Re: [PATCH v4 2/2] memcg: infrastructure to flush memcg stats

Marek Szyprowski
2021-07-16T15:03:58Z

Hi,

On 14.07.2021 03:39, Shakeel Butt wrote:
> At the moment memcg stats are read in four contexts:
>
> 1. memcg stat user interfaces
> 2. dirty throttling
> 3. page fault
> 4. memory reclaim
>
> Currently the kernel flushes the stats for first two cases. Flushing the
> stats for remaining two casese may have performance impact. Always
> flushing the memcg stats on the page fault code path may negatively
> impacts the performance of the applications. In addition flushing in the
> memory reclaim code path, though treated as slowpath, can become the
> source of contention for the global lock taken for stat flushing because
> when system or memcg is under memory pressure, many tasks may enter the
> reclaim path.
>
> This patch uses following mechanisms to solve these challenges:
>
> 1. Periodically flush the stats from root memcg every 2 seconds. This
> will time limit the out of sync stats.
>
> 2. Asynchronously flush the stats after fixed number of stat updates.
> In the worst case the stat can be out of sync by O(nr_cpus * BATCH) for
> 2 seconds.
>
> 3. For avoiding thundering herd to flush the stats particularly from the
> memory reclaim context, introduce memcg local spinlock and let only one
> flusher active at a time. This could have been done through
> cgroup_rstat_lock lock but that lock is used by other subsystem and for
> userspace reading memcg stats. So, it is better to keep flushers
> introduced by this patch decoupled from cgroup_rstat_lock.
>
> Signed-off-by: Shakeel Butt <shakeelb@google.com>

This patch landed in today's linux-next (next-20210716) as commit
42265e014ac7 ("memcg: infrastructure to flush memcg stats"). On my test
system's I found that it triggers a kernel BUG on all ARM64 boards:

BUG: sleeping function called from invalid context at
kernel/cgroup/rstat.c:200
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 7, name:
kworker/u8:0
3 locks held by kworker/u8:0/7:
#0: ffff00004000c938 ((wq_completion)events_unbound){+.+.}-{0:0}, at:
process_one_work+0x200/0x718
#1: ffff80001334bdd0 ((stats_flush_dwork).work){+.+.}-{0:0}, at:
process_one_work+0x200/0x718
#2: ffff8000124f6d40 (stats_flush_lock){+.+.}-{2:2}, at:
mem_cgroup_flush_stats+0x20/0x48
CPU: 2 PID: 7 Comm: kworker/u8:0 Tainted: G        W 5.14.0-rc1+ #3713
Hardware name: Raspberry Pi 4 Model B (DT)
Workqueue: events_unbound flush_memcg_stats_dwork
Call trace:
dump_backtrace+0x0/0x1d0
show_stack+0x14/0x20
dump_stack_lvl+0x88/0xb0
dump_stack+0x14/0x2c
___might_sleep+0x1dc/0x200
__might_sleep+0x4c/0x88
cgroup_rstat_flush+0x2c/0x58
mem_cgroup_flush_stats+0x34/0x48
flush_memcg_stats_dwork+0xc/0x38
process_one_work+0x2a8/0x718
worker_thread+0x48/0x460
kthread+0x12c/0x160
ret_from_fork+0x10/0x18

This can be also reproduced with QEmu. Please let me know if I can help
fixing this issue.

