Re: [PATCH v14 16/18] vmpressure: in-kernel notifications

[Luiz Capitulino <lcapitulino@redhat.com>]

On Fri, 20 Dec 2013 18:31:02 +0400
Glauber Costa <glommer@gmail.com> wrote:

> > I have the exact problem described above for a project I'm working on
> > and this solution seems to solve it well.
> >
> > However, I had a few issues while trying to use this interface. I'll
> > comment on them below, but please take this more as advice seeking
> > than patch review.
> >
> >> This patch extends that to also support in-kernel users. Events that
> >> should be generated for in-kernel consumption will be marked as such,
> >> and for those, we will call a registered function instead of triggering
> >> an eventfd notification.
> >>
> >> Please note that due to my lack of understanding of each shrinker user,
> >> I will stay away from converting the actual users, you are all welcome
> >> to do so.
> >>
> >> Signed-off-by: Glauber Costa <glommer@openvz.org>
> >> Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
> >> Acked-by: Anton Vorontsov <anton@enomsg.org>
> >> Acked-by: Pekka Enberg <penberg@kernel.org>
> >> Reviewed-by: Greg Thelen <gthelen@google.com>
> >> Cc: Dave Chinner <dchinner@redhat.com>
> >> Cc: John Stultz <john.stultz@linaro.org>
> >> Cc: Andrew Morton <akpm@linux-foundation.org>
> >> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> >> Cc: Michal Hocko <mhocko@suse.cz>
> >> Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> >> Cc: Johannes Weiner <hannes@cmpxchg.org>
> >> ---
> >>  include/linux/vmpressure.h |    5 +++++
> >>  mm/vmpressure.c            |   53 +++++++++++++++++++++++++++++++++++++++++---
> >>  2 files changed, 55 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/include/linux/vmpressure.h b/include/linux/vmpressure.h
> >> index 3f3788d..9102e53 100644
> >> --- a/include/linux/vmpressure.h
> >> +++ b/include/linux/vmpressure.h
> >> @@ -19,6 +19,9 @@ struct vmpressure {
> >>       /* Have to grab the lock on events traversal or modifications. */
> >>       struct mutex events_lock;
> >>
> >> +     /* False if only kernel users want to be notified, true otherwise. */
> >> +     bool notify_userspace;
> >> +
> >>       struct work_struct work;
> >>  };
> >>
> >> @@ -38,6 +41,8 @@ extern int vmpressure_register_event(struct cgroup_subsys_state *css,
> >>                                    struct cftype *cft,
> >>                                    struct eventfd_ctx *eventfd,
> >>                                    const char *args);
> >> +extern int vmpressure_register_kernel_event(struct cgroup_subsys_state *css,
> >> +                                         void (*fn)(void));
> >>  extern void vmpressure_unregister_event(struct cgroup_subsys_state *css,
> >>                                       struct cftype *cft,
> >>                                       struct eventfd_ctx *eventfd);
> >> diff --git a/mm/vmpressure.c b/mm/vmpressure.c
> >> index e0f6283..730e7c1 100644
> >> --- a/mm/vmpressure.c
> >> +++ b/mm/vmpressure.c
> >> @@ -130,8 +130,12 @@ static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
> >>  }
> >>
> >>  struct vmpressure_event {
> >> -     struct eventfd_ctx *efd;
> >> +     union {
> >> +             struct eventfd_ctx *efd;
> >> +             void (*fn)(void);
> >
> > How does the callback access its private data?
> >
> >> +     };
> >>       enum vmpressure_levels level;
> >> +     bool kernel_event;
> >>       struct list_head node;
> >>  };
> >>
> >> @@ -147,12 +151,15 @@ static bool vmpressure_event(struct vmpressure *vmpr,
> >>       mutex_lock(&vmpr->events_lock);
> >>
> >>       list_for_each_entry(ev, &vmpr->events, node) {
> >> -             if (level >= ev->level) {
> >> +             if (ev->kernel_event) {
> >> +                     ev->fn();
> >
> > I think it would be interesting to pass 'level' to the callback (I'll
> > probably use it myself), but we could wait for a in-tree user before
> > adding it.
> >
> >> +             } else if (vmpr->notify_userspace && level >= ev->level) {
> >>                       eventfd_signal(ev->efd, 1);
> >>                       signalled = true;
> >>               }
> >>       }
> >>
> >> +     vmpr->notify_userspace = false;
> >>       mutex_unlock(&vmpr->events_lock);
> >>
> >>       return signalled;
> >> @@ -222,7 +229,7 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg,
> >>        * we account it too.
> >>        */
> >>       if (!(gfp & (__GFP_HIGHMEM | __GFP_MOVABLE | __GFP_IO | __GFP_FS)))
> >> -             return;
> >> +             goto schedule;
> >>
> >>       /*
> >>        * If we got here with no pages scanned, then that is an indicator
> >> @@ -239,8 +246,15 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg,
> >>       vmpr->scanned += scanned;
> >>       vmpr->reclaimed += reclaimed;
> >>       scanned = vmpr->scanned;
> >> +     /*
> >> +      * If we didn't reach this point, only kernel events will be triggered.
> >> +      * It is the job of the worker thread to clean this up once the
> >> +      * notifications are all delivered.
> >> +      */
> >> +     vmpr->notify_userspace = true;
> >>       spin_unlock(&vmpr->sr_lock);
> >>
> >> +schedule:
> >>       if (scanned < vmpressure_win)
> >>               return;
> >>       schedule_work(&vmpr->work);
> >> @@ -324,6 +338,39 @@ int vmpressure_register_event(struct cgroup_subsys_state *css,
> >>  }
> >>
> >>  /**
> >> + * vmpressure_register_kernel_event() - Register kernel-side notification
> >> + * @css:     css that is interested in vmpressure notifications
> >> + * @fn:              function to be called when pressure happens
> >> + *
> >> + * This function register in-kernel users interested in receiving notifications
> >> + * about pressure conditions. Pressure notifications will be triggered at the
> >> + * same time as userspace notifications (with no particular ordering relative
> >> + * to it).
> >> + *
> >> + * Pressure notifications are a alternative method to shrinkers and will serve
> >> + * well users that are interested in a one-shot notification, with a
> >> + * well-defined cgroup aware interface.
> >> + */
> >> +int vmpressure_register_kernel_event(struct cgroup_subsys_state *css,
> >> +                                   void (*fn)(void))
> >> +{
> >> +     struct vmpressure *vmpr = css_to_vmpressure(css);
> >
> > This doesn't allow for css=NULL. What's the recommended way for a today's
> > shrinker (which is not related to cgroups) to register with this API?
> >
> > Also, you don't seem to provide a way to de-register from the event.
> >
> 
> The answer for all of your questions above can be summarized by noting
> that for the lack of other users (at the time), this patch does the bare minimum
> for memcg needs. I agree, for instance, that it would be good to pass the level
> but since memcg won't do anything with thta, I didn't pass it.
> 
> That should be extended if you need to.

