On 4/11/17, 10:13 PM, "linux-arm-kernel on behalf of Imran Khan" <linux-arm-kernel-bounces@xxxxxxxxxxxxxxxxxxx on behalf of kimran@xxxxxxxxxxxxxx> wrote:
On 4/7/2017 7:36 AM, Ganesh Mahendran wrote:
> 2017-04-06 23:58 GMT+08:00 Catalin Marinas <catalin.marinas@xxxxxxx>:
>> 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.
We tried different benchmarks and found none which really affects with Cache line change. If there is no correctness issue,
I think we are fine with reverting the patch.
Though I still think it is beneficiary to do some more investigation for the perf loss, who knows 32 bit align or no align might
Give even more perf benefit.
Thanks,
Tirumalesh.
>>
>> 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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