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

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Message-ID: <8641d4a4-4d60-0f31-120c-56628f477ba2@redhat.com>
Date:   Fri, 9 Dec 2022 10:58:32 -0500
From:   Waiman Long <longman@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>, 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 18:00, Jens Axboe wrote:
> 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.

Yes, you are right.  We don't need rcu_read_lock() here. I put it there 
to follow the locking pattern in cgroup_rstat_flush_locked(). I will 
remove it in the next version.

Cheers,
Longman