Re: [PATCH 0/4] crypto/arm64: reduce impact of NEON yield checks

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On 25 July 2018 at 11:09, Dave Martin <Dave.Martin@xxxxxxx> wrote:
> On Tue, Jul 24, 2018 at 06:12:20PM +0100, Ard Biesheuvel wrote:
>> Vakul reports a considerable performance hit when running the accelerated
>> arm64 crypto routines with CONFIG_PREEMPT=y configured, now that thay have
>> been updated to take the TIF_NEED_RESCHED flag into account.
>>
>> The issue appears to be caused by the fact that Cortex-A53, the core in
>> question, has a high end implementation of the Crypto Extensions, and
>> has a shallow pipeline, which means even sequential algorithms that may be
>> held back by pipeline stalls on high end out of order cores run at maximum
>> speed. This means SHA-1, SHA-2, GHASH and AES in GCM and CCM modes run at a
>> speed in the order of 2 to 4 cycles per byte, and are currently implemented
>> to check the TIF_NEED_RESCHED after each iteration, which may process as
>> little as 16 bytes (for GHASH).
>>
>> Obviously, every cycle of overhead hurts in this context, and given that
>> the A53's load/store unit is not quite high end, any delays caused by
>> memory accesses that occur in the inner loop of the algorithms are going
>> to be quite significant, hence the performance regression.
>>
>> So reduce the frequency at which the NEON yield checks are performed, so
>> that they occur roughly once every 1000 cycles, which is hopefully a
>> reasonable tradeoff between throughput and worst case scheduling latency.
>
> Is there any way to tune this automatically, or it that likely to be more
> trouble than it's worth?
>

Good question. I think A53 is a reasonable worst case, and these
changes reduce the impact to the same ballpark as the impact of
enabling CONFIG_PREEMPT in the first place.

> Also, how did you come up with 1000 cycles?  At what point does
> preemption latency become more/less important than throughput?
>

Another good question. I was hoping Sebastian or the other -rt folks
would be triggered by this. Given the above, I ended up with a ~1000
cycles quantum, and hopefully this is considered to be small enough.

> Maybe someone already invented a similar framework somewhere else in the
> kernel.  I seem to remember some automatic selection of memcpy
> implementation based on a boot-time benchmark, but I may have
> misremembered.
>

We have crypto benchmarking code in the kernel, and at some point, I
even did some work on selecting the best algo based on performance.

But to be honest, I think this is a bit overkill. If you need those
final 5% of throughput at any cost, you're better off running with
CONFIG_PREEMPT=n anyway.



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