Re: [PATCH] Revert "arm64: Increase the max granular size"

[Imran Khan <kimran@codeaurora.org>]

On 4/7/2017 7:36 AM, Ganesh Mahendran wrote:
> 2017-04-06 23:58 GMT+08:00 Catalin Marinas <catalin.marinas@arm.com>:
>> On Thu, Apr 06, 2017 at 12:52:13PM +0530, Imran Khan wrote:
>>> On 4/5/2017 10:13 AM, Imran Khan wrote:
>>>>> We may have to revisit this logic and consider L1_CACHE_BYTES the
>>>>> _minimum_ of cache line sizes in arm64 systems supported by the kernel.
>>>>> Do you have any benchmarks on Cavium boards that would show significant
>>>>> degradation with 64-byte L1_CACHE_BYTES vs 128?
>>>>>
>>>>> For non-coherent DMA, the simplest is to make ARCH_DMA_MINALIGN the
>>>>> _maximum_ of the supported systems:
>>>>>
>>>>> diff --git a/arch/arm64/include/asm/cache.h b/arch/arm64/include/asm/cache.h
>>>>> index 5082b30bc2c0..4b5d7b27edaf 100644
>>>>> --- a/arch/arm64/include/asm/cache.h
>>>>> +++ b/arch/arm64/include/asm/cache.h
>>>>> @@ -18,17 +18,17 @@
>>>>>
>>>>>  #include <asm/cachetype.h>
>>>>>
>>>>> -#define L1_CACHE_SHIFT         7
>>>>> +#define L1_CACHE_SHIFT         6
>>>>>  #define L1_CACHE_BYTES         (1 << L1_CACHE_SHIFT)
>>>>>
>>>>>  /*
>>>>>   * Memory returned by kmalloc() may be used for DMA, so we must make
>>>>> - * sure that all such allocations are cache aligned. Otherwise,
>>>>> - * unrelated code may cause parts of the buffer to be read into the
>>>>> - * cache before the transfer is done, causing old data to be seen by
>>>>> - * the CPU.
>>>>> + * sure that all such allocations are aligned to the maximum *known*
>>>>> + * cache line size on ARMv8 systems. Otherwise, unrelated code may cause
>>>>> + * parts of the buffer to be read into the cache before the transfer is
>>>>> + * done, causing old data to be seen by the CPU.
>>>>>   */
>>>>> -#define ARCH_DMA_MINALIGN      L1_CACHE_BYTES
>>>>> +#define ARCH_DMA_MINALIGN      (128)
>>>>>
>>>>>  #ifndef __ASSEMBLY__
>>>>>
>>>>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>>>>> index 392c67eb9fa6..30bafca1aebf 100644
>>>>> --- a/arch/arm64/kernel/cpufeature.c
>>>>> +++ b/arch/arm64/kernel/cpufeature.c
>>>>> @@ -976,9 +976,9 @@ void __init setup_cpu_features(void)
>>>>>         if (!cwg)
>>>>>                 pr_warn("No Cache Writeback Granule information, assuming
>>>>> cache line size %d\n",
>>>>>                         cls);
>>>>> -       if (L1_CACHE_BYTES < cls)
>>>>> -               pr_warn("L1_CACHE_BYTES smaller than the Cache Writeback Granule (%d < %d)\n",
>>>>> -                       L1_CACHE_BYTES, cls);
>>>>> +       if (ARCH_DMA_MINALIGN < cls)
>>>>> +               pr_warn("ARCH_DMA_MINALIGN smaller than the Cache Writeback Granule (%d < %d)\n",
>>>>> +                       ARCH_DMA_MINALIGN, cls);
>>>>>  }
>>>>>
>>>>>  static bool __maybe_unused
>>>>
>>>> This change was discussed at: [1] but was not concluded as apparently no one
>>>> came back with test report and numbers. After including this change in our
>>>> local kernel we are seeing significant throughput improvement. For example with:
>>>>
>>>> iperf -c 192.168.1.181 -i 1 -w 128K -t 60
>>>>
>>>> The average throughput is improving by about 30% (230Mbps from 180Mbps).
>>>> Could you please let us know if this change can be included in upstream kernel.
>>>>
>>>> [1]: https://groups.google.com/forum/#!topic/linux.kernel/P40yDB90ePs
>>>
>>> Could you please provide some feedback about the above mentioned query ?
>>
>> Do you have an explanation on the performance variation when
>> L1_CACHE_BYTES is changed? We'd need to understand how the network stack
>> is affected by L1_CACHE_BYTES, in which context it uses it (is it for
>> non-coherent DMA?).
> 
> network stack use SKB_DATA_ALIGN to align.
> ---
> #define SKB_DATA_ALIGN(X) (((X) + (SMP_CACHE_BYTES - 1)) & \
> ~(SMP_CACHE_BYTES - 1))
> 
> #define SMP_CACHE_BYTES L1_CACHE_BYTES
> ---
> I think this is the reason of performance regression.
> 

Yes this is the reason for performance regression. Due to increases L1 cache alignment the 
object is coming from next kmalloc slab and skb->truesize is changing from 2304 bytes to 
4352 bytes. This in turn increases sk_wmem_alloc which causes queuing of less send buffers.

>>
>> The Cavium guys haven't shown any numbers (IIUC) to back the
>> L1_CACHE_BYTES performance improvement but I would not revert the
>> original commit since ARCH_DMA_MINALIGN definitely needs to cover the
>> maximum available cache line size, which is 128 for them.
> 
> how about define L1_CACHE_SHIFT like below:
> ---
> #ifdef CONFIG_ARM64_L1_CACHE_SHIFT
> #define L1_CACHE_SHIFT CONFIG_ARM64_L1_CACHE_SHIFT
> #else
> #define L1_CACHE_SHIFT 7
> endif
> ---
> 
> Thanks
> 
>>
>> --
>> Catalin


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