linux-kernel - Re: [PATCH-block 3/3] blk-cgroup: Flush stats at blkgs destruction path

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Date:   Thu, 8 Dec 2022 16:00:10 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Waiman Long <longman@...hat.com>, Tejun Heo <tj@...nel.org>,
        Josef Bacik <josef@...icpanda.com>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Michal Koutný <mkoutny@...e.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@...il.com>
Subject: Re: [PATCH-block 3/3] blk-cgroup: Flush stats at blkgs destruction
 path

On 12/8/22 3:01?PM, Waiman Long wrote:
> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
> index 793ecff29038..910e633869b0 100644
> --- a/kernel/cgroup/rstat.c
> +++ b/kernel/cgroup/rstat.c
> @@ -281,6 +281,26 @@ void cgroup_rstat_flush_release(void)
>  	spin_unlock_irq(&cgroup_rstat_lock);
>  }
>  
> +/**
> + * cgroup_rstat_css_cpu_flush - flush stats for the given css and cpu
> + * @css: target css to be flush
> + * @cpu: the cpu that holds the stats to be flush
> + *
> + * A lightweight rstat flush operation for a given css and cpu.
> + * Only the cpu_lock is being held for mutual exclusion, the cgroup_rstat_lock
> + * isn't used.
> + */
> +void cgroup_rstat_css_cpu_flush(struct cgroup_subsys_state *css, int cpu)
> +{
> +	raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu);
> +
> +	raw_spin_lock_irq(cpu_lock);
> +	rcu_read_lock();
> +	css->ss->css_rstat_flush(css, cpu);
> +	rcu_read_unlock();
> +	raw_spin_unlock_irq(cpu_lock);
> +}
> +
>  int cgroup_rstat_init(struct cgroup *cgrp)
>  {
>  	int cpu;

As I mentioned last time, raw_spin_lock_irq() will be equivalent to an
RCU protected section anyway, so you don't need to do both. Just add a
comment on why rcu_read_lock()/rcu_read_unlock() isn't needed inside the
raw irq safe lock.

-- 
Jens Axboe