From: Shakeel Butt Date: Sat, 19 Apr 2025 18:35:45 +0000 (-0700) Subject: memcg: introduce non-blocking limit setting option X-Git-Tag: v6.16-rc1~92^2~123 X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=c8e6002bd611c68dab892565039b60c7ad5b8d6a;p=thirdparty%2Flinux.git memcg: introduce non-blocking limit setting option Setting the max and high limits can trigger synchronous reclaim and/or oom-kill if the usage is higher than the given limit. This behavior is fine for newly created cgroups but it can cause issues for the node controller while setting limits for existing cgroups. In our production multi-tenant and overcommitted environment, we are seeing priority inversion when the node controller dynamically adjusts the limits of running jobs of different priorities. Based on the system situation, the node controller may reduce the limits of lower priority jobs and increase the limits of higher priority jobs. However we are seeing node controller getting stuck for long period of time while reclaiming from lower priority jobs while setting their limits and also spends a lot of its own CPU. One of the workaround we are trying is to fork a new process which sets the limit of the lower priority job along with setting an alarm to get itself killed if it get stuck in the reclaim for lower priority job. However we are finding it very unreliable and costly. Either we need a good enough time buffer for the alarm to be delivered after setting limit and potentialy spend a lot of CPU in the reclaim or be unreliable in setting the limit for much shorter but cheaper (less reclaim) alarms. Let's introduce new limit setting option which does not trigger reclaim and/or oom-kill and let the processes in the target cgroup to trigger reclaim and/or throttling and/or oom-kill in their next charge request. This will make the node controller on multi-tenant overcommitted environment much more reliable. Explanation from Johannes on side-effects of O_NONBLOCK limit change: It's usually the allocating tasks inside the group bearing the cost of limit enforcement and reclaim. This allows a (privileged) updater from outside the group to keep that cost in there - instead of having to help, from a context that doesn't necessarily make sense. I suppose the tradeoff with that - and the reason why this was doing sync reclaim in the first place - is that, if the group is idle and not trying to allocate more, it can take indefinitely for the new limit to actually be met. It should be okay in most scenarios in practice. As the capacity is reallocated from group A to B, B will exert pressure on A once it tries to claim it and thereby shrink it down. If A is idle, that shouldn't be hard. If A is running, it's likely to fault/allocate soon-ish and then join the effort. It does leave a (malicious) corner case where A is just busy-hitting its memory to interfere with the clawback. This is comparable to reclaiming memory.low overage from the outside, though, which is an acceptable risk. Users of O_NONBLOCK just need to be aware. Link: https://lkml.kernel.org/r/20250419183545.1982187-1-shakeel.butt@linux.dev Signed-off-by: Shakeel Butt Acked-by: Roman Gushchin Acked-by: Johannes Weiner Acked-by: Michal Hocko Cc: Greg Thelen Cc: Michal Koutný Cc: Muchun Song Cc: Tejun Heo Cc: Christian Brauner Cc: Yosry Ahmed Signed-off-by: Andrew Morton --- diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst index 1a16ce68a4d7f..b34f1dd969e01 100644 --- a/Documentation/admin-guide/cgroup-v2.rst +++ b/Documentation/admin-guide/cgroup-v2.rst @@ -1299,6 +1299,13 @@ PAGE_SIZE multiple when read back. monitors the limited cgroup to alleviate heavy reclaim pressure. + If memory.high is opened with O_NONBLOCK then the synchronous + reclaim is bypassed. This is useful for admin processes that + need to dynamically adjust the job's memory limits without + expending their own CPU resources on memory reclamation. The + job will trigger the reclaim and/or get throttled on its + next charge request. + memory.max A read-write single value file which exists on non-root cgroups. The default is "max". @@ -1316,6 +1323,13 @@ PAGE_SIZE multiple when read back. Caller could retry them differently, return into userspace as -ENOMEM or silently ignore in cases like disk readahead. + If memory.max is opened with O_NONBLOCK, then the synchronous + reclaim and oom-kill are bypassed. This is useful for admin + processes that need to dynamically adjust the job's memory limits + without expending their own CPU resources on memory reclamation. + The job will trigger the reclaim and/or oom-kill on its next + charge request. + memory.reclaim A write-only nested-keyed file which exists for all cgroups. diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 8ed265852423f..d3b6f50e00d47 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -4269,6 +4269,9 @@ static ssize_t memory_high_write(struct kernfs_open_file *of, page_counter_set_high(&memcg->memory, high); + if (of->file->f_flags & O_NONBLOCK) + goto out; + for (;;) { unsigned long nr_pages = page_counter_read(&memcg->memory); unsigned long reclaimed; @@ -4291,7 +4294,7 @@ static ssize_t memory_high_write(struct kernfs_open_file *of, if (!reclaimed && !nr_retries--) break; } - +out: memcg_wb_domain_size_changed(memcg); return nbytes; } @@ -4318,6 +4321,9 @@ static ssize_t memory_max_write(struct kernfs_open_file *of, xchg(&memcg->memory.max, max); + if (of->file->f_flags & O_NONBLOCK) + goto out; + for (;;) { unsigned long nr_pages = page_counter_read(&memcg->memory); @@ -4345,7 +4351,7 @@ static ssize_t memory_max_write(struct kernfs_open_file *of, break; cond_resched(); } - +out: memcg_wb_domain_size_changed(memcg); return nbytes; }