Re: [PATCH] cgroup/rstat: avoid disabling irqs for O(num_cpu)

[Greg Thelen <gthelen@google.com>]

(fix mistyped CC address to Eric)

On Wed, Mar 19, 2025 at 12:13 AM Greg Thelen <gthelen@google.com> wrote:
>
> From: Eric Dumazet <edumazet@google.com>
>
> cgroup_rstat_flush_locked() grabs the irq safe cgroup_rstat_lock while
> iterating all possible cpus. It only drops the lock if there is
> scheduler or spin lock contention. If neither, then interrupts can be
> disabled for a long time. On large machines this can disable interrupts
> for a long enough time to drop network packets. On 400+ CPU machines
> I've seen interrupt disabled for over 40 msec.
>
> Prevent rstat from disabling interrupts while processing all possible
> cpus. Instead drop and reacquire cgroup_rstat_lock for each cpu. This
> approach was previously discussed in
> https://lore.kernel.org/lkml/ZBz%2FV5a7%2F6PZeM7S@slm.duckdns.org/,
> though this was in the context of an non-irq rstat spin lock.
>
> Benchmark this change with:
> 1) a single stat_reader process with 400 threads, each reading a test
>    memcg's memory.stat repeatedly for 10 seconds.
> 2) 400 memory hog processes running in the test memcg and repeatedly
>    charging memory until oom killed. Then they repeat charging and oom
>    killing.
>
> v6.14-rc6 with CONFIG_IRQSOFF_TRACER with stat_reader and hogs, finds
> interrupts are disabled by rstat for 45341 usec:
>   #  => started at: _raw_spin_lock_irq
>   #  => ended at:   cgroup_rstat_flush
>   #
>   #
>   #                    _------=> CPU#
>   #                   / _-----=> irqs-off/BH-disabled
>   #                  | / _----=> need-resched
>   #                  || / _---=> hardirq/softirq
>   #                  ||| / _--=> preempt-depth
>   #                  |||| / _-=> migrate-disable
>   #                  ||||| /     delay
>   #  cmd     pid     |||||| time  |   caller
>   #     \   /        ||||||  \    |    /
>   stat_rea-96532    52d....    0us*: _raw_spin_lock_irq
>   stat_rea-96532    52d.... 45342us : cgroup_rstat_flush
>   stat_rea-96532    52d.... 45342us : tracer_hardirqs_on <-cgroup_rstat_flush
>   stat_rea-96532    52d.... 45343us : <stack trace>
>    => memcg1_stat_format
>    => memory_stat_format
>    => memory_stat_show
>    => seq_read_iter
>    => vfs_read
>    => ksys_read
>    => do_syscall_64
>    => entry_SYSCALL_64_after_hwframe
>
> With this patch the CONFIG_IRQSOFF_TRACER doesn't find rstat to be the
> longest holder. The longest irqs-off holder has irqs disabled for
> 4142 usec, a huge reduction from previous 45341 usec rstat finding.
>
> Running stat_reader memory.stat reader for 10 seconds:
> - without memory hogs: 9.84M accesses => 12.7M accesses
> -    with memory hogs: 9.46M accesses => 11.1M accesses
> The throughput of memory.stat access improves.
>
> The mode of memory.stat access latency after grouping by of 2 buckets:
> - without memory hogs: 64 usec => 16 usec
> -    with memory hogs: 64 usec =>  8 usec
> The memory.stat latency improves.
>
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Signed-off-by: Greg Thelen <gthelen@google.com>
> Tested-by: Greg Thelen <gthelen@google.com>
> ---
>  kernel/cgroup/rstat.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
> index aac91466279f..976c24b3671a 100644
> --- a/kernel/cgroup/rstat.c
> +++ b/kernel/cgroup/rstat.c
> @@ -323,13 +323,11 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp)
>                         rcu_read_unlock();
>                 }
>
> -               /* play nice and yield if necessary */
> -               if (need_resched() || spin_needbreak(&cgroup_rstat_lock)) {
> -                       __cgroup_rstat_unlock(cgrp, cpu);
> -                       if (!cond_resched())
> -                               cpu_relax();
> -                       __cgroup_rstat_lock(cgrp, cpu);
> -               }
> +               /* play nice and avoid disabling interrupts for a long time */
> +               __cgroup_rstat_unlock(cgrp, cpu);
> +               if (!cond_resched())
> +                       cpu_relax();
> +               __cgroup_rstat_lock(cgrp, cpu);
>         }
>  }
>
> --
> 2.49.0.rc1.451.g8f38331e32-goog
>