[Devel] Re: [PATCH] Memory usage limit notification addition to memcg
Andrew Morton
akpm at linux-foundation.org
Mon Apr 13 16:08:14 PDT 2009
Please cc containers at lists.linux-foundation.org on this sort of thing
so that all the right people get to see it.
On Mon, 13 Apr 2009 18:08:32 -0400
Dan Malek <dan 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>
> ---
> Documentation/cgroups/mem_notify.txt | 129 +++++++++++++++++++++
> include/linux/memcontrol.h | 7 +
> init/Kconfig | 9 ++
> mm/memcontrol.c | 207 ++++++++++++++++++++++++++++++++++
> 4 files changed, 352 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..72d5c26
> --- /dev/null
> +++ b/Documentation/cgroups/mem_notify.txt
> @@ -0,0 +1,129 @@
> +
> +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 limit 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_limit_percent
We've run into problems in the past where a percentage number is too
coarse on large-memory systems.
Proabably that won't be an issue here, but I invite you to convince us
of this ;)
> + memory.notify_limit_usage
> + memory.notify_limit_lowait
> +
> +The notification is based upon reaching a percentage of the memory
> +resource controller limit (memory.limit_in_bytes). When the controller
> +group is created, the percentage is set to 100. Any integer percentage
> +may be set by writing to memory.notify_limit_percent, such as:
> +
> + echo 80 > memory.notify_limit_percent
> +
> +The current integer usage percentage may be read at any time from
> +the memory.notify_limit_usage file.
> +
> +The memory.notify_limit_lowait is a blocking read file. The read will
> +block until one of four conditions occurs:
> +
> + - The usage reaches or exceeds the memory.notify_limit_percent
> + - The memory.notify_limit_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)
Does it support select()/poll()/eventfd()/etc?
> +
> +1.1 Example Usage
> +
> +An application must be designed to properly take advantage of this
> +memory limit 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_limit_usage" ..)
> +and fopen("memory.notify_limit_lowait" ...), then either poll the former
> +file or block read on the latter file using fread() or fscanf() as desired.
> +
> +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 limit notification as described here.
> +
> +The only action that is needed outside of the application waiting or polling
> +is to set the memory.notify_limit_percent. To set a notification to occur
> +when memory usage of the cgroup reaches or exceeds 80 percent can be
> +simply done:
> +
> + echo 80 > memory.notify_limit_percent
> +
> +This value may be read or changed at any time. Writing a lower value once
> +the Memory Resource Controller is in operation may trigger immediate
> +notification if the usage is above the new limit.
> +
> +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_limit_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 limit 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 limit was chosen as a percentage of the
> +memory in use so the cgroup paramaters may continue to be dynamically
> +modified without the need to modify the notificaton parameters.
> +Otherwise, the notification limit 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 limit. 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
> +limit. In the cases listed above, the read() on the memory.notify_limit_lowait
> +file will not block and return "0" for the percentage. 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.
> +10 March 2009
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 18146c9..031e5d1 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -117,6 +117,13 @@ static inline bool mem_cgroup_disabled(void)
>
> extern bool mem_cgroup_oom_called(struct task_struct *task);
>
> +#ifdef CONFIG_CGROUP_MEM_NOTIFY
> +extern void test_and_wakeup_notify(struct mem_cgroup *mcg,
> + unsigned long long newlimit);
> +extern unsigned long compute_usage_percent(unsigned long long usage,
> + unsigned long long limit);
> +#endif
Stylistic trick: here, please add
#else
static inline void test_and_wakeup_notify(struct mem_cgroup *mcg,
unsigned long long newlimit)
{
}
static inline unsigned long compute_usage_percent(unsigned long long usage,
unsigned long long limit)
{
return 0; /* ? */
}
#endif
and then remove the ifdefs from the .c files.
> #else /* CONFIG_CGROUP_MEM_RES_CTLR */
> struct mem_cgroup;
>
> diff --git a/init/Kconfig b/init/Kconfig
> index f2f9b53..97138da 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -588,6 +588,15 @@ 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)
> +
> 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 2fc6d6c..d6367ed 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 {
> * statistics. This must be placed at the end of memcg.
