Re: [PATCH] vmpressure: implement strict mode

[Hyunhee Kim <hyunhee.kim@samsung.com>]

Please see "[PATCH v3] memcg: event control at vmpressure". mail
thread. (and also the thread I sent last Saturday.)
There was discussion on this mode not sending lower events when "level
!= ev->level".

Thanks,
Hyunhee Kim.

2013/6/26 Kyungmin Park <kmpark@infradead.org>:
> + 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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