linux-kernel - Re: [PATCH 3/3] block: introducing a bias over deadline's fifo

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Date: Fri, 9 Feb 2024 10:58:40 +0900
From: Damien Le Moal <dlemoal@...nel.org>
To: Zhaoyang Huang <huangzhaoyang@...il.com>, Jens Axboe <axboe@...nel.dk>
Cc: "zhaoyang.huang" <zhaoyang.huang@...soc.com>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>, Yu Zhao <yuzhao@...gle.com>,
 linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 steve.kang@...soc.com
Subject: Re: [PATCH 3/3] block: introducing a bias over deadline's fifo_time

On 2/9/24 09:28, Zhaoyang Huang wrote:
> On Fri, Feb 9, 2024 at 8:11 AM Jens Axboe <axboe@...nel.dk> wrote:
>>
>> On 2/8/24 5:02 PM, Zhaoyang Huang wrote:
>>> On Fri, Feb 9, 2024 at 1:49?AM Jens Axboe <axboe@...nel.dk> wrote:
>>>>
>>>> On 2/8/24 2:31 AM, zhaoyang.huang wrote:
>>>>> diff --git a/block/mq-deadline.c b/block/mq-deadline.c
>>>>> index f958e79277b8..43c08c3d6f18 100644
>>>>> --- a/block/mq-deadline.c
>>>>> +++ b/block/mq-deadline.c
>>>>> @@ -15,6 +15,7 @@
>>>>>  #include <linux/compiler.h>
>>>>>  #include <linux/rbtree.h>
>>>>>  #include <linux/sbitmap.h>
>>>>> +#include "../kernel/sched/sched.h"
>>>>>
>>>>>  #include <trace/events/block.h>
>>>>>
>>>>> @@ -802,6 +803,7 @@ static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
>>>>>       u8 ioprio_class = IOPRIO_PRIO_CLASS(ioprio);
>>>>>       struct dd_per_prio *per_prio;
>>>>>       enum dd_prio prio;
>>>>> +     int fifo_expire;
>>>>>
>>>>>       lockdep_assert_held(&dd->lock);
>>>>>
>>>>> @@ -840,7 +842,9 @@ static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
>>>>>               /*
>>>>>                * set expire time and add to fifo list
>>>>>                */
>>>>> -             rq->fifo_time = jiffies + dd->fifo_expire[data_dir];
>>>>> +             fifo_expire = task_is_realtime(current) ? dd->fifo_expire[data_dir] :
>>>>> +                     CFS_PROPORTION(current, dd->fifo_expire[data_dir]);
>>>>> +             rq->fifo_time = jiffies + fifo_expire;
>>>>>               insert_before = &per_prio->fifo_list[data_dir];
>>>>>  #ifdef CONFIG_BLK_DEV_ZONED
>>>>>               /*
>>>>
>>>> Hard pass on this blatant layering violation. Just like the priority
>>>> changes, this utterly fails to understand how things are properly
>>>> designed.
>>> IMHO, I don't think this is a layering violation. bio_set_ioprio is
>>> the one which introduces the scheduler thing into the block layer,
>>> this commit just wants to do a little improvement based on that. This
>>> commit helps CFS task save some IO time when preempted by RT heavily.
>>
>> Listen, both this and the previous content ioprio thing show a glaring
>> misunderstanding of how to design these kinds of things. You have no
>> grasp of what the different layers do, or how they interact. I'm not
>> sure how to put this kindly, but it's really an awful idea to hardcore
>> some CFS helper into the IO scheduler. The fact that you had to fiddle
>> around with headers to make it work was the first warning sign, and the
>> fact that you didn't stop at that point to consider how it could be
>> properly done makes it even worse.
>>
>> You need to stop sending kernel patches until you understand basic
>> software design. Neither of these patches are going anywhere until this
>> happens. There's been plenty of feedback to telling you that, but you
>> seem to just ignore it and plow on ahead. Stop.
> Ok, thanks for pointing this out, I will follow your advice. But I
> have to say that '[PATCHv9 1/1] block: introduce content activity
> based ioprio' really solved layering violation things. I would like to
> humbly ask for your kindly patient to have a look, as it is really
> helpful.

If properly designed, that patch would *not* be a block layer API/function and
so does not need review by block layer folks/Jens as it would simply set an IO
prio for a BIO issued by an FS. So that patch needs to be accepted by FS
people, for the FS you are interested in.


-- 
Damien Le Moal
Western Digital Research