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

From: Jens Axboe
Date: Thu Aug 11 2011 - 05:01:30 EST

On 2011-08-11 10:56, Kyungmin Park wrote:
> On Thu, Aug 11, 2011 at 5:23 PM, Jens Axboe <jaxboe@xxxxxxxxxxxx> wrote:
>> On 2011-08-11 03:14, Shaohua Li wrote:
>>> 2011/8/11 Shaohua Li <shli@xxxxxxxxxx>:
>>>> 2011/8/11 Kyungmin Park <kmpark@xxxxxxxxxxxxx>:
>>>>> 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.

Right, the submitter if the IO is the one that has to decide what is hot
or not.

>> 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.

No, that would be a usage bug.

Jens Axboe

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