On Thu, Mar 31, 2022 at 08:41:51AM +0000, Yosry Ahmed wrote: > From: Shakeel Butt <shakeelb@xxxxxxxxxx> > > 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@xxxxxxxxxx/ 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@xxxxxxxxxx: refreshed to current master, updated commit > message based on recent discussions and use cases] > Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx> > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> > --- > 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?
