On Fri, Jul 19, 2024 at 6:06 AM Jesper Dangaard Brouer <hawk@xxxxxxxxxx> wrote:
>
> This patch reintroduces and generalizes the "stats_flush_ongoing" concept
> to avoid redundant flushes if there is an ongoing flush, addressing lock
> contention issues on the global cgroup rstat lock.
>
> At Cloudflare, we observed significant performance degradation due to
> lock contention on the rstat lock, primarily caused by kswapd. The
> specific mem_cgroup_flush_stats() call inlined in shrink_node, which
> takes the rstat lock, is particularly problematic.
>
> On our 12 NUMA node machines, each with a kswapd kthread per NUMA node, we
> noted severe lock contention on the rstat lock, causing 12 CPUs to waste
> cycles spinning every time kswapd runs. Fleet-wide stats (/proc/N/schedstat)
> for kthreads revealed that we are burning an average of 20,000 CPU cores
> fleet-wide on kswapd, primarily due to spinning on the rstat lock.
>
> Here's a brief overview of the issue:
> - __alloc_pages_slowpath calls wake_all_kswapds, causing all kswapdN threads
> to wake up simultaneously.
> - The kswapd thread invokes shrink_node (via balance_pgdat), triggering the
> cgroup rstat flush operation as part of its work.
> - balance_pgdat() has a NULL value in target_mem_cgroup, causing
> mem_cgroup_flush_stats() to flush with root_mem_cgroup.
>
> The kernel previously addressed this with a "stats_flush_ongoing" concept,
> which was removed in commit 7d7ef0a4686a ("mm: memcg: restore subtree stats
> flushing"). This patch reintroduces and generalizes the concept to apply to
> all users of cgroup rstat, not just memcg.
>
> If there is an ongoing rstat flush and the current cgroup is a descendant, a
> new flush is unnecessary. To ensure callers still receive updated stats,
> they wait for the ongoing flush to complete before returning, but with a
> timeout, as stats may already be inaccurate due to continuous updates.
>
> Lock yielding causes complications for ongoing flushers. Therefore, we limit
> which cgroup can become ongoing flusher to top-level, as lock yielding
> allows others to obtain the lock without being the ongoing flusher, leading
> to a situation where a cgroup that isn't a descendant obtains the lock via
> yielding. Thus, we prefer an ongoing flusher with many descendants. If and
> when the lock yielding is removed, such as when changing this to a mutex,
> we can simplify this code.
>
> This change significantly reduces lock contention, especially in
> environments with multiple NUMA nodes, thereby improving overall system
> performance.
>
> Fixes: 7d7ef0a4686a ("mm: memcg: restore subtree stats flushing").
> Signed-off-by: Jesper Dangaard Brouer <hawk@xxxxxxxxxx>
> ---
> V8:
> - Updated subject+desc based on Yosry's feedback
> - Explain lock yielding challenges in comments
> - Limit ongoing flushers to cgrp level 0 and 1
>
> V7: https://lore.kernel.org/all/172070450139.2992819.13210624094367257881.stgit@firesoul
> V6: https://lore.kernel.org/all/172052399087.2357901.4955042377343593447.stgit@firesoul/
> V5: https://lore.kernel.org/all/171956951930.1897969.8709279863947931285.stgit@firesoul/
> V4: https://lore.kernel.org/all/171952312320.1810550.13209360603489797077.stgit@firesoul/
> V3: https://lore.kernel.org/all/171943668946.1638606.1320095353103578332.stgit@firesoul/
> V2: https://lore.kernel.org/all/171923011608.1500238.3591002573732683639.stgit@firesoul/
> V1: https://lore.kernel.org/all/171898037079.1222367.13467317484793748519.stgit@firesoul/
> RFC: https://lore.kernel.org/all/171895533185.1084853.3033751561302228252.stgit@firesoul/
>
> include/linux/cgroup-defs.h | 2 +
> kernel/cgroup/rstat.c | 114 ++++++++++++++++++++++++++++++++++++++-----
> 2 files changed, 104 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
> index b36690ca0d3f..a33b37514c29 100644
> --- a/include/linux/cgroup-defs.h
> +++ b/include/linux/cgroup-defs.h
> @@ -548,6 +548,8 @@ struct cgroup {
> #ifdef CONFIG_BPF_SYSCALL
> struct bpf_local_storage __rcu *bpf_cgrp_storage;
> #endif
> + /* completion queue for cgrp_rstat_ongoing_flusher */
> + struct completion flush_done;
>
> /* All ancestors including self */
> struct cgroup *ancestors[];
> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
> index fb8b49437573..eaa138f2da2f 100644
> --- a/kernel/cgroup/rstat.c
> +++ b/kernel/cgroup/rstat.c
> @@ -2,6 +2,7 @@
> #include "cgroup-internal.h"
>
> #include <linux/sched/cputime.h>
> +#include <linux/completion.h>
>
> #include <linux/bpf.h>
> #include <linux/btf.h>
> @@ -11,6 +12,7 @@
>
> static DEFINE_SPINLOCK(cgroup_rstat_lock);
> static DEFINE_PER_CPU(raw_spinlock_t, cgroup_rstat_cpu_lock);
> +static struct cgroup *cgrp_rstat_ongoing_flusher = NULL;
>
> static void cgroup_base_stat_flush(struct cgroup *cgrp, int cpu);
>
> @@ -279,17 +281,32 @@ __bpf_hook_end();
> * value -1 is used when obtaining the main lock else this is the CPU
> * number processed last.
