Re: [PATCH] arm64/fpsimd: Suppress SVE access traps when loading FPSIMD state

On Mon, Aug 21, 2023 at 01:10:50PM +0100, Will Deacon wrote:

On Mon, Aug 07, 2023 at 11:20:38PM +0100, Mark Brown wrote:

quoted

I did some instrumentation which counted the number of SVE access traps
and the number of times we loaded FPSIMD only register state for each task.
Testing with Debian Bookworm this showed that during boot the overwhelming
majority of tasks triggered another SVE access trap more than 50% of the
time after loading FPSIMD only state with a substantial number near 100%,
though some programs had a very small number of SVE accesses most likely
from startup. There were few tasks in the range 5-45%, most tasks either

Do you have any performance numbers to motivate this change? It would be
interesting, for example, to see how changing the timeout value affects
the results for some real workloads.

I agree, unfortunately I don't yet have a hardware setup so I can't do
those benchmarks myself at the minute - I just have the access trap
counting I mentioned in the commit message.  Last time I saw numbers the
access traps were about 70% of a syscall in a microbenchmark and we're
already suppressing them for non-blocking syscalls so I'd not expect
anything *too* revolutionary.