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SubjectRE: [PATCH 2/2] mmc: core: Support packed command for eMMC4.5 device
From
>> >> >> >> > +               phys_segments +=  next->nr_phys_segments;
>> >> >> >> > +               if (phys_segments > max_phys_segs) {
>> >> >> >> > +                       blk_requeue_request(q, next);
>> >> >> >> > +                       break;
>> >> >> >> > +               }
>> >> >> >> I mentioned this before - if the next request is not packable
>> and
>> >> >> requeued,
>> >> >> >> blk_fetch_request will retrieve it again and this while loop
>> will
>> >> never terminate.
>> >> >> >>
>> >> >> > If next request is not packable, it is requeued and 'break'
>> >> terminates
>> >> >> this loop.
>> >> >> > So it not infinite.
>> >> >> Right !! But that doesn't help finding the commands that are
>> packable.
>> >> Ideally, you'd need to pack all neighbouring requests into one packed
>> >> command.
>> >> >> The way CFQ works, it is not necessary that the fetch would return
>> all
>> >> outstanding
>> >> >> requests that are packable (unless you invoke a forced dispatch)
>> It
>> >> would be good to see some numbers on the number of pack hits /
>> >> misses
>> >> >> that
>> >> >> you would encounter with this logic, on a typical usecase.
>> >> > Is it considered only for CFQ scheduler? How about other I/O
>> scheduler?
>> >> If all requests are drained from scheduler queue forcedly,
>> >> > the number of candidate to be packed can be increased.
>> >> > However we may lose the unique CFQ's strength and MMC D/D may take
>> the
>> >> CFQ's duty.
>> >> > Basically, this patch accommodates the origin order requests from
>> I/O
>> >> scheduler.
>> >> >
>> >>
>> >> In order to better utilize the packed commands feature and achieve
>> the
>> >> best performance improvements I think that the command packing should
>> be
>> >> done in the block layer, according to the scheduler policy.
>> >> That is, the scheduler should be aware of the capability of the
>> device to
>> >> receive a request list and its constrains (such as maximum number of
>> >> requests, max number of sectors etc) and use this information as a
>>  factor
>> >> to its algorithm.
>> >> This way you keep the decision making in the hands of the scheduler
>> while
>> >> the MMC layer will only have to send this list as a packed command.
>> >>
>> > Yes, it would be another interesting approach.
>> > Command packing you mentioned means gathering request among same
>> direction(read/write)?
>> > Currently I/O scheduler may know device constrains which MMC driver
>> informs
>> > with the exception of order information for packed command.
>> > But I think the dependency of I/O scheduler may be able to come up.
>> > How can MMC layer treat packed command with I/O scheduler which
>> doesn't support this?
>>
>> The very act of packing presumes some sorting and re-ordering at the
>> I/O scheduler level.
>> When no such sorting is done (ex. noop), MMC should resort to
>> non-packed execution, respecting the system configuration choice.
>>
>> Looking deeper into this, I think a better approach would be to set
>> the prep_rq_fn of the request_queue, with a custom mmc function that
>> decides if the requests are packable or not, and return a
>> BLKPREP_DEFER for those that can't be packed.
>
> MMC layer anticipates the favorable requests for packing from I/O
> scheduler.
> Obviously request order from I/O scheduler might be poor for packing and
> requests can't be packed.
> But that doesn't mean mmc layer need to wait a better pack-case.
> BLKPREP_DEFER may give rise to I/O latency. Top of request will be
> deferred next time.
> If request can't be packed, it'd rather be sent at once than delayed
> by returning a BLKPREP_DEFER for better responsiveness.
>
> Thanks.
> Seungwon Jeon.
The decision whether a request is packable or not should not be made per
request, therefore I don't see the need for using req_prep_fn. The MMC
layer can peek each request and decide if to pack it or not when preparing
the packed list (as suggested in this patch). The scheduler changes will
improve the probability of getting a list that can be packed.
My suggestion for the scheduler change is:
The block layer will be notified of the packing limits via new queue
limits (in blk-settings). We can make it generic to allow all kinds of
requests lists. Example for the new queue limits can be:
Is_reqs_list_supported
Max_num_of_read_reqs_in_list
Max_num_of_write_reqs_in_list
max_blk_cnt_in_list
Is_list_interleaved (optional - to support read and write requests to be
linked together, not the case for eMMC4.5)
The scheduler, that will have all the required info in the queue limits,
will be able to decide which requests can be in the same list and move all
of them to the dispatch queue (in one elevator_dispatch_fn call).

I see 2 options for preparing the list:

Option 1. blk_fetch_request will prepare the list and return a list of
requests (will require a change in struct request to include the list but
can be more generic).

Option 2. The MMC layer will prepare the list. After fetching one request
the MMC layer can call blk_peek_request and check if the next request is
packable or not. By keeping all the calls to blk_peek_request under the
same queue lock we can guarantee to get the requests that the scheduler
pushed to the dispatch queue (this requires mmc_blk_chk_packable to move
to block.c). If the request is packable the MMC layer will call
blk_start_request to dequeue the request. This way there is no need to
re-queue the requests that cannot be packed.

Going back to this patch - the usage of blk_peek_request+blk_start_request
instead of blk_fetch_request can be done in mmc_blk_chk_packable in order
to eliminate the need to requeue commands and loose performance.

Thanks,
Maya Erez
Consultant for Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum

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