linux-kernel - Re: [PATCH v5 3/3] blk-cgroup: Optimize blkcg_rstat

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Date:   Thu, 2 Jun 2022 14:18:14 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
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>
Subject: Re: [PATCH v5 3/3] blk-cgroup: Optimize blkcg_rstat_flush()

On 6/2/22 13:46, Tejun Heo wrote:
> On Thu, Jun 02, 2022 at 01:26:10PM -0400, Waiman Long wrote:
>> On 6/2/22 12:58, Tejun Heo wrote:
>>> Hello,
>>>
>>> On Thu, Jun 02, 2022 at 09:35:43AM -0400, Waiman Long wrote:
>>>> @@ -2011,9 +2076,16 @@ void blk_cgroup_bio_start(struct bio *bio)
>>>>    	}
>>>>    	bis->cur.ios[rwd]++;
>>>> +	if (!READ_ONCE(bis->lnode.next)) {
>>>> +		struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu);
>>>> +
>>>> +		llist_add(&bis->lnode, lhead);
>>>> +		percpu_ref_get(&bis->blkg->refcnt);
>>> Hmm... what guarantees that more than one threads race here? llist assumes
>>> that there's a single writer for a given llist_node and the ref count would
>>> be off too, right?
>> The llist_add() function is atomic. It calls into llist_add_batch() in
>> lib/llist.c which uses cmpxchg() to make the change. There is a non-atomic
>> version __llist_add() which may be problematic in this case. Note that irq
>> is disabled in the u64_stats_update* critical section, there shouldn't be a
>> racing thread running in the same cpu. Other cpus will modify their own
>> version of lhead. Perhaps the non-atomic version can be used here as well.
> Ah, right, this is per-cpu, so there can be no second writer trying to add
> the same node at the same time. Can you add a comment explaining the overall
> design / behavior? Other than that, please feel free to add
>
>   Acked-by: Tejun Heo <tj@...nel.org>
>
> Thanks.

OK, I will send another patch to document the design in 
block/blk-cgroup.c. I don't want to touch this patch unless I need to 
correct some code here.

Thanks,
Longman

>