SPI: performance regression when using the common message queuing infrastructure

[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.
>>
>> 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 :)

Best regards,

Cyrille