That works for me. That is, including this minimal version first and
extending it when we get in-tree users.

> 
> > I hacked a patch to be able to use this, seems to work but it's a ugly
> > hack:
> >
> > ---
> >  include/linux/vmpressure.h |  3 ++-
> >  mm/vmpressure.c            | 13 +++++++++----
> >  2 files changed, 11 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/linux/vmpressure.h b/include/linux/vmpressure.h
> > index 9102e53..de416b6 100644
> > --- a/include/linux/vmpressure.h
> > +++ b/include/linux/vmpressure.h
> > @@ -42,7 +42,8 @@ extern int vmpressure_register_event(struct cgroup_subsys_state *css,
> >                                      struct eventfd_ctx *eventfd,
> >                                      const char *args);
> >  extern int vmpressure_register_kernel_event(struct cgroup_subsys_state *css,
> > -                                           void (*fn)(void));
> > +                                           void (*fn)(void *data, int level),
> > +                                           void *data);
> >  extern void vmpressure_unregister_event(struct cgroup_subsys_state *css,
> >                                         struct cftype *cft,
> >                                         struct eventfd_ctx *eventfd);
> > diff --git a/mm/vmpressure.c b/mm/vmpressure.c
> > index 730e7c1..4ed0e85 100644
> > --- a/mm/vmpressure.c
> > +++ b/mm/vmpressure.c
> > @@ -132,9 +132,10 @@ static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
> >  struct vmpressure_event {
> >         union {
> >                 struct eventfd_ctx *efd;
> > -               void (*fn)(void);
> > +               void (*fn)(void *data, int level);
> >         };
> >         enum vmpressure_levels level;
> > +       void *data;
> >         bool kernel_event;
> >         struct list_head node;
> >  };
> > @@ -152,7 +153,7 @@ static bool vmpressure_event(struct vmpressure *vmpr,
> >
> >         list_for_each_entry(ev, &vmpr->events, node) {
> >                 if (ev->kernel_event) {
> > -                       ev->fn();
> > +                       ev->fn(ev->data, level);
> >                 } else if (vmpr->notify_userspace && level >= ev->level) {
> >                         eventfd_signal(ev->efd, 1);
> >                         signalled = true;
> > @@ -352,21 +353,25 @@ int vmpressure_register_event(struct cgroup_subsys_state *css,
> >   * well-defined cgroup aware interface.
> >   */
> >  int vmpressure_register_kernel_event(struct cgroup_subsys_state *css,
> > -                                     void (*fn)(void))
> > +                                    void (*fn)(void *data, int level), void *data)
> >  {
> > -       struct vmpressure *vmpr = css_to_vmpressure(css);
> > +       struct vmpressure *vmpr;
> >         struct vmpressure_event *ev;
> >
> > +       vmpr = css ? css_to_vmpressure(css) : memcg_to_vmpressure(NULL);
> > +
> >         ev = kzalloc(sizeof(*ev), GFP_KERNEL);
> >         if (!ev)
> >                 return -ENOMEM;
> >
> >         ev->kernel_event = true;
> > +       ev->data = data;
> >         ev->fn = fn;
> >
> >         mutex_lock(&vmpr->events_lock);
> >         list_add(&ev->node, &vmpr->events);
> >         mutex_unlock(&vmpr->events_lock);
> > +
> 
> Your patch makes sense.
> 
> 
> 

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