SPI: performance regression when using the common message queuing infrastructure

[Heiko Schocher]

Hello Cyrille,

sorry for the late answer, but just back from holidays ...

Am 07.07.2016 um 10:12 schrieb Cyrille Pitchen:
> Hi Grygorii,
>
> Le 06/07/2016 12:03, Grygorii Strashko a écrit :
>> On 07/06/2016 12:50 PM, Cyrille Pitchen wrote:
>>> Hi Mark,
>>>
>>> recently Heiko reported to us a performance regression with Atmel SPI
>>> controllers. He noticed the issue on a sam9g15ek board and I was also able to
>>> reproduce it on a sama5d36ek board.
>>>
>>> We found out that the performance regression was introduced in 3.14 by commit:
>>> 8090d6d1a415d3ae1a7208995decfab8f60f4f36
>>> spi: atmel: Refactor spi-atmel to use SPI framework queue
>>>
>>> For the test, I connected a Spansion S25FL512 memory on the SPI1 controller of
>>> a sama5d36ek board. Then with an oscilloscope I monitored the chip-select, clock
>>> and MOSI signals on the SPI bus.
>>>
>>>
>>> 1 - Reading 512 bytes from the memory
>>>
>>> # dd if=/dev/mtd6 bs=512 count=1 of=/dev/null
>>>
>>> With the oscilloscope, I measured the time between the chip-select fell before
>>> the Read Status command (05h) and the chip-select rose after all data had been
>>> read by the 4-byte address Fast Read 1-1-1 command (13h).
>>>
>>> 3.14 vanilla                      : 305 µs
>>> 3.14 commit 8090d6d1a415 reverted : 242 µs   -21%
>>>
>>> 2 - Reading 1000 x 1024 bytes from the memory
>>>
>>> # dd if=/dev/mtd6 bs=1024 count=1000 of=/dev/null
>>>
>>> Still with the scope, I measured the time to read all data.
>>>
>>> 3.14 vanilla                      : 435 ms
>>> 3.14 commit 8090d6d1a415 reverted : 361 ms   -17%
>>>
>>>
>>> Indeed the oscilloscope shows that more time is spent between messages and
>>> transfers.

Yes this fits with my observations.

>>> commit 8090d6d1a415 replaced the tasklet used to manage a SPI message/transfer
>>> queue by a workqueue provided by the SPI framework.
>>>
>>> The support of this (optional) workqueue was introduced by commit:
>>> ffbbdd21329f3e15eeca6df2d4bc11c04d9d91c0
>>> spi: create a message queuing infrastructure
>>>
>>> Though the commit message claims that is common infrastructure is optional,
>>> the patch also claims the .transfer() hook is deprecated, suggesting drivers
>>> should implement the new .transfer_one_message() hook instead.
>>>
>>> This is the reason why commit 8090d6d1a415 was submitted. However we lost
>>> quite amount of performances moving from our tasklet to the generic workqueue.
>>>
>>> So do you recommend us to keep our current generic implementation relying on
>>> the SPI framework workqueue or to go back to a custom implementation using
>>> tasklet?
>>> If we keep the current implementation, is there a way to improve the
>>> performances so we go back to something close to what he had before?
>>>
>>> We saw in commit ffbbdd21329f that we can change the workqueue thread
>>> scheduling policy to SCHED_FIFO by setting master->rt.
>>>
>>
>> master->rt is not a good choice as i know and
>> you may find thread [1] useful for you.
>>
>> [1] http://www.spinics.net/lists/linux-rt-users/msg14347.html
>>
>
> thanks for the link, I'll look at it :)

Thanks for digging into this issue and your tests!

Do you have some new results? Can I help you?

bye,
Heiko
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