diff mbox series

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

Message ID 163157838437.13293.15392955714346973750.stgit@noble.brown (mailing list archive)
State New
Headers show
Series congestion_wait() and GFP_NOFAIL | expand

Commit Message

NeilBrown Sept. 14, 2021, 12:13 a.m. UTC
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.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 include/linux/backing-dev.h |    7 +++++++
 mm/backing-dev.c            |    9 +++++++++
 2 files changed, 16 insertions(+)

Comments

Michal Hocko Sept. 15, 2021, 11:56 a.m. UTC | #1
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)
>  {
>
NeilBrown Sept. 16, 2021, 10:13 p.m. UTC | #2
On Wed, 15 Sep 2021, Michal Hocko wrote:
> 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.

That is a valid concern.  Discouraging the use of an interface without
providing a clear alternative risks people doing worse things.

At lease if people continue to use congestion_wait(), then we will be
able to find those uses when we are able to provide a better approach.

I'll drop this patch.

Thanks,
NeilBrown


>  
> > 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
> 
>
diff mbox series

Patch

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)
 {