> */
> struct mem_cgroup_stat stat;
> +
> +#ifdef CONFIG_CGROUP_MEM_NOTIFY
> + unsigned long notify_limit_percent;
> + wait_queue_head_t notify_limit_wait;
> +#endif
> };
>
> enum charge_type {
> @@ -934,6 +943,21 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
>
> VM_BUG_ON(mem_cgroup_is_obsolete(mem));
>
> +#ifdef CONFIG_CGROUP_MEM_NOTIFY
> + /* We check on the way in so we don't have to duplicate code
> + * in both the normal and error exit path.
> + */
> + if (likely(mem->res.limit != (unsigned long long)LLONG_MAX)) {
> + unsigned long usage_pct;
> +
> + usage_pct = compute_usage_percent(mem->res.usage + PAGE_SIZE,
> + mem->res.limit);
> + if ((usage_pct >= mem->notify_limit_percent) &&
> + waitqueue_active(&mem->notify_limit_wait))
> + wake_up(&mem->notify_limit_wait);
> + }
> +#endif
It would be nicer to pull this out into a separate function, I expect.
> while (1) {
> int ret;
> bool noswap = false;
> @@ -1663,6 +1687,13 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
> int children = mem_cgroup_count_children(memcg);
> u64 curusage, oldusage;
>
> +#ifdef CONFIG_CGROUP_MEM_NOTIFY
> + /* Test and notify ahead of the necessity to free pages, as
> + * applications giving up pages may help this reclaim procedure.
> + */
> + test_and_wakeup_notify(memcg, val);
> +#endif
ifdefs-in-c make kernel developers sad.
> /*
> * For keeping hierarchical_reclaim simple, how long we should retry
> * is depends on callers. We set our retry-count to be function
> @@ -2215,6 +2246,147 @@ static int mem_cgroup_swappiness_write(struct cgroup *cgrp, struct cftype *cft,
> return 0;
> }
>
> +#ifdef CONFIG_CGROUP_MEM_NOTIFY
> +#define CGROUP_LOCAL_BUFFER_SIZE 64 /* Would be nice if this was in cgroup.h */
> +
> +/* The resource counters are defined as long long, but few processors
> + * handle 64-bit divisor in hardware, and the software to do it isn't
> + * present in the kernel. It would be nice if the resource counters were
> + * platform specific configurable typedefs, but for now we'll just divide
> + * down the byte counters by the page size to get 32-bit arithmetic.
> + * With a 4K page size, this will work up to about 16384G resource limit.
> + */
> +unsigned long compute_usage_percent(unsigned long long usage,
> + unsigned long long limit)
This is a poor choice of identifier for a global symbol. Please prefix
it with some string which identifies its subsystem.
> +{
> + unsigned long lim;
> + unsigned long long usage_pct;
> +
> + usage_pct = (usage / PAGE_SIZE) * 100;
> + lim = (unsigned long)(limit / PAGE_SIZE);
> +
> + do_div(usage_pct, lim);
> +
> + return (unsigned long)usage_pct;
> +}
> +
> +void test_and_wakeup_notify(struct mem_cgroup *mcg, unsigned long long newlimit)
Ditto.
> +{
> + unsigned long usage_pct;
> +
> + /* Check to see if the new limit should cause notification.
> + */
> + usage_pct = compute_usage_percent(mcg->res.usage, newlimit);
> +
> + if ((usage_pct >= mcg->notify_limit_percent) &&
> + waitqueue_active(&mcg->notify_limit_wait))
> + wake_up(&mcg->notify_limit_wait);
> +}
Also, identifiers such as "newlimit" are a bit unclear because they
don't communicate the units, and (less seriously) they don't
communicate what quantity they are measuring. Something like
memory_newlimit_bytes would be clearer, although a bit silly.
I _assume_ these things are all operating in units of bytes. Perhaps
it was pages? That's my point...
> +static ssize_t notify_limit_read(struct cgroup *cgrp, struct cftype *cft,
> + struct file *file,
> + char __user *buf, size_t nbytes,
> + loff_t *ppos)
> +{
> + struct mem_cgroup *memcg = mem_cgroup_from_cont(cgrp);
> + char tmp[CGROUP_LOCAL_BUFFER_SIZE];
> + int len;
> +
> + len = sprintf(tmp, "%lu\n", memcg->notify_limit_percent);
The reader has to run around the tree to find out if there's a buffer
overflow here. scnprintf() would set minds at ease.
