Re: [PATCH 1/2] vmpressure: in-kernel notifications

[Greg Thelen <gthelen@google.com>]

On Tue, Apr 23 2013, Glauber Costa wrote:

> From: Glauber Costa <glommer@parallels.com>
>
> During the past weeks, it became clear to us that the shrinker interface
> we have right now works very well for some particular types of users,
> but not that well for others. The later are usually people interested in
> one-shot notifications, that were forced to adapt themselves to the
> count+scan behavior of shrinkers. To do so, they had no choice than to
> greatly abuse the shrinker interface producing little monsters all over.
>
> During LSF/MM, one of the proposals that popped out during our session
> was to reuse Anton Voronstsov's vmpressure for this. They are designed
> for userspace consumption, but also provide a well-stablished,
> cgroup-aware entry point for notifications.
>
> 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>
> Cc: Dave Chinner <david@fromorbit.com>
> Cc: Anton Vorontsov <anton.vorontsov@linaro.org>
> Cc: John Stultz <john.stultz@linaro.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Joonsoo Kim <js1304@gmail.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 |  6 ++++++
>  mm/vmpressure.c            | 48 ++++++++++++++++++++++++++++++++++++++++++----
>  2 files changed, 50 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/vmpressure.h b/include/linux/vmpressure.h
> index 76be077..1862012 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;
>  };
>  
> @@ -36,6 +39,9 @@ extern struct vmpressure *css_to_vmpressure(struct cgroup_subsys_state *css);
>  extern int vmpressure_register_event(struct cgroup *cg, struct cftype *cft,
>  				     struct eventfd_ctx *eventfd,
>  				     const char *args);
> +
> +extern int vmpressure_register_kernel_event(struct cgroup *cg,
> +					    void (*fn)(void));
>  extern void vmpressure_unregister_event(struct cgroup *cg, struct cftype *cft,
>  					struct eventfd_ctx *eventfd);
>  #else
> diff --git a/mm/vmpressure.c b/mm/vmpressure.c
> index 736a601..8d77ad0 100644
> --- a/mm/vmpressure.c
> +++ b/mm/vmpressure.c
> @@ -135,8 +135,12 @@ 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);
> +	};
>  	enum vmpressure_levels level;
> +	bool kernel_event;
>  	struct list_head node;
>  };
>  
> @@ -152,7 +156,9 @@ 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();
> +		else if (vmpr->notify_userspace && (level >= ev->level)) {
>  			eventfd_signal(ev->efd, 1);
>  			signalled = true;
>  		}
> @@ -227,7 +233,7 @@ 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
> @@ -238,14 +244,16 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg,
>  	 * through vmpressure_prio(). But so far, keep calm.
>  	 */
>  	if (!scanned)
> -		return;
> +		goto schedule;
>  
>  	mutex_lock(&vmpr->sr_lock);
>  	vmpr->scanned += scanned;
>  	vmpr->reclaimed += reclaimed;
> +	vmpr->notify_userspace = true;

Should notify_userspace get cleared sometime?  Seems like we might need
to clear or decrement notify_userspace in vmpressure_event() when
calling eventfd_signal().

>  	scanned = vmpr->scanned;
>  	mutex_unlock(&vmpr->sr_lock);
>  
> +schedule:
>  	if (scanned < vmpressure_win || work_pending(&vmpr->work))
>  		return;
>  	schedule_work(&vmpr->work);
> @@ -328,6 +336,38 @@ int vmpressure_register_event(struct cgroup *cg, struct cftype *cft,
>  }
>  
>  /**
> + * vmpressure_register_kernel_event() - Register kernel-side notification
> + * @cg:		cgroup 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 *cg, void (*fn)(void))
> +{
> +	struct vmpressure *vmpr = cg_to_vmpressure(cg);
> +	struct vmpressure_event *ev;
> +
> +	ev = kzalloc(sizeof(*ev), GFP_KERNEL);
> +	if (!ev)
> +		return -ENOMEM;
> +
> +	ev->kernel_event = true;
> +	ev->fn = fn;
> +
> +	mutex_lock(&vmpr->events_lock);
> +	list_add(&ev->node, &vmpr->events);
> +	mutex_unlock(&vmpr->events_lock);
> +	return 0;
> +}
> +
> +/**
>   * vmpressure_unregister_event() - Unbind eventfd from vmpressure
>   * @cg:		cgroup handle
>   * @cft:	cgroup control files handle

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