Re: [PATCH V8 1/2] cgroup/rstat: Avoid flushing if there is an ongoing overlapping flush

[Yosry Ahmed]

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;
>  }
>
>
>
>