Re: [PATCH] vmpressure: implement strict mode

[Luiz Capitulino <lcapitulino@redhat.com>]

On Wed, 26 Jun 2013 10:08:27 +0200
Michal Hocko <mhocko@suse.cz> wrote:

> On Tue 25-06-13 17:51:29, Luiz Capitulino wrote:
> > Currently, applications are notified for the level they registered for
> > _plus_ higher levels.
> > 
> > This is a problem if the application wants to implement different
> > actions for different levels. For example, an application might want
> > to release 10% of its cache on level low, 50% on medium and 100% on
> > critical. To do this, the application has to register a different fd
> > for each event. However, fd low is always going to be notified and
> > and all fds are going to be notified on level critical.
> 
> OK, I am not user of this interface but I thought that the application
> would take an action of the highest level it gets notification. But yes
> this might get clumsy to implement.
> 
> > Strict mode solves this problem by strictly notifiying the event
> > an fd has registered for. It's optional. By default we still notify
> > on higher levels.
> 
> OK, makes some sense to me and it should work with the proposed edge
> trigerring as well.
> 
> > Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> > ---
> > 
> > PS: I'm following the discussion on the event storm problem, but I believe
> >     strict mode is orthogonal to what has been suggested (although the
> >     patches conflict)
> > 
> >  Documentation/cgroups/memory.txt | 10 ++++++----
> >  mm/vmpressure.c                  | 19 +++++++++++++++++--
> >  2 files changed, 23 insertions(+), 6 deletions(-)
> > 
> > diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
> > index ddf4f93..3c589cf 100644
> > --- a/Documentation/cgroups/memory.txt
> > +++ b/Documentation/cgroups/memory.txt
> > @@ -807,12 +807,14 @@ register a notification, an application must:
> >  
> >  - create an eventfd using eventfd(2);
> >  - open memory.pressure_level;
> > -- write string like "<event_fd> <fd of memory.pressure_level> <level>"
> > +- write string like "<event_fd> <fd of memory.pressure_level> <level> [strict]"
> >    to cgroup.event_control.
> >  
> > -Application will be notified through eventfd when memory pressure is at
> > -the specific level (or higher). Read/write operations to
> > -memory.pressure_level are no implemented.
> > +Applications will be notified through eventfd when memory pressure is at
> > +the specific level or higher. If strict is passed, then applications
> > +will only be notified when memory pressure reaches the specified level.
> 
> It would be good to describe when is strick and when the default
> appropriate.

Yeah, Anton asked for the same thing. Will do it.

> > +
> > +Read/write operations to memory.pressure_level are no implemented.
> >  
> >  Test:
> >  
> > diff --git a/mm/vmpressure.c b/mm/vmpressure.c
> > index 736a601..6289ede 100644
> > --- a/mm/vmpressure.c
> > +++ b/mm/vmpressure.c
> > @@ -137,6 +137,7 @@ static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
> >  struct vmpressure_event {
> >  	struct eventfd_ctx *efd;
> >  	enum vmpressure_levels level;
> > +	bool strict_mode;
> 
> Any reason to not using a flag for this? If there are any other modes to
> come them we would end up with zilions of bools which is not very nice.

Good point, I'll change it.

> 
> >  	struct list_head node;
> >  };
> >  
> > @@ -153,6 +154,9 @@ static bool vmpressure_event(struct vmpressure *vmpr,
> >  
> >  	list_for_each_entry(ev, &vmpr->events, node) {
> >  		if (level >= ev->level) {
> > +			/* strict mode ensures level == ev->level */
> > +			if (ev->strict_mode && level != ev->level)
> > +				continue;
> >  			eventfd_signal(ev->efd, 1);
> >  			signalled = true;
> >  		}
> > @@ -292,7 +296,7 @@ void vmpressure_prio(gfp_t gfp, struct mem_cgroup *memcg, int prio)
> >   * infrastructure, so that the notifications will be delivered to the
> >   * @eventfd. The @args parameter is a string that denotes pressure level
> >   * threshold (one of vmpressure_str_levels, i.e. "low", "medium", or
> > - * "critical").
> > + * "critical") and optionally a different operating mode (i.e. "strict")
> >   *
> >   * This function should not be used directly, just pass it to (struct
> >   * cftype).register_event, and then cgroup core will handle everything by
> > @@ -303,22 +307,33 @@ int vmpressure_register_event(struct cgroup *cg, struct cftype *cft,
> >  {
> >  	struct vmpressure *vmpr = cg_to_vmpressure(cg);
> >  	struct vmpressure_event *ev;
> > +	bool smode = false;
> > +	const char *p;
> >  	int level;
> >  
> >  	for (level = 0; level < VMPRESSURE_NUM_LEVELS; level++) {
> > -		if (!strcmp(vmpressure_str_levels[level], args))
> > +		p = vmpressure_str_levels[level];
> > +		if (!strncmp(p, args, strlen(p)))
> >  			break;
> >  	}
> >  
> >  	if (level >= VMPRESSURE_NUM_LEVELS)
> >  		return -EINVAL;
> >  
> > +	p = strchr(args, ' ');
> > +	if (p) {
> > +		if (strncmp(++p, "strict", 6))
> > +			return -EINVAL;
> > +		smode = true;
> > +	}
> > +
> >  	ev = kzalloc(sizeof(*ev), GFP_KERNEL);
> >  	if (!ev)
> >  		return -ENOMEM;
> >  
> >  	ev->efd = eventfd;
> >  	ev->level = level;
> > +	ev->strict_mode = smode;
> >  
> >  	mutex_lock(&vmpr->events_lock);
> >  	list_add(&ev->node, &vmpr->events);
> > -- 
> > 1.8.1.4
> > 
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