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    SubjectRe: [PATCH 1/1] Memory usage limit notification addition to memcg
    A few comments. Maybe adding linux-mm@kvack.org in CC. list makes it easier to
    find this thread in the next post.

    On Tue, 7 Jul 2009 13:25:10 -0700
    Vladislav Buzov <vbuzov@embeddedalley.com> wrote:

    > This patch updates the Memory Controller cgroup to add
    > a configurable memory usage limit notification. The feature
    > was presented at the April 2009 Embedded Linux Conference.
    >
    > Signed-off-by: Dan Malek <dan@embeddedalley.com>
    > Signed-off-by: Vladislav Buzov <vbuzov@embeddedalley.com>
    > ---
    > Documentation/cgroups/mem_notify.txt | 140 ++++++++++++++++++++++++++
    > include/linux/memcontrol.h | 21 ++++
    > init/Kconfig | 9 ++
    > mm/memcontrol.c | 178 ++++++++++++++++++++++++++++++++++
    > 4 files changed, 348 insertions(+), 0 deletions(-)
    > create mode 100644 Documentation/cgroups/mem_notify.txt
    >
    > diff --git a/Documentation/cgroups/mem_notify.txt b/Documentation/cgroups/mem_notify.txt
    > new file mode 100644
    > index 0000000..b4f20d0
    > --- /dev/null
    > +++ b/Documentation/cgroups/mem_notify.txt
    > @@ -0,0 +1,140 @@
    > +
    > +Memory Limit Notificiation
    > +
    > +Attempts have been made in the past to provide a mechanism for
    > +the notification to processes (task, an address space) when memory
    > +usage is approaching a high limit. The intention is that it gives
    > +the application an opportunity to release some memory and continue
    > +operation rather than be OOM killed. The CE Linux Forum requested
    > +a more comtemporary implementation, and this is the result.
    > +
    > +The memory threshold notification is a configurable extension to the
    > +existing Memory Resource Controller. Please read memory.txt in this
    > +directory to understand its operation before continuing here.
    > +
    > +1. Operation
    > +
    > +When a kernel is configured with CGROUP_MEM_NOTIFY, three additional
    > +files will appear in the memory resource controller:
    > +
    > + memory.notify_threshold_in_bytes
    > + memory.notify_available_in_bytes
    > + memory.notify_threshold_lowait
    > +
    > +The notification is based upon reaching a threshold below the memory
    > +resouce controller limit (memory.limit_in_bytes). The threshold
    > +represents the minimal number of bytes that should be available under
    > +the limit. When the controller group is created, the threshold is set
    > +to zero which triggers notification when the memory resource controller
    > +limit is reached.
    > +
    > +The threshold may be set by writing to memory.notify_threshold_in_bytes,
    > +such as:
    > +
    > + echo 10M > memory.notify_threshold_in_bytes
    > +
    > +The current number of available bytes may be read at any time from
    > +the memory.notify_available_in_bytes
    > +
    > +The memory.notify_threshold_lowait is a blocking read file. The read will
    > +block until one of four conditions occurs:
    > +
    > + - The amount of available memory is equal or less than the threshold
    > + defined in memory.notify_threshold_in_bytes
    > + - The memory.notify_threshold_lowait file is written with any value (debug)
    > + - A thread is moved to another controller group
    > + - The cgroup is destroyed or forced empty (memory.force_empty)
    > +

    I don't think notify_available_in_bytes is necessary.

    For making this kind of threashold useful, I think some relaxing margin is good.
    for example) Once triggered, "notiry" will not be triggered in next 1ms
    Do you have an idea ?

    I know people likes to wait for file descriptor to get notification in these days.
    Can't we have "event" file descriptor in cgroup layer and make it reusable for
    other purposes ?

