Re: [PATCH v3 2/6] mm: shrinkers: introduce debugfs interface for memory shrinkers

[Roman Gushchin <roman.gushchin@linux.dev>]

On Fri, May 20, 2022 at 06:58:12PM +0200, Christophe JAILLET wrote:
> Le 09/05/2022 à 20:38, Roman Gushchin a écrit :
> > This commit introduces the /sys/kernel/debug/shrinker debugfs
> > interface which provides an ability to observe the state of
> > individual kernel memory shrinkers.
> > 
> > Because the feature adds some memory overhead (which shouldn't be
> > large unless there is a huge amount of registered shrinkers), it's
> > guarded by a config option (enabled by default).
> > 
> > This commit introduces the "count" interface for each shrinker
> > registered in the system.
> > 
> > The output is in the following format:
> > <cgroup inode id> <nr of objects on node 0> <nr of objects on node 1>...
> > <cgroup inode id> <nr of objects on node 0> <nr of objects on node 1>...
> > ...
> > 
> > To reduce the size of output on machines with many thousands cgroups,
> > if the total number of objects on all nodes is 0, the line is omitted.
> > 
> > If the shrinker is not memcg-aware or CONFIG_MEMCG is off, 0 is
> > printed as cgroup inode id. If the shrinker is not numa-aware, 0's are
> > printed for all nodes except the first one.
> > 
> > This commit gives debugfs entries simple numeric names, which are not
> > very convenient. The following commit in the series will provide
> > shrinkers with more meaningful names.
> > 
> > Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> > ---
> >   include/linux/shrinker.h |  19 ++++-
> >   lib/Kconfig.debug        |   9 +++
> >   mm/Makefile              |   1 +
> >   mm/shrinker_debug.c      | 171 +++++++++++++++++++++++++++++++++++++++
> >   mm/vmscan.c              |   6 +-
> >   5 files changed, 203 insertions(+), 3 deletions(-)
> >   create mode 100644 mm/shrinker_debug.c
> > 
> > diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> > index 76fbf92b04d9..2ced8149c513 100644
> > --- a/include/linux/shrinker.h
> > +++ b/include/linux/shrinker.h
> > @@ -72,6 +72,10 @@ struct shrinker {
> >   #ifdef CONFIG_MEMCG
> >   	/* ID in shrinker_idr */
> >   	int id;
> > +#endif
> > +#ifdef CONFIG_SHRINKER_DEBUG
> > +	int debugfs_id;
> > +	struct dentry *debugfs_entry;
> >   #endif
> >   	/* objs pending delete, per node */
> >   	atomic_long_t *nr_deferred;
> > @@ -94,4 +98,17 @@ extern int register_shrinker(struct shrinker *shrinker);
> >   extern void unregister_shrinker(struct shrinker *shrinker);
> >   extern void free_prealloced_shrinker(struct shrinker *shrinker);
> >   extern void synchronize_shrinkers(void);
> > -#endif
> > +
> > +#ifdef CONFIG_SHRINKER_DEBUG
> > +extern int shrinker_debugfs_add(struct shrinker *shrinker);
> > +extern void shrinker_debugfs_remove(struct shrinker *shrinker);
> > +#else /* CONFIG_SHRINKER_DEBUG */
> > +static inline int shrinker_debugfs_add(struct shrinker *shrinker)
> > +{
> > +	return 0;
> > +}
> > +static inline void shrinker_debugfs_remove(struct shrinker *shrinker)
> > +{
> > +}
> > +#endif /* CONFIG_SHRINKER_DEBUG */
> > +#endif /* _LINUX_SHRINKER_H */
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index 3fd7a2e9eaf1..5fa65a649798 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -733,6 +733,15 @@ config SLUB_STATS
> >   	  out which slabs are relevant to a particular load.
> >   	  Try running: slabinfo -DA
> > +config SHRINKER_DEBUG
> > +	default y
> 
> The previous version of the serie had default 'n'.
> Is it intentional to have it now activated by default? It looked more like a
> tuning functionality when fine grained mangement of shrinker is needed.

Yes, it is intentional.
The overhead is small, so I don't think we have a good reason to hide it
by default behind a config option. In my opinion, enabling it be default
will increase the chances to gather a useful data.
It was the feedback I've received for one of the previous versions of
the patchset, and I think it's totally valid.
And preserving the config option allows to have a zero overhead for
really constrained systems.

Thanks!