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Message-ID: <22a70cc4-2150-4296-84d5-f0bf6617613a@acm.org>
Date: Fri, 1 Dec 2023 14:14:49 -0800
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Subject: Re: [PATCH v6 1/4] block: Make fair tag sharing configurable
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To: Johannes Thumshirn <Johannes.Thumshirn@wdc.com>,
 Jens Axboe <axboe@kernel.dk>
Cc: "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
 "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
 "Martin K . Petersen" <martin.petersen@oracle.com>,
 Christoph Hellwig <hch@lst.de>, Ming Lei <ming.lei@redhat.com>,
 Keith Busch <kbusch@kernel.org>,
 Damien Le Moal <damien.lemoal@opensource.wdc.com>,
 Yu Kuai <yukuai1@huaweicloud.com>, Ed Tsai <ed.tsai@mediatek.com>,
 Matthias Brugger <matthias.bgg@gmail.com>,
 AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
References: <20231130193139.880955-1-bvanassche@acm.org>
 <20231130193139.880955-2-bvanassche@acm.org>
 <e728ac4b-bce4-4c2e-88bb-8874c73f0c8e@wdc.com>
From: Bart Van Assche <bvanassche@acm.org>
In-Reply-To: <e728ac4b-bce4-4c2e-88bb-8874c73f0c8e@wdc.com>
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On 12/1/23 04:52, Johannes Thumshirn wrote:
> On 30.11.23 20:31, Bart Van Assche wrote:
>> +void blk_mq_update_fair_sharing(struct blk_mq_tag_set *set, bool enable)
>> +{
>> +	const unsigned int DFTS_BIT = ilog2(BLK_MQ_F_DISABLE_FAIR_TAG_SHARING);
>> +	struct blk_mq_hw_ctx *hctx;
>> +	struct request_queue *q;
>> +	unsigned long i;
>> +
>> +	/*
>> +	 * Serialize against blk_mq_update_nr_hw_queues() and
>> +	 * blk_mq_realloc_hw_ctxs().
>> +	 */
>> +	mutex_lock(&set->tag_list_lock);
>> +	list_for_each_entry(q, &set->tag_list, tag_set_list)
>> +		blk_mq_freeze_queue(q);
>> +	assign_bit(DFTS_BIT, &set->flags, !enable);
>> +	list_for_each_entry(q, &set->tag_list, tag_set_list)
>> +		queue_for_each_hw_ctx(q, hctx, i)
>> +			assign_bit(DFTS_BIT, &hctx->flags, !enable);
>> +	list_for_each_entry(q, &set->tag_list, tag_set_list)
>> +		blk_mq_unfreeze_queue(q);
>> +	mutex_unlock(&set->tag_list_lock);
> 
> Hi Bart,
> 
> The above code adds a 3rd user (at least) of the following pattern to
> the kernel:
> 
> 	list_for_each_entry(q, &set->tag_list, tag_set_list)
> 		blk_mq_freeze_queue(q);
> 
> 	/* do stuff */
> 
> 	list_for_each_entry(q, &set->tag_list, tag_set_list)
> 		blk_mq_unfreeze_queue(q);
> 
> Would it maybe be beneficial if we'd introduce functions for this, like:
> 
> static inline void blk_mq_freeze_tag_set(struct blk_mq_tag_set *set)
> {
> 	lockdep_assert_held(&set->tag_list_lock);
> 
> 	list_for_each_entry(q, &set->tag_list, tag_set_list)
> 		blk_mq_freeze_queue(q);
> }
> 
> static inline void blk_mq_unfreeze_tag_set(struct blk_mq_tag_set *set)
> {
> 	lockdep_assert_held(&set->tag_list_lock);
> 
> 	list_for_each_entry(q, &set->tag_list, tag_set_list)
> 		blk_mq_unfreeze_queue(q);
> }

Hi Johannes,

That sounds like a good idea to me. I will make this change.

Thanks,

Bart.