linux-kernel - Re: [RFC V4 1/6] blk: prepare to make blk-rq-qos pluggable and modular

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Date:   Fri, 18 Feb 2022 11:34:52 +0800
From:   Wang Jianchao <jianchao.wan9@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jens Axboe <axboe@...nel.dk>, Josef Bacik <jbacik@...com>,
        Tejun Heo <tj@...nel.org>,
        Bart Van Assche <bvanassche@....org>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC V4 1/6] blk: prepare to make blk-rq-qos pluggable and
 modular



On 2022/2/17 4:48 下午, Christoph Hellwig wrote:
>>  {
>>  	struct request_queue *q = rqos->q;
>> -	const char *dir_name = rq_qos_id_to_name(rqos->id);
>> +	const char *dir_name;
>> +
>> +	dir_name = rqos->ops->name ? rqos->ops->name : rq_qos_id_to_name(rqos->id);
> 
> Overly long line here.  And it would be much more readable if you used
> a good old if/else.
> 
>> +static DEFINE_IDA(rq_qos_ida);
>> +static int nr_rqos_blkcg_pols;
>> +static DEFINE_MUTEX(rq_qos_mutex);
>> +static LIST_HEAD(rq_qos_list);
> 
> Please use an allocating xarray instead of an IDA plus list.
> 
>> +	/*
>> +	 * queue must have been unregistered here, it is safe to iterate
>> +	 * the list w/o lock
>> +	 */
> 
> Please capitalize multi-line comments.
> 
>> + * After the pluggable blk-qos, rqos's life cycle become complicated,
>> + * as we may modify the rqos list there. Except for the places where
>> + * queue is not registered, there are following places may access rqos
>> + * list concurrently:
> 
> Code comments are not the place to explain history.  PLease explain the
> current situation.
> 
>> +struct rq_qos *rq_qos_get(struct request_queue *q, int id)
>> +{
>> +	struct rq_qos *rqos;
>> +
>> +	spin_lock_irq(&q->queue_lock);
> 
> Please don't use the grab all queue_lock for new code.  It badly needs
> to be split and documented, and new code is the best place to start
> that.
> 
> Also with all the new code please add a new config option that is
> selected by all rq-pos implementations so that blk-rq-qos.c only gets
> built when actually needed.
> 
>> +static inline struct rq_qos *rq_qos_by_id(struct request_queue *q, int id)
>> +{
>> +	struct rq_qos *rqos;
>> +
>> +	WARN_ON(!mutex_is_locked(&q->sysfs_lock) && !spin_is_locked(&q->queue_lock));
> 
> Another overly long line.  And in doubt split this into two helpers
> so that you cna use lockdep_assert_held instead of doing the incorrect
> asserts.

Thanks so much for your kindly comment. I'd change the code in next version.

Regards
Jianchao