linux-kernel - Re: [RFC PATCH v2] Add new elevator ops for request hint

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Message-ID: <CAH9JG2WOrB5HTh2JiuVfPsXN2Z+R0gDGQeEoecxrrj9FpGSR+g@mail.gmail.com>
Date:	Thu, 11 Aug 2011 17:56:22 +0900
From:	Kyungmin Park <kmpark@...radead.org>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	Shaohua Li <shli@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"arnd@...db.de" <arnd@...db.de>,
	"jh80.chung@...sung.com" <jh80.chung@...sung.com>,
	"linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>
Subject: Re: [RFC PATCH v2] Add new elevator ops for request hint

On Thu, Aug 11, 2011 at 5:23 PM, Jens Axboe <jaxboe@...ionio.com> wrote:
> On 2011-08-11 03:14, Shaohua Li wrote:
>> 2011/8/11 Shaohua Li <shli@...nel.org>:
>>> 2011/8/11 Kyungmin Park <kmpark@...radead.org>:
>>>> 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.
>>>>
>>>> Sample usage is following in drivers/mmc/card/block.c
>>>>
>>>>        struct elevator_queue *e = md->queue.queue->elevator;
>>>>        struct request_hint hint;
>>>>        int ret;
>>>>
>>>>        if (e->ops->elevator_get_req_hint_fn && req)
>>>>                ret = e->ops->elevator_get_req_hint_fn(req, &hint);
>>> please put this to blkdev.h or similar. directly using it here
>>> is abnormal.
>> BTW, we can add a (rq->cmd_flags & REQ_ELVPRIV) check here to make
>> sure the request is at io scheduler.
>
> Yep, that should all go inside elv_get_request_context() or whatever is
> a good name. I don't want the hint structure, the caller can just check
> the request flags himself.
Okay I see, it has to modify the filesystem to send the meta request,
REQ_META flags to know it from device drivers.
>
> So something like:
>
> int elv_get_request_context(struct request_queue *q, struct request *rq)
> {
>        struct elevator_queue *e = q->elevator;
>
>        if (!(rq->cmd_flags & REQ_ELVPRIV))
>                return -1;
Also need to check the !rq case, mmc send the NULL request to wait the
previous request wait.
>
>        if (e->ops->elevator_get_req_context_fn)
>                return e->ops->elevator_get_req_context_fn(q, rq);
>
>        return -1;
> }
>
> and then cfq/others adding that helper to provide the mapping.
>
> Context is a bad name, but so is hint. Perhaps app_key would be better,
> as it more direcly infers what is being returned.

I'll send the new version as you suggested.

Thank you,
Kyungmin Park
>
> --
> Jens Axboe
>
>
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