Messages in this thread | | | Date | Fri, 1 Apr 2022 17:48:59 +0800 | Subject | Re: [PATCH resend] memcg: introduce per-memcg reclaim interface | From | Chen Wandun <> |
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在 2022/4/1 17:20, Yosry Ahmed 写道: > On Thu, Mar 31, 2022 at 8:05 PM Chen Wandun <chenwandun@huawei.com> wrote: >> >> >> 在 2022/3/31 16:41, Yosry Ahmed 写道: >>> 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/ >>> >>> 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. >>> >>> For now, let's keep things simple by adding the basic functionality. >>> >>> [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); >> In some scenario there are lots of page cache, and we only want to >> reclaim page cache, >> how about add may_swap option? > Thanks for taking a look at this! > > The first listed extension is an argument/flags to specify the type of do you mean nbytes in memory_reclaim? it decide the amount of memory to reclaim.
one more argument such as may_swap can be add into memory_reclaim, and pass this argument to try_to_free_mem_cgroup_pages in order to replace the default "true"
Thanks.
> memory that we want to reclaim, I think this covers this use case, or > am I missing something? > >>> + >>> + 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, >>> + }, >>> { } /* terminate */ >>> }; >>> > .
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