1 From 0b3d6e6f2dd0a7b697b1aa8c167265908940624b Mon Sep 17 00:00:00 2001
2 From: Greg Thelen <gthelen@google.com>
3 Date: Fri, 5 Apr 2019 18:39:18 -0700
4 Subject: mm: writeback: use exact memcg dirty counts
6 From: Greg Thelen <gthelen@google.com>
8 commit 0b3d6e6f2dd0a7b697b1aa8c167265908940624b upstream.
10 Since commit a983b5ebee57 ("mm: memcontrol: fix excessive complexity in
11 memory.stat reporting") memcg dirty and writeback counters are managed
14 1) per-memcg per-cpu values in range of [-32..32]
16 2) per-memcg atomic counter
18 When a per-cpu counter cannot fit in [-32..32] it's flushed to the
19 atomic. Stat readers only check the atomic. Thus readers such as
20 balance_dirty_pages() may see a nontrivial error margin: 32 pages per
24 4k x86 page_size: 13 MiB error per memcg
25 64k ppc page_size: 200 MiB error per memcg
27 Considering that dirty+writeback are used together for some decisions the
30 This inaccuracy can lead to undeserved oom kills. One nasty case is
31 when all per-cpu counters hold positive values offsetting an atomic
32 negative value (i.e. per_cpu[*]=32, atomic=n_cpu*-32).
33 balance_dirty_pages() only consults the atomic and does not consider
34 throttling the next n_cpu*32 dirty pages. If the file_lru is in the
35 13..200 MiB range then there's absolutely no dirty throttling, which
36 burdens vmscan with only dirty+writeback pages thus resorting to oom
39 It could be argued that tiny containers are not supported, but it's more
40 subtle. It's the amount the space available for file lru that matters.
41 If a container has memory.max-200MiB of non reclaimable memory, then it
42 will also suffer such oom kills on a 100 cpu machine.
44 The following test reliably ooms without this patch. This patch avoids
48 mount -t cgroup2 none /dev/cgroup
50 echo +io +memory > cgroup.subtree_control
54 (echo $BASHPID > cgroup.procs && exec /memcg-writeback-stress /foo)
55 (echo $BASHPID > cgroup.procs && exec dd if=/dev/zero of=/foo bs=2M count=100)
57 $ cat memcg-writeback-stress.c
59 * Dirty pages from all but one cpu.
60 * Clean pages from the non dirtying cpu.
61 * This is to stress per cpu counter imbalance.
62 * On a 100 cpu machine:
63 * - per memcg per cpu dirty count is 32 pages for each of 99 cpus
64 * - per memcg atomic is -99*32 pages
65 * - thus the complete dirty limit: sum of all counters 0
66 * - balance_dirty_pages() only sees atomic count -99*32 pages, which
68 * - So a workload can dirty -99*32 pages before balance_dirty_pages()
78 #include <sys/sysinfo.h>
79 #include <sys/types.h>
85 static void set_affinity(int cpu)
90 CPU_SET(cpu, &affinity);
91 if (sched_setaffinity(0, sizeof(affinity), &affinity))
92 err(1, "sched_setaffinity");
95 static void dirty_on(int output_fd, int cpu)
100 for (i = 0; i < 32; i++) {
101 for (wrote = 0; wrote < bufSize; ) {
102 int ret = write(output_fd, buf+wrote, bufSize-wrote);
110 int main(int argc, char **argv)
112 int cpu, flush_cpu = 1, output_fd;
116 errx(1, "usage: output_file");
119 bufSize = getpagesize();
120 buf = malloc(getpagesize());
122 errx(1, "malloc failed");
124 output_fd = open(output, O_CREAT|O_RDWR);
126 err(1, "open(%s)", output);
128 for (cpu = 0; cpu < get_nprocs(); cpu++) {
129 if (cpu != flush_cpu)
130 dirty_on(output_fd, cpu);
133 set_affinity(flush_cpu);
134 if (fsync(output_fd))
135 err(1, "fsync(%s)", output);
136 if (close(output_fd))
137 err(1, "close(%s)", output);
141 Make balance_dirty_pages() and wb_over_bg_thresh() work harder to
142 collect exact per memcg counters. This avoids the aforementioned oom
145 This does not affect the overhead of memory.stat, which still reads the
146 single atomic counter.
148 Why not use percpu_counter? memcg already handles cpus going offline, so
149 no need for that overhead from percpu_counter. And the percpu_counter
150 spinlocks are more heavyweight than is required.
152 It probably also makes sense to use exact dirty and writeback counters
153 in memcg oom reports. But that is saved for later.
155 Link: http://lkml.kernel.org/r/20190329174609.164344-1-gthelen@google.com
156 Signed-off-by: Greg Thelen <gthelen@google.com>
157 Reviewed-by: Roman Gushchin <guro@fb.com>
158 Acked-by: Johannes Weiner <hannes@cmpxchg.org>
159 Cc: Michal Hocko <mhocko@kernel.org>
160 Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
161 Cc: Tejun Heo <tj@kernel.org>
162 Cc: <stable@vger.kernel.org> [4.16+]
163 Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
164 Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
165 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
168 include/linux/memcontrol.h | 5 ++++-
169 mm/memcontrol.c | 20 ++++++++++++++++++--
170 2 files changed, 22 insertions(+), 3 deletions(-)
172 --- a/include/linux/memcontrol.h
173 +++ b/include/linux/memcontrol.h
174 @@ -559,7 +559,10 @@ struct mem_cgroup *lock_page_memcg(struc
175 void __unlock_page_memcg(struct mem_cgroup *memcg);
176 void unlock_page_memcg(struct page *page);
178 -/* idx can be of type enum memcg_stat_item or node_stat_item */
180 + * idx can be of type enum memcg_stat_item or node_stat_item.
181 + * Keep in sync with memcg_exact_page_state().
183 static inline unsigned long memcg_page_state(struct mem_cgroup *memcg,
186 --- a/mm/memcontrol.c
187 +++ b/mm/memcontrol.c
188 @@ -3897,6 +3897,22 @@ struct wb_domain *mem_cgroup_wb_domain(s
189 return &memcg->cgwb_domain;
193 + * idx can be of type enum memcg_stat_item or node_stat_item.
194 + * Keep in sync with memcg_exact_page().
196 +static unsigned long memcg_exact_page_state(struct mem_cgroup *memcg, int idx)
198 + long x = atomic_long_read(&memcg->stat[idx]);
201 + for_each_online_cpu(cpu)
202 + x += per_cpu_ptr(memcg->stat_cpu, cpu)->count[idx];
209 * mem_cgroup_wb_stats - retrieve writeback related stats from its memcg
210 * @wb: bdi_writeback in question
211 @@ -3922,10 +3938,10 @@ void mem_cgroup_wb_stats(struct bdi_writ
212 struct mem_cgroup *memcg = mem_cgroup_from_css(wb->memcg_css);
213 struct mem_cgroup *parent;
215 - *pdirty = memcg_page_state(memcg, NR_FILE_DIRTY);
216 + *pdirty = memcg_exact_page_state(memcg, NR_FILE_DIRTY);
218 /* this should eventually include NR_UNSTABLE_NFS */
219 - *pwriteback = memcg_page_state(memcg, NR_WRITEBACK);
220 + *pwriteback = memcg_exact_page_state(memcg, NR_WRITEBACK);
221 *pfilepages = mem_cgroup_nr_lru_pages(memcg, (1 << LRU_INACTIVE_FILE) |
222 (1 << LRU_ACTIVE_FILE));
223 *pheadroom = PAGE_COUNTER_MAX;