Re: [PATCH RFC 22/39] mmc: block: Prepare CQE data

Adrian Hunter
2017-03-10T08:29:36Z

On 10/03/17 00:39, Linus Walleij wrote:
> On Fri, Mar 3, 2017 at 1:22 PM, Adrian Hunter <adrian.hunter@intel.com> wrote:
>> On 15/02/17 15:49, Linus Walleij wrote:
>>> On Fri, Feb 10, 2017 at 1:55 PM, Adrian Hunter <adrian.hunter@intel.com> wrote:
>>>
>>>> Enhance mmc_blk_data_prep() to support CQE requests.
>>>>
>>>> Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
>>>
>>> Hey:
>>>
>>>> +#include <linux/ioprio.h>
>>> (...)
>>>> + if (IOPRIO_PRIO_CLASS(req_get_ioprio(req)) == IOPRIO_CLASS_RT)
>>>> + brq->data.flags |= MMC_DATA_PRIO;
>>>
>>> What is this?
>>
>> It is the command queue priority.
>>
>> The command queue supports 2 priorities: "high" (1) and "simple" (0). The
>> eMMC will give "high" priority tasks priority over "simple" priority tasks.
>>
>> So here we give priority to IOPRIO_CLASS_RT which seems appropriate.
>
> So if I understand correctly, you are obtaining the block layer scheduling
> priorities, that can (only?) be set by ionice has from the command line?

AFAICS it is the ioprio_set() system call .

>
> We need to discuss this with the block maintainers.
>
> I'm not so sure about the future of this. The IOPRIO is only used with the CFQ
> scheduler, only two other sites in the kernel use this and MQ and its schedulers
> surely does not have ionice handling as far as I know.
>
> The BFQ does not use it, AFAIK it is using different heuristics to prioritize
> block traffic, and that does not include using ionice hints.
>
> Is ionice:ing something we're really going to do going forward?
> Should this be repurposed so that the block scheduler use this prio to
> communicate to the driver layer to prioritize certain traffic?

That seems like a separate issue. At the moment, I/O priorities are what we
have, and giving priority to RT seems appropriate.