vchiq: Performance regression since 5.18-rc1

[Stefan Wahren]

Hi Phil,

Am 23.05.22 um 11:29 schrieb Phil Elwell:
> Hi Stefan,
>
> On 23/05/2022 07:19, Stefan Wahren wrote:
>> Hi Paul,
>>
>> Am 23.05.22 um 06:48 schrieb Paul E. McKenney:
>>> On Sun, May 22, 2022 at 05:11:36PM +0200, Stefan Wahren wrote:
>>>> Hi Paul,
>>>>
>>>> Am 22.05.22 um 01:46 schrieb Paul E. McKenney:
>>>>> On Sun, May 22, 2022 at 01:22:00AM +0200, Stefan Wahren wrote:
>>>>>> Hi,
>>>>>>
>>>>>> while testing the staging/vc04_services/interface/vchiq_arm 
>>>>>> driver with my
>>>>>> Raspberry Pi 3 B+ (multi_v7_defconfig) i noticed a huge performance
>>>>>> regression since [ff042f4a9b050895a42cae893cc01fa2ca81b95c] mm:
>>>>>> lru_cache_disable: replace work queue synchronization with 
>>>>>> synchronize_rcu
>>>>>>
>>>>>> Usually i run "vchiq_test -f 1" to see the driver is still 
>>>>>> working [1].
>>>>>>
>>>>>> Before commit:
>>>>>>
>>>>>> real    0m1,500s
>>>>>> user    0m0,068s
>>>>>> sys    0m0,846s
>>>>>>
>>>>>> After commit:
>>>>>>
>>>>>> real    7m11,449s
>>>>>> user    0m2,049s
>>>>>> sys    0m0,023s
>>>>>>
>>>>>> Best regards
>>>>>>
>>>>>> [1] - https://github.com/raspberrypi/userland
>>>>> Please feel free to try the patch shown below.  Or the pair of 
>>>>> patches
>>>>> from Rik here:
>>>>>
>>>>> https://lore.kernel.org/lkml/20220218183114.2867528-2-riel@surriel.com/ 
>>>>>
>>>>> https://lore.kernel.org/lkml/20220218183114.2867528-3-riel@surriel.com/ 
>>>>>
>>>> I tried your patch and Rik's patches but in both cases vchiq_test 
>>>> runs 7
>>>> minutes instead of ~ 1 second.
>>> That is surprising.  Do you boot with rcupdate.rcu_normal=1?
>> No, not explicit.
>>>    That would
>>> nullify my patch, but I would expect that Rik's patch would still 
>>> provide
>>> increased performance even in that case.
>> I will retest with a fresh SD card image.
>>>
>>> Could you please characterize where the slowdown is occurring?
>>
>> Unfortunately i don't have a deep insight into driver and vchiq_test 
>> tool. Just a user view.
>>
>> Do you think an strace would be a good starting point?
>>
>> @Phil Any advices to analyse this issue?
>
> Sending many small control packets:
>
>    vchiq_test -c 1 10000
>
> essentially tests interrupt latency. Using a small number of large 
> bulk transfers:
>
>    vchiq_test -b 10000 1
>
> becomes a test of how long it takes to lock down pages. It also tests 
> DMA transfer speeds, but since the DMA is run by the firmware (which 
> you aren't changing), I think you can rule that.
Thanks i will try.
>
> You may also find it helpful to include "force_turbo=1" in config.txt 
> for more predictable results.
>
> By the way, running our 5.18-rc7-based branch on a 3B+ I'm not seeing 
> any performance problems:
I assume you are using arm/bcm2709_defconfig and not 
arm/multi_v7_defconfig as me?
>
> pi at raspberrypi:~$ time vchiq_test -f 1
> Functional test - iters:1
> ======== iteration 1 ========
> Testing bulk transfer for alignment.
> Testing bulk transfer at PAGE_SIZE.
>
> real    0m0.512s
> user    0m0.042s
> sys     0m0.165s
>
> Phil