On 25 July 2018 at 11:09, Dave Martin <dave.mar...@arm.com> 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.
