On 25 July 2018 at 11:45, Dave Martin <dave.mar...@arm.com> wrote: > On Wed, Jul 25, 2018 at 10:23:00AM +0100, Ard Biesheuvel wrote: >> 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. > > Can't really argue with any of that -- I was just wondering whether > there was precedent. > > Hopefully the ~1000 cycles ballpark will satisfy most people. For > the rest, it's too bad: if somebody is relying on the last 1-2% of > performance, they probably have a broken use case. >
Indeed. OTOH, if the -rt people (Sebastian?) turn up and say that a 1000 cycle limit to the quantum of work performed with preemption disabled is unreasonably low, we can increase the yield block counts and approach the optimal numbers a bit closer. But with diminishing returns. Also, if the cost of enabling CONFIG_PREEMPT by itself is significantly reduced, e.g., by moving the per-CPU offset into a GPR, we can always revisit this of course.