    > +
    > +1.1 Example Usage
    > +
    > +An application must be designed to properly take advantage of this
    > +memory threshold notification feature. It is a powerful management component
    > +of some operating systems and embedded devices that must provide
    > +highly available and reliable computing services. The application works
    > +in conjunction with information provided by the operating system to
    > +control limited resource usage. Since many programmers still think
    > +memory is infinite and never check the return value from malloc(), it
    > +may come as a surprise that such mechanisms have been utilized long ago.
    > +
    > +A typical application will be multithreaded, with one thread either
    > +polling or waiting for the notification event. When the event occurs,
    > +the thread will take whatever action is appropriate within the application
    > +design. This could be actually running a garbage collection algorithm
    > +or to simply signal other processing threads they must do something to
    > +reduce their memory usage. The notification thread will then be required
    > +to poll the actual usage until the low limit of its choosing is met,
    > +at which time the reclaim of memory can stop and the notification thread
    > +will wait for the next event.
    > +
    > +Internally, the application only needs to
    > +fopen("memory.notify_available_in_bytes" ..) or
    > +fopen("memory.notify_threshold_lowait" ...), then either poll the former
    > +file or block read on the latter file using fread() or fscanf() as desired.
    > +Comparing the value returned from either of these read function with the
    > +value obtained by reading memory.notify_threshold_in_bytes will be an
    > +indication of the amount of memory used over the threshold limit.
    > +

    I hope this application will not block rmdir() ;)



    > +2. Configuration
    > +
    > +Follow the instructions in memory.txt for the configuration and usage of
    > +the Memory Resource Controller cgroup. Once this is created and tasks
    > +assigned, use the memory threshold notification as described here.
    > +
    > +The only action that is needed outside of the application waiting or polling
    > +is to set the memory.notify_threshold_in_bytes. To set a notification to occur
    > +when memory usage of the cgroup reaches or exceeds 1 MByte below the limit
    > +can be simply done:
    > +
    > + echo 1M > memory.notify_threshold_in_bytes
    > +
    > +This value may be read or changed at any time. Writing a higher value once
    > +the Memory Resource Controller is in operation may trigger immediate
    > +notification if the usage is above the new threshold.
    > +

    One question is how this works under hierarchical accounting.

    Considering following.

    /cgroup/A/ no thresh
    001/ thresh=5M
    John thresh=1M
    002/ no thresh
    Hiroyuki no thresh

    If Hiroyuki use too much and hit /cgroup/A's limit, memory will be reclaimed from all
    A,001,John,002,Hiroyuki and OOM Killer may kill processes in John.
    But 001/John's notifier will not fire. Right ?


