Re: [PATCH v3] memcg: Add memory.pressure_level events

Andrew Morton
2013-03-26T20:47:00Z

On Fri, 22 Mar 2013 00:13:51 -0700 Anton Vorontsov <anton.vorontsov@linaro.org> wrote:

> With this patch userland applications that want to maintain the
> interactivity/memory allocation cost can use the pressure level
> notifications. The levels are defined like this:
>
> The "low" level means that the system is reclaiming memory for new
> allocations. Monitoring this reclaiming activity might be useful for
> maintaining cache level. Upon notification, the program (typically
> "Activity Manager") might analyze vmstat and act in advance (i.e.
> prematurely shutdown unimportant services).
>
> The "medium" level means that the system is experiencing medium memory
> pressure, the system might be making swap, paging out active file caches,
> etc. Upon this event applications may decide to further analyze
> vmstat/zoneinfo/memcg or internal memory usage statistics and free any
> resources that can be easily reconstructed or re-read from a disk.
>
> The "critical" level means that the system is actively thrashing, it is
> about to out of memory (OOM) or even the in-kernel OOM killer is on its
> 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.)
>
> Performance wise, the memory pressure notifications feature itself is
> lightweight and does not require much of bookkeeping, in contrast to the
> rest of memcg features. Unfortunately, as of current memcg implementation,
> pages accounting is an inseparable part and cannot be turned off. The good
> news is that there are some efforts[1] to improve the situation; plus,
> implementing the same, fully API-compatible[2] interface for
> CONFIG_MEMCG=n case (e.g. embedded) is also a viable option, so it will
> not require any changes on the userland side.

Nicely presented patch, thanks.

The interface still seems a bit of a toy, but is a conservative
approach: anything less toy-like would expose (and hence be dependent
upon) more VM internals.

> --- a/Documentation/cgroups/memory.txt
> +++ b/Documentation/cgroups/memory.txt
> @@ -40,6 +40,7 @@ Features:
> - soft limit
> - moving (recharging) account at moving a task is selectable.
> - usage threshold notifier
> + - memory pressure notifier
> - oom-killer disable knob and oom-notifier
> - Root cgroup has no limit controls.
>
> @@ -65,6 +66,7 @@ Brief summary of control files.
> memory.stat # show various statistics
> memory.use_hierarchy # set/show hierarchical account enabled
> memory.force_empty # trigger forced move charge to parent
> + memory.pressure_level # set memory pressure notifications
> memory.swappiness # set/show swappiness parameter of vmscan
> (See sysctl's vm.swappiness)
> memory.move_charge_at_immigrate # set/show controls of moving charges
> @@ -778,7 +780,64 @@ At reading, current status of OOM is shown.
> under_oom 0 or 1 (if 1, the memory cgroup is under OOM, tasks may
> be stopped.)
>
> -11. TODO
> +11. Memory Pressure
> +
> +The pressure level notifications can be used to monitor the memory
> +allocation cost; based on the pressure, applications can implement
> +different strategies of managing their memory resources. The pressure
> +levels are defined as following:
> +
> +The "low" level means that the system is reclaiming memory for new
> +allocations. Monitoring this reclaiming activity might be useful for
> +maintaining cache level. Upon notification, the program (typically
> +"Activity Manager") might analyze vmstat and act in advance (i.e.
> +prematurely shutdown unimportant services).
> +
> +The "medium" level means that the system is experiencing medium memory
> +pressure, the system might be making swap, paging out active file caches,
> +etc. Upon this event applications may decide to further analyze
> +vmstat/zoneinfo/memcg or internal memory usage statistics and free any
> +resources that can be easily reconstructed or re-read from a disk.
> +
> +The "critical" level means that the system is actively thrashing, it is
> +about to out of memory (OOM) or even the in-kernel OOM killer is on its
> +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.)
> +
> +The file memory.pressure_level is only used to setup an eventfd,
> +read/write operations are no implemented.
> +
> +Test:
> +
> + Here is a small script example that makes a new cgroup, sets up a
> + memory limit, sets up a notification in the cgroup and then makes child
> + cgroup experience a critical pressure:
> +
> + # cd /sys/fs/cgroup/memory/
> + # mkdir foo
> + # cd foo
> + # cgroup_event_listener memory.pressure_level low &
> + # echo 8000000 > memory.limit_in_bytes
> + # echo 8000000 > memory.memsw.limit_in_bytes
> + # echo $$ > tasks
> + # dd if=/dev/zero | read x
> +
> + (Expect a bunch of notifications, and eventually, the oom-killer will
> + trigger.)
> +
> +12. TODO

Did we tell people how to use the eventfd interface anywhere?

