Re: [PATCH 2/6] MM: annotate congestion_wait() and wait_iff_congested() as ineffective.

Michal Hocko
2021-09-15T11:56:41Z

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@suse.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