linux-kernel - Re: [RFC PATCH] request: teach the device more intelligent

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Message-ID: <4E438B2A.2000002@kernel.dk>
Date:	Thu, 11 Aug 2011 09:56:26 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Shaohua Li <shli@...nel.org>
CC:	Kyungmin Park <kmpark@...radead.org>, linux-kernel@...r.kernel.org,
	arnd@...db.de, jh80.chung@...sung.com, linux-mmc@...r.kernel.org
Subject: Re: [RFC PATCH] request: teach the device more intelligent

On 2011-08-11 02:29, Shaohua Li wrote:
> 2011/8/10 Jens Axboe <axboe@...nel.dk>:
>> On 2011-08-10 10:47, Shaohua Li wrote:
>>> 2011/8/10 Kyungmin Park <kmpark@...radead.org>:
>>>> On Wed, Aug 10, 2011 at 5:08 PM, Jens Axboe <axboe@...nel.dk> wrote:
>>>>> On 2011-08-10 01:43, Kyungmin Park wrote:
>>>>>> On Wed, Aug 10, 2011 at 3:52 AM, Jens Axboe <axboe@...nel.dk> wrote:
>>>>>>> On 2011-08-09 05:47, Kyungmin Park wrote:
>>>>>>>> Hi Jens
>>>>>>>>
>>>>>>>> Now eMMC device requires the upper layer information to improve the data
>>>>>>>> performance and reliability.
>>>>>>>>
>>>>>>>> . Context ID
>>>>>>>> Using the context information, it can sort out the data internally and improve the performance.
>>>>>>>> The main problem is that it's needed to define "What's the context"..
>>>>>>>> Actually I expect cfq queue has own unique ID but it doesn't so decide to use the pid instead
>>>>>>>>
>>>>>>>> . Data Tag
>>>>>>>> Using the Data Tag (1-bit information), It writes the data at SLC area when it's hot data. So it can make the chip more reliable.
>>>>>>>> First I expect the REQ_META but current ext4 doesn't pass the WRITE_META. only use the READ_META. so it needs to investigate it.
>>>>>>>>
>>>>>>>> With these characteristics, it's helpful to teach the device. After some consideration. it's needed to pass out these information at request data structure.
>>>>>>>>
>>>>>>>> Can you give your opinions and does it proper fields at requests?
>>>>>>>
>>>>>>> You need this to work on all IO schedulers, not just cfq.
>>>>>> Of course if the concept is acceptable, I'll add the other IO schedulers also.
>>>>>>
>>>>>>> And since that's the case, there's no need to add this field since you can just
>>>>>>> retrieve it if the driver asks for it. For CFQ, it could look like this:
>>>>>>>
>>>>>>> static int cfq_foo(struct request *rq)
>>>>>>> {
>>>>>>>        struct cfq_queue *cfqq = rq->elevator_private[1];
>>>>>>>
>>>>>>>        if (cfqq)
>>>>>>>                return cfqq->pid;
>>>>>>>
>>>>>>>        return -1;
>>>>>>> }
>>>>>>
>>>>>> The actual user of these information is device driver. e.g.,
>>>>>> drivers/mmc/card/block.c
>>>>>> So it's not good to use cfq data structure at D/D. some time later
>>>>>> these are also used at scsi device drivers.
>>>>>
>>>>> No, what I'm suggesting above is the CFQ implementation. You would need
>>>>> to wire up an elv_ops->get_foo() and have the IO schedulers fill it in..
>>>>> If you notice, the above function does not take or output anything
>>>>> related to CFQ in particular, it'll just return you the unique key you
>>>>> need.
>>>>>
>>>>> It's either the above, or a field in the request that the schedulers
>>>>> fill out. However, it'd be somewhat annoying to grow struct request for
>>>>> something that has a narrow scope of use. Hence the suggestion to add a
>>>>> strategy helper for this.
>>>> Okay, I'll add new elevator function one for getting context or more hints.
>>>> BTW, does it okay to call elevator function call at D/D?
>>>>
>>>> The quick-n-dirty call is like this at "drivers/mmc/card/block.c"
>>>>
>>>>                struct elevator_queue *e = md->queue.queue->elevator;
>>>>                int context = -1;
>>>>
>>>>                if (e->ops->elevator_get_req_hint_fn && req) {
>>>>                        context = e->ops->elevator_get_req_hint_fn(req);
>>> I'm wondering how the driver deal with elevator switch. A context id from
>>> one elevator might just be garbage for another elevator.
>>
>> Any request with sched private data is drained prior to switching over.
>> This problem isn't unique to this context id, we have other per-request
>> IO scheduler data structures associated with the request, too.
> what I'm afraid is the context id isn't consistent. Say in cfq, context id
> for app1 is 1, app2 2. Then switching to deadline, context id for app1
> is 2, app2 1. Will the driver be confused about this?

It's a hint, so should not be a worry at all. Things should function
perfectly fine with just returning 1 all the time, the idea is to allow
some more efficiency in scheduling on the hw side if we can.
Realistically, the device isn't going to be tracking a ton of pids
anyway. In the rare event of an IO scheduler switch, things will settle
down very quickly.

-- 
Jens Axboe

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