Re: [PATCH] vmpressure: implement strict mode

[Michal Hocko <mhocko@suse.cz>]

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.

> +
> +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.

>  	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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-- 
Michal Hocko
SUSE Labs

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