This is a user API visible change and as such linux-api should be CCed.
On Wed 31-05-17 14:22:30, David Rientjes wrote:
> By default, vmpressure events are not pass-through, i.e. they propagate
> up through the memcg hierarchy until an event notifier is found for any
> threshold level.
>
> This presents a difficulty when a thread waiting on a read(2) for a
> vmpressure event cannot distinguish between local memory pressure and
> memory pressure in a descendant memcg, especially when that thread may
> not control the memcg hierarchy.
>
> Consider a user-controlled child memcg with a smaller limit than a
> top-level memcg controlled by the "Activity Manager" specified in
> Documentation/cgroup-v1/memory.txt. It may register for memory pressure
> notification for descendant memcgs to make a policy decision: oom kill a
> low priority job, increase the limit, decrease other limits, etc.
> If it registers for memory pressure notification on the top-level memcg,
> it currently cannot distinguish between memory pressure in its own memcg
> or a descendant memcg, which is user-controlled.
>
> Conversely, if a user registers for memory pressure notification on their
> own descendant memcg, the Activity Manager does not receive any pressure
> notification for that child memcg hierarchy. Vmpressure events are not
> received for ancestor memcgs if the memcg experiencing pressure have
> notifiers registered, perhaps outside the knowledge of the thread
> waiting on read(2) at the top level.
>
> Both of these are consequences of vmpressure notification not being
> pass-through.
>
> This implements a pass-through behavior for vmpressure events. When
> writing to control.event_control, vmpressure event handlers may
> optionally specify a mode. There are two new modes:
>
> - "hierarchy": always propagate memory pressure events up the
> hierarchy regardless if descendant memcgs have their own notifiers
> registered, and
>
> - "local": only receive notifications when the memcg for which the
> event is registered experiences memory pressure.
>
> Of course, processes may register for one notification of "low,local",
> for example, and another for "low".
>
> If no mode is specified, the current behavior is maintained for
> backwards compatibility.
>
> See the change to Documentation/cgroup-v1/memory.txt for full
> specification.
>
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
> Documentation/cgroup-v1/memory.txt | 47 ++++++++++----
> mm/vmpressure.c | 122 ++++++++++++++++++++++++++++---------
> 2 files changed, 128 insertions(+), 41 deletions(-)
>
> diff --git a/Documentation/cgroup-v1/memory.txt b/Documentation/cgroup-v1/memory.txt
> --- a/Documentation/cgroup-v1/memory.txt
> +++ b/Documentation/cgroup-v1/memory.txt
> @@ -789,23 +789,46 @@ way to trigger. Applications should do whatever they can to help the
> system. It might be too late to consult with vmstat or any other
> statistics, so it's advisable to take an immediate action.
>
> -The events are propagated upward until the event is handled, i.e. the
> -events are not pass-through. Here is what this means: for example you have
> -three cgroups: A->B->C. Now you set up an event listener on cgroups A, B
> -and C, and suppose group C experiences some pressure. In this situation,
> -only group C will receive the notification, i.e. groups A and B will not
> -receive it. This is done to avoid excessive "broadcasting" of messages,
> -which disturbs the system and which is especially bad if we are low on
> -memory or thrashing. So, organize the cgroups wisely, or propagate the
> -events manually (or, ask us to implement the pass-through events,
> -explaining why would you need them.)
> +By default, events are propagated upward until the event is handled, i.e. the
> +events are not pass-through. For example, you have three cgroups: A->B->C. Now
> +you set up an event listener on cgroups A, B and C, and suppose group C
> +experiences some pressure. In this situation, only group C will receive the
> +notification, i.e. groups A and B will not receive it. This is done to avoid
> +excessive "broadcasting" of messages, which disturbs the system and which is
> +especially bad if we are low on memory or thrashing. Group B, will receive
> +notification only if there are no event listers for group C.
> +
> +There are three optional modes that specify different propagation behavior:
> +
> + - "default": this is the default behavior specified above. This mode is the
> + same as omitting the optional mode parameter, preserved by backwards
> + compatibility.
> +
> + - "hierarchy": events always propagate up to the root, similar to the default
> + behavior, except that propagation continues regardless of whether there are
> + event listeners at each level, with the "hierarchy" mode. In the above
> + example, groups A, B, and C will receive notification of memory pressure.