> */
> -static inline void __cgroup_rstat_lock(struct cgroup *cgrp, int cpu_in_loop)
> +static inline bool __cgroup_rstat_trylock(struct cgroup *cgrp, int cpu_in_loop)
> +{
> + bool locked;
> +
> + locked = spin_trylock_irq(&cgroup_rstat_lock);
> + if (!locked)
> + trace_cgroup_rstat_lock_contended(cgrp, cpu_in_loop, true);
> + else
> + trace_cgroup_rstat_locked(cgrp, cpu_in_loop, false);
> +
> + return locked;
> +}
> +
> +static inline void __cgroup_rstat_lock(struct cgroup *cgrp, int cpu_in_loop,
> + bool already_contended)
> __acquires(&cgroup_rstat_lock)
> {
> - bool contended;
> + bool locked = false;
>
> - contended = !spin_trylock_irq(&cgroup_rstat_lock);
> - if (contended) {
> - trace_cgroup_rstat_lock_contended(cgrp, cpu_in_loop, contended);
> + if (already_contended) /* Skip trylock if already contended */
> + locked = __cgroup_rstat_trylock(cgrp, cpu_in_loop);
Should this be the other way around?
> +
> + if (!locked) {
> spin_lock_irq(&cgroup_rstat_lock);
> + trace_cgroup_rstat_locked(cgrp, cpu_in_loop, true);
> }
> - trace_cgroup_rstat_locked(cgrp, cpu_in_loop, contended);
> }
>
> static inline void __cgroup_rstat_unlock(struct cgroup *cgrp, int cpu_in_loop)
> @@ -299,6 +316,72 @@ static inline void __cgroup_rstat_unlock(struct cgroup *cgrp, int cpu_in_loop)
> spin_unlock_irq(&cgroup_rstat_lock);
> }
>
> +#define MAX_WAIT msecs_to_jiffies(100)
> +/**
> + * cgroup_rstat_trylock_flusher - Trylock that checks for on ongoing flusher
> + * @cgrp: target cgroup
> + *
> + * Function return value follow trylock semantics. Returning true when lock is
> + * obtained. Returning false when not locked and it detected flushing can be
> + * skipped as another ongoing flusher took care of the flush.
> + */
> +static bool cgroup_rstat_trylock_flusher(struct cgroup *cgrp)
> +{
> + struct cgroup *ongoing;
> + bool locked;
> +
> + /*
> + * Check if ongoing flusher is already taking care of this, if
> + * we are a descendant skip work, but wait for ongoing flusher
> + * to complete work.
> + */
> +retry:
> + ongoing = READ_ONCE(cgrp_rstat_ongoing_flusher);
> + if (ongoing && cgroup_is_descendant(cgrp, ongoing)) {
The discussion about cgrp_rstat_ongoing_flusher possibly going away in
parallel never reached a conclusion AFAICT. Should we use
cgroup_tryget() here to get a ref on 'ongoing' until wait completes?
This shouldn't add much complexity AFAICT.
I think just using RCU here wouldn't be enough as we can flush rstat
after the RCU grace period when a cgroup is being freed.
