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Date: Wed, 26 Jun 2013 09:28:31 +0900
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Subject: Re: [PATCH] vmpressure: implement strict mode
From: Kyungmin Park <kmpark@infradead.org>
To: Luiz Capitulino <lcapitulino@redhat.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, mhocko@suse.cz,
        minchan@kernel.org, anton@enomsg.org, akpm@linux-foundation.org,
        Hyunhee Kim <hyunhee.kim@samsung.com>
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+ Ms. Kim,

she already raised this issue at another mail thread.

Thank you,
Kyungmin Park

On Wed, Jun 26, 2013 at 6:51 AM, Luiz Capitulino <lcapitulino@redhat.com> 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.
>
> 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.
>
> 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.
> +
> +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;
>         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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