> +
> + - "local": events are pass-through, i.e. they only receive notifications when
> + memory pressure is experienced in the memcg for which the notification is
> + registered. In the above example, group C will receive notification if
> + registered for "local" notification and the group experiences memory
> + pressure. However, group B will never receive notification, regardless if
> + there is an event listener for group C or not, if group B is registered for
> + local notification.
> +
> +The level and event notification mode ("hierarchy" or "local", if necessary) are
> +specified by a comma-delimited string, i.e. "low,hierarchy" specifies
> +hierarchical, pass-through, notification for all ancestor memcgs. Notification
> +that is the default, non pass-through behavior, does not specify a mode.
> +"medium,local" specifies pass-through notification for the medium level.
>
> The file memory.pressure_level is only used to setup an eventfd. To
> 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 as "<event_fd> <fd of memory.pressure_level> <level[,mode]>"
> to cgroup.event_control.
>
> Application will be notified through eventfd when memory pressure is at
> @@ -821,7 +844,7 @@ Test:
> # cd /sys/fs/cgroup/memory/
> # mkdir foo
> # cd foo
> - # cgroup_event_listener memory.pressure_level low &
> + # cgroup_event_listener memory.pressure_level low,hierarchy &
> # echo 8000000 > memory.limit_in_bytes
> # echo 8000000 > memory.memsw.limit_in_bytes
> # echo $$ > tasks
> diff --git a/mm/vmpressure.c b/mm/vmpressure.c
> --- a/mm/vmpressure.c
> +++ b/mm/vmpressure.c
> @@ -93,12 +93,25 @@ enum vmpressure_levels {
> VMPRESSURE_NUM_LEVELS,
> };
>
> +enum vmpressure_modes {
> + VMPRESSURE_NO_PASSTHROUGH = 0,
> + VMPRESSURE_HIERARCHY,
> + VMPRESSURE_LOCAL,
> + VMPRESSURE_NUM_MODES,
> +};
> +
> static const char * const vmpressure_str_levels[] = {
> [VMPRESSURE_LOW] = "low",
> [VMPRESSURE_MEDIUM] = "medium",
> [VMPRESSURE_CRITICAL] = "critical",
> };
>
> +static const char * const vmpressure_str_modes[] = {
> + [VMPRESSURE_NO_PASSTHROUGH] = "default",
> + [VMPRESSURE_HIERARCHY] = "hierarchy",
> + [VMPRESSURE_LOCAL] = "local",
> +};
> +
> static enum vmpressure_levels vmpressure_level(unsigned long pressure)
> {
> if (pressure >= vmpressure_level_critical)
> @@ -141,27 +154,31 @@ static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
> struct vmpressure_event {
> struct eventfd_ctx *efd;
> enum vmpressure_levels level;
> + enum vmpressure_modes mode;
> struct list_head node;
> };
>
> static bool vmpressure_event(struct vmpressure *vmpr,
> - enum vmpressure_levels level)
> + const enum vmpressure_levels level,
> + bool ancestor, bool signalled)
> {
> struct vmpressure_event *ev;
> - bool signalled = false;
> + bool ret = false;
>
> mutex_lock(&vmpr->events_lock);
> -
> list_for_each_entry(ev, &vmpr->events, node) {
> - if (level >= ev->level) {
> - eventfd_signal(ev->efd, 1);
> - signalled = true;
> - }
> + if (ancestor && ev->mode == VMPRESSURE_LOCAL)
> + continue;
> + if (signalled && ev->mode == VMPRESSURE_NO_PASSTHROUGH)
> + continue;
> + if (level < ev->level)
> + continue;
> + eventfd_signal(ev->efd, 1);
> + ret = true;
> }
> -
> mutex_unlock(&vmpr->events_lock);
>
> - return signalled;
> + return ret;
> }
>
> static void vmpressure_work_fn(struct work_struct *work)
> @@ -170,6 +187,8 @@ static void vmpressure_work_fn(struct work_struct *work)
> unsigned long scanned;
> unsigned long reclaimed;
> enum vmpressure_levels level;
> + bool ancestor = false;
> + bool signalled = false;
>
> spin_lock(&vmpr->sr_lock);
> /*
> @@ -194,12 +213,9 @@ static void vmpressure_work_fn(struct work_struct *work)
> level = vmpressure_calc_level(scanned, reclaimed);
>
> do {
> - if (vmpressure_event(vmpr, level))
> - break;
> - /*
> - * If not handled, propagate the event upward into the
> - * hierarchy.
