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Date: Wed, 26 Jun 2013 10:08:27 +0200
From: Michal Hocko <mhocko@...e.cz>
To: Luiz Capitulino <lcapitulino@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
minchan@...nel.org, anton@...msg.org, akpm@...ux-foundation.org
Subject: Re: [PATCH] vmpressure: implement strict mode
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@...hat.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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