    > +3. Debug and Testing
    > +
    > +The design of cgroups makes it easier to perform some debugging or
    > +monitoring tasks without modification to the application. For example,
    > +a write of any value to memory.notify_threshold_lowait will wake up all
    > +threads waiting for notifications regardless of current memory usage.
    > +
    > +Collecting performance data about the cgroup is also simplified, as
    > +no application modifications are necessary. A separate task can be
    > +created that will open and monitor any necessary files of the cgroup
    > +(such as current limits, usage and usage percentages and even when
    > +notification occurs). This task can also operate outside of the cgroup,
    > +so its memory usage is not charged to the cgroup.
    > +
    > +4. Design
    > +
    > +The memory threshold notification is a configurable extension to the
    > +existing Memory Resource Controller, which operates as described to
    > +track and manage the memory of the Control Group. The Memory Resource
    > +Controller will still continue to reclaim memory under pressure
    > +of the limits, and may OOM kill tasks within the cgroup according to
    > +the OOM Killer configuration.
    > +
    > +The memory notification threshold was chosen as a number of bytes of the
    > +memory not in use so the cgroup paramaters may continue to be dynamically
    > +modified without the need to modify the notificaton parameters.
    > +Otherwise, the notification threshold would have to also be computed
    > +and modified on any Memory Resource Controller operating parameter change.
    > +
    > +The cgroup file semantics are not well suited for this type of notificaton
    > +mechanism. While applications may choose to simply poll the current
    > +usage at their convenience, it was also desired to have a notification
    > +event that would trigger when the usage attained the threshold. The
    > +blocking read() was chosen, as it is the only current useful method.
    > +This presented the problems of "out of band" notification, when you want
    > +to return some exceptional status other than reaching the notification
    > +threshold. In the cases listed above, the read() on the
    > +memory.notify_threshold_lowait file will not block and return "0" for
    > +the remaining size. When this occurs, the thread must determine if the task
    > +has moved to a new cgroup or if the cgroup has been destroyed. Due to
    > +the usage model of this cgroup, neither is likely to happen during normal
    > +operation of a product.
    > +
    > +Dan Malek <dan@embeddedalley.com>
    > +Embedded Alley Solutions, Inc.
    > +6 July 2009
    > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
    > index e46a073..78205a3 100644
    > --- a/include/linux/memcontrol.h
    > +++ b/include/linux/memcontrol.h
    > @@ -118,6 +118,27 @@ static inline bool mem_cgroup_disabled(void)
    >
    > extern bool mem_cgroup_oom_called(struct task_struct *task);
    > void mem_cgroup_update_mapped_file_stat(struct page *page, int val);
    > +
    > +#ifdef CONFIG_CGROUP_MEM_NOTIFY
    > +void mem_cgroup_notify_test_and_wakeup(struct mem_cgroup *mcg,
    > + unsigned long long usage, unsigned long long limit);
    > +void mem_cgroup_notify_new_limit(struct mem_cgroup *mcg,
    > + unsigned long long newlimit);
    > +void mem_cgroup_notify_move_task(struct cgroup *old_cont);
    > +#else
    > +static inline void mem_cgroup_notify_test_and_wakeup(struct mem_cgroup *mcg,
    > + unsigned long long usage, unsigned long long limit)
    > +{
    > +}
    > +static inline void mem_cgroup_notify_new_limit(struct mem_cgroup *mcg,
    > + unsigned long long newlimit)
    > +{
    > +}
    > +static inline void mem_cgroup_notify_move_task(struct cgroup *old_cont)
    > +{
    > +}
    > +#endif
    > +
    > #else /* CONFIG_CGROUP_MEM_RES_CTLR */
    > struct mem_cgroup;
    >
    > diff --git a/init/Kconfig b/init/Kconfig
    > index 1ce05a4..fb2f7d5 100644
    > --- a/init/Kconfig
    > +++ b/init/Kconfig
    > @@ -594,6 +594,15 @@ config CGROUP_MEM_RES_CTLR
    > This config option also selects MM_OWNER config option, which
    > could in turn add some fork/exit overhead.
    >
    > +config CGROUP_MEM_NOTIFY
    > + bool "Memory Usage Limit Notification"
    > + depends on CGROUP_MEM_RES_CTLR
    > + help
    > + Provides a memory notification when usage reaches a preset limit.
    > + It is an extenstion to the memory resource controller, since it
    > + uses the memory usage accounting of the cgroup to test against
    > + the notification limit. (See Documentation/cgroups/mem_notify.txt)
    > +

    I don't think CONFIG is necessary. Let this always used.


    > config CGROUP_MEM_RES_CTLR_SWAP
    > bool "Memory Resource Controller Swap Extension(EXPERIMENTAL)"
    > depends on CGROUP_MEM_RES_CTLR && SWAP && EXPERIMENTAL
    > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    > index e2fa20d..cf04279 100644
    > --- a/mm/memcontrol.c
    > +++ b/mm/memcontrol.c
    > @@ -6,6 +6,10 @@
    > * Copyright 2007 OpenVZ SWsoft Inc
    > * Author: Pavel Emelianov <xemul@openvz.org>
    > *
    > + * Memory Limit Notification update
    > + * Copyright 2009 CE Linux Forum and Embedded Alley Solutions, Inc.
    > + * Author: Dan Malek <dan@embeddedalley.com>
    > + *
    > * This program is free software; you can redistribute it and/or modify
    > * it under the terms of the GNU General Public License as published by
    > * the Free Software Foundation; either version 2 of the License, or
    > @@ -180,6 +184,11 @@ struct mem_cgroup {
    > /* set when res.limit == memsw.limit */
    > bool memsw_is_minimum;
    >
    > +#ifdef CONFIG_CGROUP_MEM_NOTIFY
    > + unsigned long long notify_threshold_bytes;
    > + wait_queue_head_t notify_threshold_wait;
    > +#endif
    > +
    > /*
    > * statistics. This must be placed at the end of memcg.
    > */
    > @@ -995,6 +1004,13 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
    >
    > VM_BUG_ON(css_is_removed(&mem->css));
    >
    > + /*
    > + * We check on the way in so we don't have to duplicate code
    > + * in both the normal and error exit path.
    > + */
    > + mem_cgroup_notify_test_and_wakeup(mem, mem->res.usage + PAGE_SIZE,
    > + mem->res.limit);
    > +

    2 points.
    - Do we have to check this always we account ?
    - This will not catch hierarchical accounting threshold because this check
    only local cgroup, no ancestors.

