is rarely reported (see calls
> + * to clear_bdi_congested).  So this should not be assumed to ever wake before
> + * the timeout.
>   */
>  long congestion_wait(int sync, long timeout)
>  {
> @@ -1054,6 +1059,10 @@ EXPORT_SYMBOL(congestion_wait);
>   * The return value is 0 if the sleep is for the full timeout. Otherwise,
>   * it is the number of jiffies that were still remaining when the function
>   * returned. return_value == timeout implies the function did not sleep.
> + *
> + * NOTE: in the current implementation, hardly any backing_devs are ever
> + * marked as congested, and write-completion is rarely reported (see calls
> + * to clear_bdi_congested).  So this should not be assumed to sleep at all.
>   */
>  long wait_iff_congested(int sync, long timeout)
>  {
> 

-- 
Michal Hocko
SUSE Labs

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