> + return simple_read_from_buffer(buf, nbytes, ppos, tmp, len);
> +}
> +
> +static int notify_limit_write(struct cgroup *cgrp, struct cftype *cft,
> + const char *buffer)
> +{
> + struct mem_cgroup *memcg = mem_cgroup_from_cont(cgrp);
> + unsigned long val;
> + char *endptr;
> +
> + val = simple_strtoul(buffer, &endptr, 0);
strict_strtoul() please. It's stricter.
> + if (val > 100)
> + return -EINVAL;
> +
> + memcg->notify_limit_percent = val;
> +
> + /* Check to see if the new percentage limit should cause notification.
> + */
> + test_and_wakeup_notify(memcg, memcg->res.limit);
> +
> + return 0;
> +}
> +
> +static ssize_t notify_limit_usage_read(struct cgroup *cgrp, struct cftype *cft,
> + struct file *file,
> + char __user *buf, size_t nbytes,
> + loff_t *ppos)
> +{
> + struct mem_cgroup *mem = mem_cgroup_from_cont(cgrp);
> + char tmp[CGROUP_LOCAL_BUFFER_SIZE];
> + unsigned long usage_pct;
> + int len;
> +
> + usage_pct = compute_usage_percent(mem->res.usage, mem->res.limit);
> +
> + len = sprintf(tmp, "%lu\n", usage_pct);
scnprintf()
> + return simple_read_from_buffer(buf, nbytes, ppos, tmp, len);
> +}
> +
> +static ssize_t notify_limit_lowait(struct cgroup *cgrp, struct cftype *cft,
Perhaps this function would benefit from a nice comment explaining its
design. It's the core thing.
Than again, the .txt file has good coverage.
> + struct file *file,
> + char __user *buf, size_t nbytes,
> + loff_t *ppos)
> +{
> + struct mem_cgroup *mem = mem_cgroup_from_cont(cgrp);
> + char tmp[CGROUP_LOCAL_BUFFER_SIZE];
> + unsigned long usage_pct;
> + int len;
> + 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.
> + */
> + usage_pct = compute_usage_percent(mem->res.usage, mem->res.limit);
> + if (likely(mem->res.usage != 0)) {
> +
> + prepare_to_wait(&mem->notify_limit_wait, ¬ify_lowait,
> + TASK_INTERRUPTIBLE);
> +
> + if (usage_pct < mem->notify_limit_percent) {
> + schedule();
> +
> + /* Compute percentage we have now and return it.
> + */
> + usage_pct = compute_usage_percent(mem->res.usage,
> + mem->res.limit);
> + }
> + finish_wait(&mem->notify_limit_wait, ¬ify_lowait);
> + }
> +
> + len = sprintf(tmp, "%lu\n", usage_pct);
scnprintf()
> + return simple_read_from_buffer(buf, nbytes, ppos, tmp, len);
> +}
What is the behavior of this read when someone sends the process a
signal? Seems that it will return early, giving userspace a number
which it didn't expect to see. I guess that's OK.
> +/* 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 notify_limit_wake_em_up(struct cgroup *cgrp, unsigned int event)
> +{
> + struct mem_cgroup *mem;
> +
> + mem = mem_cgroup_from_cont(cgrp);
> + wake_up(&mem->notify_limit_wait);
> + return 0;
> +}
> +#endif /* CONFIG_CGROUP_MEM_NOTIFY */
> +
>
> static struct cftype mem_cgroup_files[] = {
> {
> @@ -2258,6 +2430,22 @@ 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_limit_percent",
> + .write_string = notify_limit_write,
> + .read = notify_limit_read,
> + },
> + {
> + .name = "notify_limit_usage",
> + .read = notify_limit_usage_read,
> + },
> + {
> + .name = "notify_limit_lowait",
> + .trigger = notify_limit_wake_em_up,
> + .read = notify_limit_lowait,
> + },
> +#endif
> };
>
> #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
> @@ -2461,6 +2649,11 @@ 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_limit_wait);
> + mem->notify_limit_percent = 100;
> +#endif
> +
> if (parent)
> mem->swappiness = get_swappiness(parent);
> atomic_set(&mem->refcnt, 1);
> @@ -2504,6 +2697,20 @@ static void mem_cgroup_move_task(struct cgroup_subsys *ss,
> struct cgroup *old_cont,
> struct task_struct *p)
> {
> +#ifdef CONFIG_CGROUP_MEM_NOTIFY
> + /* 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.
> + */
> + if (old_cont->parent != NULL)
> + notify_limit_wake_em_up(old_cont, 0);
> +#endif
> +
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