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

["Chalamarla, Tirumalesh" <Tirumalesh.Chalamarla@cavium.com>]

On 4/11/17, 10:13 PM, "linux-arm-kernel on behalf of Imran Khan" <linux-arm-kernel-bounces@lists.infradead.org on behalf of kimran@codeaurora.org> wrote:

    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.
    
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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