linux-kernel - Re: [PATCH v2 2/4] block, bfq: do not idle if only one cgroup is activated

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Message-Id: <F1ADC992-11AE-4511-9033-D233CBCA6F26@linaro.org>
Date:   Tue, 7 Sep 2021 11:10:02 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     "yukuai (C)" <yukuai3@...wei.com>
Cc:     Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com
Subject: Re: [PATCH v2 2/4] block, bfq: do not idle if only one cgroup is
 activated



> Il giorno 2 set 2021, alle ore 15:31, yukuai (C) <yukuai3@...wei.com> ha scritto:
> 
> On 2021/08/27 1:00, Paolo Valente wrote:
>> Why do you make these extensive changes, while you can leave all the
>> function unchanged and just modify the above condition to something
>> like
>> || bfqd->num_groups_with_pending_reqs > 1
>> || (bfqd->num_groups_with_pending_reqs && bfqd->num_queues_with_pending_reqs_in_root)
> 
> Hi, Paolo
> 
> I was thinking that if CONFIG_BFQ_GROUP_IOSCHED is enabled, there is no
> need to caculate smallest_weight, varied_queue_weights, and
> multiple_classes_busy:
> 
> If we count root group into num_groups_with_pending_reqs
> - If num_groups_with_pending_reqs <= 1, idle is not needed

Unfortunately, if active queues have different weights or belong to
different classes, then idling is needed to preserve per-queue
bandwidths.

Thanks,
Paolo

> - If num_groups_with_pending_reqs  > 1, idle is needed
> 
> Thus such changes can save some additional overhead.
> 
> Thanks
> Yu Kuai
> 
>> In addition, I still wonder whether you can simply add also the root
>> group to bfqd->num_groups_with_pending_reqs (when the root group is
>> active).  This would make the design much cleaner.
>> Thanks,
>> Paolo
>>> -#endif
>>> -		;
>>> +	return varied_queue_weights || multiple_classes_busy;
>>> }
>>> 
>>> /*
>>> -- 
>>> 2.31.1
>>> 
>> .