On Thu, Mar 31, 2022 at 10:25 AM Roman Gushchin <roman.gushchin@xxxxxxxxx> wrote: > > 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. A timeout is a good idea. I think it can be added as an extension, similar to other extensions. > 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. +1 > > > > 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. For the first two extensions (flags and nodemask), they can be implemented as additional positional arguments of memory.reclaim. The non-memcg use cases will need a different interface, which can be either a sysfs file or a syscall. > 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. sys_reclaim() syscall is a good proposal for non-memcg use cases. But for memcg-based proactive reclaim, memory.reclaim should be more natural. It is not common to have cgroup as a syscall argument. > > > > [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?