    I don't want to say this but you need to add hook to res_counter itself.


    > while (1) {
    > int ret;
    > bool noswap = false;
    > @@ -1744,6 +1760,12 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
    > u64 curusage, oldusage;
    >
    > /*
    > + * Test and notify ahead of the necessity to free pages, as
    > + * applications giving up pages may help this reclaim procedure.
    > + */
    > + mem_cgroup_notify_new_limit(memcg, val);
    > +
    > + /*
    > * For keeping hierarchical_reclaim simple, how long we should retry
    > * is depends on callers. We set our retry-count to be function
    > * of # of children which we should visit in this loop.
    > @@ -2308,6 +2330,139 @@ static int mem_cgroup_swappiness_write(struct cgroup *cgrp, struct cftype *cft,
    > return 0;
    > }
    >
    > +#ifdef CONFIG_CGROUP_MEM_NOTIFY
    > +/*
    > + * Check if a task exceeded notification threshold set for a memory cgroup.
    > + * Wake up waiting notification threads, if any.
    > + */
    > +void mem_cgroup_notify_test_and_wakeup(struct mem_cgroup *mcg,
    > + unsigned long long usage,
    > + unsigned long long limit)
    > +{
    > + if (unlikely(usage == RESOURCE_MAX))
    > + return;
    What this means ?? Can happen ?

    > +
    > + if ((limit - usage <= mcg->notify_threshold_bytes) &&
    > + waitqueue_active(&mcg->notify_threshold_wait))
    > + wake_up(&mcg->notify_threshold_wait);
    > +}
    > +/*
    > + * Check if current notification threshold exceeds new memory usage
    > + * limit set for a memory cgroup. If so, set threshold to zero to
    > + * notify tasks in the group when maximal memory usage is achieved.
    > + */
    > +void mem_cgroup_notify_new_limit(struct mem_cgroup *mcg,
    > + unsigned long long newlimit)
    > +{
    > + if (newlimit <= mcg->notify_threshold_bytes)
    > + mcg->notify_threshold_bytes = 0;
    > +
    > + mem_cgroup_notify_test_and_wakeup(mcg, mcg->res.usage, newlimit);
    > +}
    > +
    > +static u64 mem_cgroup_notify_threshold_read(struct cgroup *cgrp,
    > + struct cftype *cft)
    > +{
    > + struct mem_cgroup *memcg = mem_cgroup_from_cont(cgrp);
    > + return memcg->notify_threshold_bytes;
    > +}
    > +
    > +static int mem_cgroup_notify_threshold_write(struct cgroup *cgrp,
    > + struct cftype *cft,
    > + const char *buffer)
    > +{
    > + struct mem_cgroup *memcg = mem_cgroup_from_cont(cgrp);
    > + unsigned long long val;
    > + int ret;
    > +
    > + /* This function does all necessary parse...reuse it */
    > + ret = res_counter_memparse_write_strategy(buffer, &val);
    > + if (ret)
    > + return ret;
    > +
    > + /* Threshold must be lower than usage limit */
    > + if (val >= memcg->res.limit)
    > + return -EINVAL;

    If this is true, "set limit" should be checked to guarantee this.
    plz allow minus this for avoiding mess.

