Re: [PATCH resend] memcg: introduce per-memcg reclaim interface

Roman Gushchin
2022-03-31T17:25:37Z

On Thu, Mar 31, 2022 at 08:41:51AM +0000, Yosry Ahmed wrote:
> From: Shakeel Butt <shakeelb@google.com>
>
> Introduce an memcg interface to trigger memory reclaim on a memory cgroup.
>
> Use case: Proactive Reclaim
> ---------------------------
>
> A userspace proactive reclaimer can continuously probe the memcg to
> reclaim a small amount of memory. This gives more accurate and
> up-to-date workingset estimation as the LRUs are continuously
> sorted and can potentially provide more deterministic memory
> overcommit behavior. The memory overcommit controller can provide
> more proactive response to the changing behavior of the running
> applications instead of being reactive.
>
> A userspace reclaimer's purpose in this case is not a complete replacement
> for kswapd or direct reclaim, it is to proactively identify memory savings
> opportunities and reclaim some amount of cold pages set by the policy
> to free up the memory for more demanding jobs or scheduling new jobs.
>
> A user space proactive reclaimer is used in Google data centers.
> Additionally, Meta's TMO paper recently referenced a very similar
> interface used for user space proactive reclaim:
> https://dl.acm.org/doi/pdf/10.1145/3503222.3507731
>
> Benefits of a user space reclaimer:
> -----------------------------------
>
> 1) More flexible on who should be charged for the cpu of the memory
> reclaim. For proactive reclaim, it makes more sense to be centralized.
>
> 2) More flexible on dedicating the resources (like cpu). The memory
> overcommit controller can balance the cost between the cpu usage and
> the memory reclaimed.
>
> 3) Provides a way to the applications to keep their LRUs sorted, so,
> under memory pressure better reclaim candidates are selected. This also
> gives more accurate and uptodate notion of working set for an
> application.
>
> Why memory.high is not enough?
> ------------------------------
>
> - memory.high can be used to trigger reclaim in a memcg and can
> potentially be used for proactive reclaim.
> However there is a big downside in using memory.high. It can potentially
> introduce high reclaim stalls in the target application as the
> allocations from the processes or the threads of the application can hit
> the temporary memory.high limit.
>
> - Userspace proactive reclaimers usually use feedback loops to decide
> how much memory to proactively reclaim from a workload. The metrics
> used for this are usually either refaults or PSI, and these metrics
> will become messy if the application gets throttled by hitting the
> high limit.
>
> - memory.high is a stateful interface, if the userspace proactive
> reclaimer crashes for any reason while triggering reclaim it can leave
> the application in a bad state.
>
> - If a workload is rapidly expanding, setting memory.high to proactively
> reclaim memory can result in actually reclaiming more memory than
> intended.
>
> The benefits of such interface and shortcomings of existing interface
> were further discussed in this RFC thread:
> https://lore.kernel.org/linux-mm/5df21376-7dd1-bf81-8414-32a73cea45dd@google.com/

Hello!

I'm totally up for the proposed feature! It makes total sense and is proved
to be useful, let's add it.

>
> Interface:
> ----------
>
> Introducing a very simple memcg interface 'echo 10M > memory.reclaim' to
> trigger reclaim in the target memory cgroup.
>
>
> Possible Extensions:
> --------------------
>
> - This interface can be extended with an additional parameter or flags
> to allow specifying one or more types of memory to reclaim from (e.g.
> file, anon, ..).
>
> - The interface can also be extended with a node mask to reclaim from
> specific nodes. This has use cases for reclaim-based demotion in memory
> tiering systens.
>
> - A similar per-node interface can also be added to support proactive
> reclaim and reclaim-based demotion in systems without memcg.

Maybe an option to specify a timeout? That might simplify the userspace part.
Also, please please add a test to selftests/cgroup/memcg tests.
It will also provide an example on how the userspace can use the feature.

>
> For now, let's keep things simple by adding the basic functionality.

What I'm worried about is how we gonna extend it? How do you see the interface
with 2-3 extensions from the list above? All these extensions look very
reasonable to me, so we'll likely have to implement them soon. So let's think
about the extensibility now.

I wonder if it makes more sense to introduce a sys_reclaim() syscall instead?
In the end, such a feature might make sense on the system level too.
Yes, there is the drop_caches sysctl, but it's too radical for many cases.

>
> [yosryahmed@google.com: refreshed to current master, updated commit
> message based on recent discussions and use cases]
> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> ---
> Documentation/admin-guide/cgroup-v2.rst | 9 ++++++
> mm/memcontrol.c | 37 +++++++++++++++++++++++++
> 2 files changed, 46 insertions(+)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 69d7a6983f78..925aaabb2247 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1208,6 +1208,15 @@ PAGE_SIZE multiple when read back.
> high limit is used and monitored properly, this limit's
> utility is limited to providing the final safety net.
>
> + memory.reclaim
> + A write-only file which exists on non-root cgroups.
> +
> + This is a simple interface to trigger memory reclaim in the
> + target cgroup. Write the number of bytes to reclaim to this
> + file and the kernel will try to reclaim that much memory.
> + Please note that the kernel can over or under reclaim from
> + the target cgroup.
> +
> memory.oom.group
> A read-write single value file which exists on non-root
> cgroups. The default value is "0".
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 725f76723220..994849fab7df 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6355,6 +6355,38 @@ static ssize_t memory_oom_group_write(struct kernfs_open_file *of,
> return nbytes;
> }
>
> +static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
> + size_t nbytes, loff_t off)
> +{
> + struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
> + unsigned int nr_retries = MAX_RECLAIM_RETRIES;
> + unsigned long nr_to_reclaim, nr_reclaimed = 0;
> + int err;
> +
> + buf = strstrip(buf);
> + err = page_counter_memparse(buf, "", &nr_to_reclaim);
> + if (err)
> + return err;
> +
> + while (nr_reclaimed < nr_to_reclaim) {
> + unsigned long reclaimed;
> +
> + if (signal_pending(current))
> + break;
> +
> + reclaimed = try_to_free_mem_cgroup_pages(memcg,
> + nr_to_reclaim - nr_reclaimed,
> + GFP_KERNEL, true);
> +
> + if (!reclaimed && !nr_retries--)
> + break;
> +
> + nr_reclaimed += reclaimed;
> + }
> +
> + return nbytes;
> +}
> +
> static struct cftype memory_files[] = {
> {
> .name = "current",
> @@ -6413,6 +6445,11 @@ static struct cftype memory_files[] = {
> .seq_show = memory_oom_group_show,
> .write = memory_oom_group_write,
> },
> + {
> + .name = "reclaim",
> + .flags = CFTYPE_NOT_ON_ROOT | CFTYPE_NS_DELEGATABLE,
> + .write = memory_reclaim,

Btw, why not on root?