> 1. Add support for accounting huge pages (as a separate controller)
> 2. Make per-cgroup scanner reclaim not-shared pages first
> diff --git a/include/linux/vmpressure.h b/include/linux/vmpressure.h
> --- /dev/null
> +++ b/include/linux/vmpressure.h
> @@ -0,0 +1,47 @@
> +#ifndef __LINUX_VMPRESSURE_H
> +#define __LINUX_VMPRESSURE_H
> +
> +#include <linux/mutex.h>
> +#include <linux/list.h>
> +#include <linux/workqueue.h>
> +#include <linux/gfp.h>
> +#include <linux/types.h>
> +#include <linux/cgroup.h>
> +
> +struct vmpressure {
> + unsigned int scanned;
> + unsigned int reclaimed;
> + /* The lock is used to keep the scanned/reclaimed above in sync. */
> + struct mutex sr_lock;
> +
> + struct list_head events;

A comment describing what goes at `events' would be nice. Reference
"struct vmpressure_event".

> + /* Have to grab the lock on events traversal or modifications. */
> + struct mutex events_lock;
> +
> + struct work_struct work;
> +};
>
> ...
>
> +/*
> + * The window size is the number of scanned pages before we try to analyze
> + * the scanned/reclaimed ratio (or difference).
> + *
> + * It is used as a rate-limit tunable for the "low" level notification,
> + * and for averaging medium/critical levels. Using small window sizes can
> + * cause lot of false positives, but too big window size will delay the
> + * notifications.
> + *
> + * TODO: Make the window size depend on machine size, as we do for vmstat
> + * thresholds.

Here "the window size" refers to vmpressure_win, yes?

> + */
> +static const unsigned int vmpressure_win = SWAP_CLUSTER_MAX * 16;
> +static const unsigned int vmpressure_level_med = 60;
> +static const unsigned int vmpressure_level_critical = 95;
> +static const unsigned int vmpressure_level_critical_prio = 3;

vmpressure_level_critical_prio is a bit mysterious and undocumented.
Please document it here and/or at vmpressure_prio().

> +
> +enum vmpressure_levels {
> + VMPRESSURE_LOW = 0,
> + VMPRESSURE_MEDIUM,
> + VMPRESSURE_CRITICAL,
> + VMPRESSURE_NUM_LEVELS,
> +};
>
> ...
>
> +static enum vmpressure_levels vmpressure_calc_level(unsigned int scanned,
> + unsigned int reclaimed)
> +{
> + unsigned long scale = scanned + reclaimed;
> + unsigned long pressure;
> +
> + if (!scanned)
> + return VMPRESSURE_LOW;
> +
> + /*
> + * We calculate the ratio (in percents) of how many pages were
> + * scanned vs. reclaimed in a given time frame (window). Note that
> + * time is in VM reclaimer's "ticks", i.e. number of pages
> + * scanned. This makes it possible to set desired reaction time
> + * and serves as a ratelimit.
> + */
> + pressure = scale - (reclaimed * scale / scanned);
> + pressure = pressure * 100 / scale;
> +
> + pr_debug("%s: %3lu (s: %6u r: %6u)\n", __func__, pressure,
> + scanned, reclaimed);
> +
> + return vmpressure_level(pressure);
> +}

The types worry me. The patch switches the vm's unsigned longs into
unsigneds. We have a long and sorry history of overflowing 32-bit
counters in VM corner cases. I suggest that this patch should
carefully use `unsigned longs' in all the appropriate places and do
away with its present truncation and overflow risks.

