On 2 December 2017 at 13:59, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > On Sat, Dec 02, 2017 at 11:15:14AM +0000, Ard Biesheuvel wrote: >> On 2 December 2017 at 09:11, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote: > >> > They consume the entire input in a single go, yes. But making it more >> > granular than that is going to hurt performance, unless we introduce >> > some kind of kernel_neon_yield(), which does a end+begin but only if >> > the task is being scheduled out. >> > >> > For example, the SHA256 keeps 256 bytes of round constants in NEON >> > registers, and reloading those from memory for each 64 byte block of >> > input is going to be noticeable. The same applies to the AES code >> > (although the numbers are slightly different) >> >> Something like below should do the trick I think (apologies for the >> patch soup). I.e., check TIF_NEED_RESCHED at a point where only very >> few NEON registers are live, and preserve/restore the live registers >> across calls to kernel_neon_end + kernel_neon_begin. Would that work >> for RT? > > Probably yes. The important point is that preempt latencies (and thus by > extension NEON regions) are bounded and preferably small. > > Unbounded stuff (like depends on the amount of data fed) are a complete > no-no for RT since then you cannot make predictions on how long things > will take. > OK, that makes sense. But I do wonder what the parameters should be here. For instance, the AES instructions on ARMv8 operate at <1 cycle per byte, and so checking the TIF_NEED_RESCHED flag for every iteration of the inner loop (i.e., every 64 bytes ~ 64 cycles) is clearly going to be noticeable, and is probably overkill. The pure NEON version (which is instantiated from the same block mode wrappers) uses ~25 cycles per byte, and the bit sliced NEON version runs at ~20 cycles per byte but can only operate at 8 blocks (128 bytes) at a time. So rather than simply polling the bit at each iteration of the inner loop in each algorithm, I'd prefer to aim for a ballpark number of cycles to execute, in the order 1000 - 2000. Would that be OK or too coarse?