linux-kernel - Re: [PATCH] cgroup/rstat: avoid disabling irqs for O(num

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Message-ID: <u5kcjffhyrjsxagpdzas7q463ldgqtptaafozea3bv64odn2xt@agx42ih5m76l>
Date: Wed, 19 Mar 2025 11:47:32 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: Greg Thelen <gthelen@...gle.com>
Cc: Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Koutný <mkoutny@...e.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Eric Dumazet <edumzaet@...gle.com>, Yosry Ahmed <yosryahmed@...gle.com>, cgroups@...r.kernel.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH] cgroup/rstat: avoid disabling irqs for O(num_cpu)

On Wed, Mar 19, 2025 at 12:13:30AM -0700, Greg Thelen wrote:
> From: Eric Dumazet <edumazet@...gle.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@...gle.com>
> Signed-off-by: Greg Thelen <gthelen@...gle.com>
> Tested-by: Greg Thelen <gthelen@...gle.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);
>  	}
>  }

Is not this going a little too far?

the lock + irq trip is quite expensive in its own right and now is
going to be paid for each cpu, as in the total time spent executing
cgroup_rstat_flush_locked is going to go up.

Would your problem go away toggling this every -- say -- 8 cpus?

Just a suggestion.