> ---
> Changes since v3:
> - Add back the sigoff
>
> Changes since v2:
> - Changed the subject of the patch
> - Added mechanism to bound errors to nr_cpus instead of nr_cgroups
> - memcg local lock to let one active flusher
>
> Changes since v1:
> - use system_unbound_wq for flushing the memcg stats
>
> include/linux/memcontrol.h | 6 ++++++
> mm/memcontrol.c | 34 ++++++++++++++++++++++++++++++++++
> mm/vmscan.c | 6 ++++++
> 3 files changed, 46 insertions(+)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 0bfa0409af22..fa095a94ae56 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -991,6 +991,8 @@ static inline unsigned long lruvec_page_state_local(struct lruvec *lruvec,
> return x;
> }
>
> +void mem_cgroup_flush_stats(void);
> +
> void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
> int val);
> void __mod_lruvec_kmem_state(void *p, enum node_stat_item idx, int val);
> @@ -1400,6 +1402,10 @@ static inline unsigned long lruvec_page_state_local(struct lruvec *lruvec,
> return node_page_state(lruvec_pgdat(lruvec), idx);
> }
>
> +static inline void mem_cgroup_flush_stats(void)
> +{
> +}
> +
> static inline void __mod_memcg_lruvec_state(struct lruvec *lruvec,
> enum node_stat_item idx, int val)
> {
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 848d711bf576..39a00991fc80 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -103,6 +103,14 @@ static bool do_memsw_account(void)
> return !cgroup_subsys_on_dfl(memory_cgrp_subsys) && !cgroup_memory_noswap;
> }
>
> +/* memcg and lruvec stats flushing */
> +static void flush_memcg_stats_dwork(struct work_struct *w);
> +static DECLARE_DEFERRABLE_WORK(stats_flush_dwork, flush_memcg_stats_dwork);
> +static void flush_memcg_stats_work(struct work_struct *w);
> +static DECLARE_WORK(stats_flush_work, flush_memcg_stats_work);
> +static DEFINE_PER_CPU(unsigned int, stats_flush_threshold);
> +static DEFINE_SPINLOCK(stats_flush_lock);
> +
> #define THRESHOLDS_EVENTS_TARGET 128
> #define SOFTLIMIT_EVENTS_TARGET 1024
>
> @@ -685,6 +693,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
>
> /* Update lruvec */
> __this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
> + if (!(__this_cpu_inc_return(stats_flush_threshold) % MEMCG_CHARGE_BATCH))
> + queue_work(system_unbound_wq, &stats_flush_work);
> }
>
> /**
> @@ -5248,6 +5258,10 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
> /* Online state pins memcg ID, memcg ID pins CSS */
> refcount_set(&memcg->id.ref, 1);
> css_get(css);
> +
> + if (unlikely(mem_cgroup_is_root(memcg)))
> + queue_delayed_work(system_unbound_wq, &stats_flush_dwork,
> + 2UL*HZ);
> return 0;
> }
>
> @@ -5339,6 +5353,26 @@ static void mem_cgroup_css_reset(struct cgroup_subsys_state *css)
> memcg_wb_domain_size_changed(memcg);
> }
>
> +void mem_cgroup_flush_stats(void)
> +{
> + if (!spin_trylock(&stats_flush_lock))
> + return;
> +
> + cgroup_rstat_flush(root_mem_cgroup->css.cgroup);
> + spin_unlock(&stats_flush_lock);
> +}
> +
> +static void flush_memcg_stats_dwork(struct work_struct *w)
> +{
> + mem_cgroup_flush_stats();
> + queue_delayed_work(system_unbound_wq, &stats_flush_dwork, 2UL*HZ);
> +}
> +
> +static void flush_memcg_stats_work(struct work_struct *w)
> +{
> + mem_cgroup_flush_stats();
> +}
> +
> static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu)
> {
> struct mem_cgroup *memcg = mem_cgroup_from_css(css);
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a7602f71ec04..1cc05ab8ca15 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2893,6 +2893,12 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> target_lruvec = mem_cgroup_lruvec(sc->target_mem_cgroup, pgdat);
>
> again:
> + /*
> + * Flush the memory cgroup stats, so that we read accurate per-memcg
> + * lruvec stats for heuristics.
> + */
> + mem_cgroup_flush_stats();
> +
> memset(&sc->nr, 0, sizeof(sc->nr));
>
> nr_reclaimed = sc->nr_reclaimed;

Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland