linux-kernel - Re: [PATCH v9 3/3] blk-cgroup: Flush stats at blkgs destruction path

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Message-ID: <Y2VvboMSxgF0pYpX@slm.duckdns.org>
Date:   Fri, 4 Nov 2022 10:00:46 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ming Lei <ming.lei@...hat.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Koutný <mkoutny@...e.com>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH v9 3/3] blk-cgroup: Flush stats at blkgs destruction path

On Fri, Nov 04, 2022 at 02:20:50PM -0400, Waiman Long wrote:
> +/**
> + * cgroup_rstat_css_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_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);
> +}

Would it make sense to itereate CPUs within the helper rather than asking
the caller to do it? Also, in terms of patch sequencing, this introduces a
bug and then fixes it. Prolly better to not introduce the bug in the first
place.

Thanks.

-- 
tejun