On Tue, Aug 16, 2022 at 8:02 PM Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Tue, Aug 16, 2022 at 10:49 AM Jon Nettleton <jon@xxxxxxxxxxxxx> wrote: > > > > It is moot if Linus has already taken the patch, but with a stock > > kernel config I am > > still seeing a slight performance dip but only ~1-2% in the specific > > tests I was running. > > It would be interesting to hear if you can pinpoint in the profiles > where the time is spent. > > It might be some random place that really doesn't care about ordering > at all, and then we could easily rewrite _that_ particular case to do > the unordered test explicitly, ie something like > > - if (test_and_set_bit()) ... > + if (test_bit() || test_and_set_bit()) ... > > or even introduce an explicitly unordered "test_and_set_bit_relaxed()" thing. > > Linus This is very interesting, the additional performance overhead doesn't seem to be coming from within the kernel but from userspace. Comparing patched and unpatched kernels I am seeing more cycles being taken up by glibc atomics like __aarch64_cas4_acq and __aarch64_ldadd4_acq_rel. I need to test further to see if there is less effect on a system with less cores, This is a 16-core Cortex-A72, it is possible this is less of an issue on 4 core A72's and A53's. -Jon
