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From: Bart Van Assche <bvanassche@acm.org>
To: Hannes Reinecke <hare@suse.de>, Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
	Jaegeuk Kim <jaegeuk@kernel.org>,
	Damien Le Moal <damien.lemoal@wdc.com>,
	Ming Lei <ming.lei@redhat.com>,
	Johannes Thumshirn <johannes.thumshirn@wdc.com>,
	Himanshu Madhani <himanshu.madhani@oracle.com>
Subject: Re: [PATCH 04/14] block: Introduce the ioprio rq-qos policy
Date: Thu, 10 Jun 2021 10:14:20 -0700	[thread overview]
Message-ID: <1ffd5799-ac6d-5ed8-c0aa-e2481e05de32@acm.org> (raw)
In-Reply-To: <d68c5b3d-362f-08e0-180e-bfc59b4a9d07@suse.de>

On 6/9/21 11:20 PM, Hannes Reinecke wrote:
> On 6/9/21 1:06 AM, Bart Van Assche wrote:
>> +static void blkcg_ioprio_track(struct rq_qos *rqos, struct request *rq,
>> +			       struct bio *bio)
>> +{
>> +	struct ioprio_blkcg *blkcg = ioprio_blkcg_from_bio(bio);
>> +
>> +	/*
>> +	 * Except for IOPRIO_CLASS_NONE, higher I/O priority numbers
>> +	 * correspond to a lower priority. Hence, the max_t() below selects
>> +	 * the lower priority of bi_ioprio and the cgroup I/O priority class.
>> +	 * If the cgroup priority has been set to IOPRIO_CLASS_NONE == 0, the
>> +	 * bio I/O priority is not modified. If the bio I/O priority equals
>> +	 * IOPRIO_CLASS_NONE, the cgroup I/O priority is assigned to the bio.
>> +	 */
>> +	bio->bi_ioprio = max_t(u16, bio->bi_ioprio,
>> +			       IOPRIO_PRIO_VALUE(blkcg->prio_class, 0));
>> +}
> 
> Sheesh. Now that is cheeky.
> First defining a (conceptually) complex policy setting (where people
> wonder where these policies came from), which then devolve into a simple
> max() setting of the priority value.
> This _really_ could do with a better explanation in the documentation,
> as then it's far easier to understand _why_ certain policies override
> others.
> IE this comment belongs in the documentation, as explanation of the
> underlying mechanics of the ioprio policies.

Hi Hannes,

blkcg_ioprio_track() is called from the hot path so I want this function
to be fast. Since the desired behavior can be implemented with a max() I
chose max() instead of e.g. using a lookup table.

I will address your comments about the documentation and the code.

Bart.

  reply	other threads:[~2021-06-10 17:14 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-08 23:06 [PATCH 00/14] Improve I/O priority support Bart Van Assche
2021-06-08 23:06 ` [PATCH 01/14] block/Kconfig: Make the BLK_WBT and BLK_WBT_MQ entries consecutive Bart Van Assche
2021-06-09  4:14   ` Damien Le Moal
2021-06-09  7:03   ` Johannes Thumshirn
2021-06-10  6:02   ` Hannes Reinecke
2021-06-08 23:06 ` [PATCH 02/14] block/blk-cgroup: Swap the blk_throtl_init() and blk_iolatency_init() calls Bart Van Assche
2021-06-09  4:19   ` Damien Le Moal
2021-06-09  7:05   ` Johannes Thumshirn
2021-06-10  6:02   ` Hannes Reinecke
2021-06-08 23:06 ` [PATCH 03/14] block/blk-rq-qos: Move a function from a header file into a C file Bart Van Assche
2021-06-09  4:22   ` Damien Le Moal
2021-06-10  6:03   ` Hannes Reinecke
2021-06-08 23:06 ` [PATCH 04/14] block: Introduce the ioprio rq-qos policy Bart Van Assche
2021-06-09  4:40   ` Damien Le Moal
2021-06-09 16:48     ` Bart Van Assche
2021-06-10  0:29       ` Damien Le Moal
2021-06-10  6:20   ` Hannes Reinecke
2021-06-10 17:14     ` Bart Van Assche [this message]
2021-06-08 23:06 ` [PATCH 05/14] block/mq-deadline: Add several comments Bart Van Assche
2021-06-10  6:21   ` Hannes Reinecke
2021-06-08 23:06 ` [PATCH 06/14] block/mq-deadline: Add two lockdep_assert_held() statements Bart Van Assche
2021-06-10  6:21   ` Hannes Reinecke
2021-06-08 23:06 ` [PATCH 07/14] block/mq-deadline: Remove two local variables Bart Van Assche
2021-06-10  6:22   ` Hannes Reinecke
2021-06-08 23:06 ` [PATCH 08/14] block/mq-deadline: Rename dd_init_queue() and dd_exit_queue() Bart Van Assche
2021-06-08 23:06 ` [PATCH 09/14] block/mq-deadline: Improve compile-time argument checking Bart Van Assche
2021-06-08 23:06 ` [PATCH 10/14] block/mq-deadline: Improve the sysfs show and store macros Bart Van Assche
2021-06-09  4:46   ` Damien Le Moal
2021-06-09 16:54     ` Bart Van Assche
2021-06-10  0:31       ` Damien Le Moal
2021-06-09  7:11   ` Johannes Thumshirn
2021-06-09 16:59     ` Bart Van Assche
2021-06-10  6:23   ` Hannes Reinecke
2021-06-08 23:07 ` [PATCH 11/14] block/mq-deadline: Reserve 25% of scheduler tags for synchronous requests Bart Van Assche
2021-06-10  6:26   ` Hannes Reinecke
2021-06-08 23:07 ` [PATCH 12/14] block/mq-deadline: Add I/O priority support Bart Van Assche
2021-06-09  5:03   ` Damien Le Moal
2021-06-09 17:25     ` Bart Van Assche
2021-06-10  0:39       ` Damien Le Moal
2021-06-10  6:35   ` Hannes Reinecke
2021-06-08 23:07 ` [PATCH 13/14] block/mq-deadline: Add cgroup support Bart Van Assche
2021-06-10  6:37   ` Hannes Reinecke
2021-06-08 23:07 ` [PATCH 14/14] block/mq-deadline: Prioritize high-priority requests Bart Van Assche
2021-06-09  5:10   ` Damien Le Moal
2021-06-10  6:43   ` Hannes Reinecke

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