On 09/04/2025 16:18, Mateusz Guzik wrote:
On Wed, Apr 09, 2025 at 03:31:38PM +0100, Colin Ian King wrote:Adding an unlikely() hint on the fd < 0 comparison return path improves run-time performance of the mincore system call. gcov based coverage analysis shows that this path return path is highly unlikely. Benchmarking on an Debian based Intel(R) Core(TM) Ultra 9 285K with a 6.15-rc1 kernel and a poll of 1024 file descriptors with zero timeout shows an call reduction from 32818 ns down to 32635 ns, which is a ~0.5% performance improvement. Results based on running 25 tests with turbo disabled (to reduce clock freq turbo changes), with 30 second run per test and comparing the number of poll() calls per second. The % standard deviation of the 25 tests was 0.08%, so results are reliable.I don't think adding a branch hint warrants benchmarking of the sort. Instead the thing to do is to check if the prediction matches real world uses. While it is impossible to check this for all programs out there, it should not be a significant time investment to look to check some of the popular ones out there. Normally I would do it with bpftrace, but this comes from a user-backed area instead of func args, so involved hackery may be needed which is not warranted the change. Perhaps running strace on a bunch of network progs would also do it (ssh, browser?). I have to say I did not even know one can legally pass a fd < 0 to poll and I never seen it in action, so I don't expect many users. ;)
I did check this based on gcov coverage (mentioned in the commit message) and this is based on running gcov data from running stress-ng and kernel builds and I've been looking for branch hint performance wins based on the top 250 if statements that are not already hinted using likely/unlikely.
Colin
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