From mboxrd@z Thu Jan  1 00:00:00 1970
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Subject: Re: testing io.low limit for blk-throttle
To: Paolo Valente <paolo.valente@linaro.org>
Cc: linux-block <linux-block@vger.kernel.org>, Jens Axboe <axboe@kernel.dk>,
 Shaohua Li <shli@fb.com>, Mark Brown <broonie@kernel.org>,
 Linus Walleij <linus.walleij@linaro.org>,
 Ulf Hansson <ulf.hansson@linaro.org>, LKML <linux-kernel@vger.kernel.org>,
 Tejun Heo <tj@kernel.org>,
 'Paolo Valente' via bfq-iosched <bfq-iosched@googlegroups.com>
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 <A0424504-2778-41F4-B1C6-BE1B0253E524@linaro.org>
 <18accc1e-c7b3-86a7-091b-1d4b631fcd4a@gmail.com>
 <536A1B1D-575F-4193-ADA6-BA832AEC7179@linaro.org>
 <871b8d16-a172-af68-1aae-92ae55c0cce7@gmail.com>
 <5FEFF82B-4160-4F00-A60A-D3A6D9DDE66C@linaro.org>
From: Joseph Qi <jiangqi903@gmail.com>
Message-ID: <b8c4c2b1-dafa-703b-f322-0ea3bbc768f1@gmail.com>
Date: Fri, 27 Apr 2018 11:27:35 +0800
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Hi Paolo,

On 18/4/27 01:27, Paolo Valente wrote:
> 
> 
>> Il giorno 25 apr 2018, alle ore 14:13, Joseph Qi <jiangqi903@gmail.com> ha scritto:
>>
>> Hi Paolo,
>>
> 
> Hi Joseph
> 
>> ...
>> Could you run blktrace as well when testing your case? There are several
>> throtl traces to help analyze whether it is caused by frequently
>> upgrade/downgrade.
> 
> Certainly.  You can find a trace attached.  Unfortunately, I'm not
> familiar with the internals of blk-throttle and low limit, so, if you
> want me to analyze the trace, give me some hints on what I have to
> look for.  Otherwise, I'll be happy to learn from your analysis.
> 

I've taken a glance at your blktrace attached. It is only upgrade at first and
then downgrade (just adjust limit, not to LIMIT_LOW) frequently.
But I don't know why it always thinks throttle group is not idle.

For example:
fio-2336  [004] d...   428.458249:   8,16   m   N throtl avg_idle=90, idle_threshold=1000, bad_bio=10, total_bio=84, is_idle=0, scale=9
fio-2336  [004] d...   428.458251:   8,16   m   N throtl downgrade, scale 4

In throtl_tg_is_idle():
is_idle = ... ||
	(tg->latency_target && tg->bio_cnt &&
	 tg->bad_bio_cnt * 5 < tg->bio_cnt);

It should be idle and allow run more bandwidth. But here the result shows not
idle (is_idle=0). I have to do more investigation to figure it out why. 

You can also filter these logs using:
grep throtl trace | grep -E 'upgrade|downgrade|is_idle'

Thanks,
Joseph