> - */
> + if (vmpressure_event(vmpr, level, ancestor, signalled))
> + signalled = true;
> + ancestor = true;
> } while ((vmpr = vmpressure_parent(vmpr)));
> }
>
> @@ -326,17 +342,40 @@ void vmpressure_prio(gfp_t gfp, struct mem_cgroup *memcg, int prio)
> vmpressure(gfp, memcg, true, vmpressure_win, 0);
> }
>
> +static enum vmpressure_levels str_to_level(const char *arg)
> +{
> + enum vmpressure_levels level;
> +
> + for (level = 0; level < VMPRESSURE_NUM_LEVELS; level++)
> + if (!strcmp(vmpressure_str_levels[level], arg))
> + return level;
> + return -1;
> +}
> +
> +static enum vmpressure_modes str_to_mode(const char *arg)
> +{
> + enum vmpressure_modes mode;
> +
> + for (mode = 0; mode < VMPRESSURE_NUM_MODES; mode++)
> + if (!strcmp(vmpressure_str_modes[mode], arg))
> + return mode;
> + return -1;
> +}
> +
> +#define MAX_VMPRESSURE_ARGS_LEN (strlen("critical") + strlen("hierarchy") + 2)
> +
> /**
> * vmpressure_register_event() - Bind vmpressure notifications to an eventfd
> * @memcg: memcg that is interested in vmpressure notifications
> * @eventfd: eventfd context to link notifications with
> - * @args: event arguments (used to set up a pressure level threshold)
> + * @args: event arguments (pressure level threshold, optional mode)
> *
> * This function associates eventfd context with the vmpressure
> * 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").
> + * @eventfd. The @args parameter is a comma-delimited string that denotes a
> + * pressure level threshold (one of vmpressure_str_levels, i.e. "low", "medium",
> + * or "critical") and an optional mode (one of vmpressure_str_modes, i.e.
> + * "hierarchy" or "local").
> *
> * To be used as memcg event method.
> */
> @@ -345,28 +384,53 @@ int vmpressure_register_event(struct mem_cgroup *memcg,
> {
> struct vmpressure *vmpr = memcg_to_vmpressure(memcg);
> struct vmpressure_event *ev;
> - int level;
> + enum vmpressure_modes mode = VMPRESSURE_NO_PASSTHROUGH;
> + enum vmpressure_levels level = -1;
> + char *spec = NULL;
> + char *token;
> + int ret = 0;
> +
> + spec = kzalloc(MAX_VMPRESSURE_ARGS_LEN + 1, GFP_KERNEL);
> + if (!spec) {
> + ret = -ENOMEM;
> + goto out;
> + }
> + strncpy(spec, args, MAX_VMPRESSURE_ARGS_LEN);
>
> - for (level = 0; level < VMPRESSURE_NUM_LEVELS; level++) {
> - if (!strcmp(vmpressure_str_levels[level], args))
> - break;
> + /* Find required level */
> + token = strsep(&spec, ",");
> + level = str_to_level(token);
> + if (level == -1) {
> + ret = -EINVAL;
> + goto out;
> }
>
> - if (level >= VMPRESSURE_NUM_LEVELS)
> - return -EINVAL;
> + /* Find optional mode */
> + token = strsep(&spec, ",");
> + if (token) {
> + mode = str_to_mode(token);
> + if (mode == -1) {
> + ret = -EINVAL;
> + goto out;
> + }
> + }
>
> ev = kzalloc(sizeof(*ev), GFP_KERNEL);
> - if (!ev)
> - return -ENOMEM;
> + if (!ev) {
> + ret = -ENOMEM;
> + goto out;
> + }
>
> ev->efd = eventfd;
> ev->level = level;
> + ev->mode = mode;
>
> mutex_lock(&vmpr->events_lock);
> list_add(&ev->node, &vmpr->events);
> mutex_unlock(&vmpr->events_lock);
> -
> - return 0;
> +out:
> + kfree(spec);
> + return ret;
> }
>
> /**
>
> --
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--
Michal Hocko
SUSE Labs
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