    > +
    > + memcg->notify_threshold_bytes = val;
    > +
    > + /* Check to see if the new threshold should cause notification */
    > + mem_cgroup_notify_test_and_wakeup(memcg, memcg->res.usage,
    > + memcg->res.limit);
    > +
    > + return 0;
    > +}
    > +
    > +static u64 mem_cgroup_notify_available_read(struct cgroup *cgrp,
    > + struct cftype *cft)
    > +{
    > + struct mem_cgroup *memcg = mem_cgroup_from_cont(cgrp);
    > + return memcg->res.limit - memcg->res.usage;
    > +}
    > +
    > +static u64 mem_cgroup_notify_threshold_lowait(struct cgroup *cgrp,
    > + struct cftype *cft)
    > +{
    > + struct mem_cgroup *mem = mem_cgroup_from_cont(cgrp);
    > + unsigned long long available_bytes;
    > + DEFINE_WAIT(notify_lowait);
    > +
    > + /*
    > + * A memory resource usage of zero is a special case that
    > + * causes us not to sleep. It normally happens when the
    > + * cgroup is about to be destroyed, and we don't want someone
    > + * trying to sleep on a queue that is about to go away. This
    > + * condition can also be forced as part of testing.
    > + */
    > + available_bytes = mem->res.limit - mem->res.usage;
    > + if (likely(mem->res.usage != 0)) {
    > +
    > + prepare_to_wait(&mem->notify_threshold_wait, &notify_lowait,
    > + TASK_INTERRUPTIBLE);
    > +
    > + if (available_bytes > mem->notify_threshold_bytes)
    > + schedule();
    > +
    > + available_bytes = mem->res.limit - mem->res.usage;
    > +
    > + finish_wait(&mem->notify_threshold_wait, &notify_lowait);
    > + }
    > +
    > + return available_bytes;
    > +}
    > +
    > +/*
    > + * This is used to wake up all threads that may be hanging
    > + * out waiting for a low memory condition prior to that happening.
    > + * Useful for triggering the event to assist with debug of applications.
    > + */
    > +static int mem_cgroup_notify_threshold_wake_em_up(struct cgroup *cgrp,
    > + unsigned int event)
    > +{
    > + struct mem_cgroup *mem;
    > +
    > + mem = mem_cgroup_from_cont(cgrp);
    > + wake_up(&mem->notify_threshold_wait);
    > + return 0;
    > +}
    > +
    > +/*
    > + * We wake up all notification threads any time a migration takes
    > + * place. They will have to check to see if a move is needed to
    > + * a new cgroup file to wait for notification.
    > + * This isn't so much a task move as it is an attach. A thread not
    > + * a child of an existing task won't have a valid parent, which
    > + * is necessary to test because it won't have a valid mem_cgroup
    > + * either. Which further means it won't have a proper wait queue
    > + * and we can't do a wakeup.
    > + */
    > +void mem_cgroup_notify_move_task(struct cgroup *old_cont)
    > +{
    > + if (old_cont->parent != NULL)
    > + mem_cgroup_notify_threshold_wake_em_up(old_cont, 0);
    > +}
    > +#endif /* CONFIG_CGROUP_MEM_NOTIFY */
    > +
    >

    plz call wake_em_up at pre_destroy(), too.

    Thanks,
    -Kame


    > static struct cftype mem_cgroup_files[] = {
    > {
    > @@ -2351,6 +2506,22 @@ static struct cftype mem_cgroup_files[] = {
    > .read_u64 = mem_cgroup_swappiness_read,
    > .write_u64 = mem_cgroup_swappiness_write,
    > },
    > +#ifdef CONFIG_CGROUP_MEM_NOTIFY
    > + {
    > + .name = "notify_threshold_in_bytes",
    > + .write_string = mem_cgroup_notify_threshold_write,
    > + .read_u64 = mem_cgroup_notify_threshold_read,
    > + },
    > + {
    > + .name = "notify_available_in_bytes",
    > + .read_u64 = mem_cgroup_notify_available_read,
    > + },
    > + {
    > + .name = "notify_threshold_lowait",
    > + .trigger = mem_cgroup_notify_threshold_wake_em_up,
    > + .read_u64 = mem_cgroup_notify_threshold_lowait,
    > + },
    > +#endif
    > };
    >
    > #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
    > @@ -2554,6 +2725,11 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
    > mem->last_scanned_child = 0;
    > spin_lock_init(&mem->reclaim_param_lock);
    >
    > +#ifdef CONFIG_CGROUP_MEM_NOTIFY
    > + init_waitqueue_head(&mem->notify_threshold_wait);
    > + mem->notify_threshold_bytes = 0;
    > +#endif
    > +
    > if (parent)
    > mem->swappiness = get_swappiness(parent);
    > atomic_set(&mem->refcnt, 1);
    > @@ -2597,6 +2773,8 @@ static void mem_cgroup_move_task(struct cgroup_subsys *ss,
    > struct cgroup *old_cont,
    > struct task_struct *p)
    > {
    > + mem_cgroup_notify_move_task(old_cont);
    > +
    > mutex_lock(&memcg_tasklist);
    > /*
    > * FIXME: It's better to move charges of this process from old
    > --




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