vchiq: Performance regression since 5.18-rc1

[Stefan Wahren]

Hi Phil,

Am 23.05.22 um 13:01 schrieb Phil Elwell:
> Hi Stefan,
>
> On 23/05/2022 11:48, Stefan Wahren wrote:
>> 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?
>
> That's correct. Simply switching to multi_v7_defconfig breaks vchiq 
> completely, presumably because it doesn't define CONFIG_BCM2835_VCHIQ.
sorry, forgot to mention. I that i enable VCHIQ as module on top of 
multi_v7_defconfig.
>
> Phil
>
>>>
>>> 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