> +void vmpressure(gfp_t gfp, struct mem_cgroup *memcg,
> + unsigned long scanned, unsigned long reclaimed)

Exported function and a primary inteface. Needs nice documentation, please ;)

> +{
> + struct vmpressure *vmpr = memcg_to_vmpr(memcg);
> +
> + /*
> + * So far we are only interested application memory, or, in case

"interested in"

> + * of low pressure, in FS/IO memory reclaim. We are also
> + * interested indirect reclaim (kswapd sets sc->gfp_mask to
> + * GFP_KERNEL).

This is all pretty obvious from reading the code. Good comments
explain "why", not "what". *why* did we make these decisions?

> + */
> + if (!(gfp & (__GFP_HIGHMEM | __GFP_MOVABLE | __GFP_IO | __GFP_FS)))

I'm surprised at __GFP_HIGHMEM's inclusion. On some machines the great
majority of user memory is in highmem. What's up?

> + return;
> +
> + if (!scanned)
> + return;
> +
> + mutex_lock(&vmpr->sr_lock);
> + vmpr->scanned += scanned;
> + vmpr->reclaimed += reclaimed;

See, here we're accumulating into a 32-bit variable quantities which used
to be held in 64-bit variables. The overflow risk gets higher...

> + mutex_unlock(&vmpr->sr_lock);
> +
> + if (scanned < vmpressure_win || work_pending(&vmpr->work))
> + return;
> + schedule_work(&vmpr->work);
> +}
> +
> +void vmpressure_prio(gfp_t gfp, struct mem_cgroup *memcg, int prio)

Documentation please.

> +{
> + if (prio > vmpressure_level_critical_prio)
> + return;
> +
> + /*
> + * OK, the prio is below the threshold, updating vmpressure

But you never told me what that threshold is for! And I have no means
of working out why you chose "3", nor the effects of altering it, etc.

> + * information before diving into long shrinking of long range
> + * vmscan.
> + */
> + vmpressure(gfp, memcg, vmpressure_win, 0);
> +}
> +
> +static struct vmpressure *wk_to_vmpr(struct work_struct *wk)
> +{
> + return container_of(wk, struct vmpressure, work);
> +}
> +
> +static struct vmpressure *cg_to_vmpr(struct cgroup *cg)
> +{
> + return css_to_vmpr(cgroup_subsys_state(cg, mem_cgroup_subsys_id));
> +}
> +
> +struct vmpressure_event {
> + struct eventfd_ctx *efd;
> + enum vmpressure_levels level;
> + struct list_head node;
> +};
> +
> +static bool vmpressure_event(struct vmpressure *vmpr,
> + unsigned long scanned, unsigned long reclaimed)
> +{
> + struct vmpressure_event *ev;
> + int level = vmpressure_calc_level(scanned, reclaimed);

Here's where we go from 64-bit to 32-bit.

> + bool signalled = 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;
> + }
> + }
> +
> + mutex_unlock(&vmpr->events_lock);
> +
> + return signalled;
> +}
>
> ...
>
> +int vmpressure_register_event(struct cgroup *cg, struct cftype *cft,
> + struct eventfd_ctx *eventfd, const char *args)

Document the interface, please.

> +{
> + struct vmpressure *vmpr = cg_to_vmpr(cg);
> + struct vmpressure_event *ev;
> + int lvl;

These abbreviations are rather unlinuxy. wk->work, vmpr->vmpressure,
lvl->level, etc.

> + for (lvl = 0; lvl < VMPRESSURE_NUM_LEVELS; lvl++) {
> + if (!strcmp(vmpressure_str_levels[lvl], args))
> + break;
> + }
> +
> + if (lvl >= VMPRESSURE_NUM_LEVELS)
> + return -EINVAL;
> +
> + ev = kzalloc(sizeof(*ev), GFP_KERNEL);
> + if (!ev)
> + return -ENOMEM;
> +
> + ev->efd = eventfd;
> + ev->level = lvl;
> +
> + mutex_lock(&vmpr->events_lock);
> + list_add(&ev->node, &vmpr->events);

What's the upper bound on the length of this list?

> + mutex_unlock(&vmpr->events_lock);
> +
> + return 0;
> +}
> +
>
> ...
>