[Devel] Re: [PATCH 1/1] Memory usage limit notification addition to memcg
KAMEZAWA Hiroyuki
kamezawa.hiroyu at jp.fujitsu.com
Tue Jul 7 17:56:16 PDT 2009
A few comments. Maybe adding linux-mm at 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 at 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 at embeddedalley.com>
> Signed-off-by: Vladislav Buzov <vbuzov at 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 at 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 at openvz.org>
> *
> + * Memory Limit Notification update
> + * Copyright 2009 CE Linux Forum and Embedded Alley Solutions, Inc.
> + * Author: Dan Malek <dan at 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, ¬ify_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, ¬ify_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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