> + wait_for_completion_interruptible_timeout(
> + &ongoing->flush_done, MAX_WAIT);
> + /* TODO: Add tracepoint here */
> + return false;
> + }
> +
> + locked = __cgroup_rstat_trylock(cgrp, -1);
> + if (!locked) {
> + /* Contended: Handle losing race for ongoing flusher */
> + if (!ongoing && READ_ONCE(cgrp_rstat_ongoing_flusher))
> + goto retry;
> +
> + __cgroup_rstat_lock(cgrp, -1, true);
> + }
> + /*
> + * Obtained lock, record this cgrp as the ongoing flusher.
> + * Due to lock yielding, we might obtain lock while another
> + * ongoing flusher (that isn't a parent) owns ongoing_flusher.
> + */
> + ongoing = READ_ONCE(cgrp_rstat_ongoing_flusher);
> + if (!ongoing) {
I think we don't need protection here since we never dereference
'cgrp_rstat_ongoing_flusher', but I think it may be clearer to
directly check it to make this obvious:
if (!READ_ONCE(cgrp_rstat_ongoing_flusher)) {
Perhaps we can also explicitly mention in the comment why we do not
need any protection here, but I am not sure how helpful that will be.
> + /*
> + * Limit to top-level as lock yielding allows others to obtain
> + * lock without being ongoing_flusher. Leading to cgroup that
> + * isn't descendant to obtain lock via yielding. So, prefer
> + * ongoing_flusher with many descendants.
> + */
> + if (cgrp->level < 2) {
This covers roots and top-level cgroups under them, right? Did them
improve the numbers you were observing?
AFAICT, we can remove this restriction completely if/when we use a
mutex and support a single ongoing flusher. If so, let's explicitly
mention this, perhaps:
XXX: Remove this restriction if/when lock yielding is removed
> + reinit_completion(&cgrp->flush_done);
> + WRITE_ONCE(cgrp_rstat_ongoing_flusher, cgrp);
> + }
> + }
> + return true;
> +}
> +
> +static void cgroup_rstat_unlock_flusher(struct cgroup *cgrp)
> +{
> + if (cgrp == READ_ONCE(cgrp_rstat_ongoing_flusher)) {
> + WRITE_ONCE(cgrp_rstat_ongoing_flusher, NULL);
> + complete_all(&cgrp->flush_done);
> + }
> + __cgroup_rstat_unlock(cgrp, -1);
> +}
> +
> /* see cgroup_rstat_flush() */
> static void cgroup_rstat_flush_locked(struct cgroup *cgrp)
> __releases(&cgroup_rstat_lock) __acquires(&cgroup_rstat_lock)
> @@ -328,7 +411,7 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp)
> __cgroup_rstat_unlock(cgrp, cpu);
> if (!cond_resched())
> cpu_relax();
> - __cgroup_rstat_lock(cgrp, cpu);
> + __cgroup_rstat_lock(cgrp, cpu, false);
> }
> }
> }
> @@ -350,9 +433,11 @@ __bpf_kfunc void cgroup_rstat_flush(struct cgroup *cgrp)
> {
> might_sleep();
>
> - __cgroup_rstat_lock(cgrp, -1);
> + if (!cgroup_rstat_trylock_flusher(cgrp))
> + return;
> +
> cgroup_rstat_flush_locked(cgrp);
> - __cgroup_rstat_unlock(cgrp, -1);
> + cgroup_rstat_unlock_flusher(cgrp);
> }
>
> /**
> @@ -368,8 +453,11 @@ void cgroup_rstat_flush_hold(struct cgroup *cgrp)
> __acquires(&cgroup_rstat_lock)
> {
> might_sleep();
> - __cgroup_rstat_lock(cgrp, -1);
> - cgroup_rstat_flush_locked(cgrp);
> +
> + if (cgroup_rstat_trylock_flusher(cgrp))
> + cgroup_rstat_flush_locked(cgrp);
> + else
> + __cgroup_rstat_lock(cgrp, -1, true);
> }
>
> /**
> @@ -379,7 +467,7 @@ void cgroup_rstat_flush_hold(struct cgroup *cgrp)
> void cgroup_rstat_flush_release(struct cgroup *cgrp)
> __releases(&cgroup_rstat_lock)
> {
> - __cgroup_rstat_unlock(cgrp, -1);
> + cgroup_rstat_unlock_flusher(cgrp);
> }
>
> int cgroup_rstat_init(struct cgroup *cgrp)
> @@ -401,6 +489,8 @@ int cgroup_rstat_init(struct cgroup *cgrp)
> u64_stats_init(&rstatc->bsync);
> }
>
> + init_completion(&cgrp->flush_done);
> +
> return 0;
> }
>
>
>
>
