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Guzik wrote: > So I had a look where the timing difference is coming from and I think > I have the answer: init_ipc_ns does not have a guaranteed cacheline > placement and things get moved around with the patch. > > On my kernels (nm vmlinux-newbits | sort -nk 1 | less) > > before: > ffffffff839ffb60 T init_ipc_ns > ffffffff83a00020 t event_exit__msgrcv > > after: > ffffffff839ffbc0 T init_ipc_ns > ffffffff83a00080 t event_exit__msgrcv > > This is the pervasive problem of vars from all .o files placed > adjacent to each other, meaning changes in one .o file result in > offsets changing in other files and then you get performance > fluctuations as not-explicitly-padded variables share (or no longer > share) cachelines. > > I brought this up a year ago elsewhere: > https://gcc.gnu.org/pipermail/gcc/2024-October/245004.html > > maybe i should pick it up again and see it through > > as for the thing at hand, someone(tm) will want to make sure the > namespace is cacheline aligned and possibly pad its own internals > afterwards. Personally I can't be bothered. Thanks! Looking it seems we accumulate a bunch of offsets in: print_fmt_dax_pte_fault_class print_fmt_dax_pmd_load_hole_class print_fmt_dax_pmd_fault_class (entries that the bloat-o-meter confirms changed in size) That continue on to eventually offset init_ipc_ns. It actually looked in my testing like performance _improved_ with the change, and I notice locally I end up aligned on the struct: ffffffff82be16a0 T init_ipc_ns -> ffffffff82be1700 T init_ipc_ns Examining stress-ng before 2b6a3f061f11: Command is: $ stress-ng --timeout 60 --times --verify --metrics --no-rand-seed --msg $(nproc) Results: stress-ng: metrc: [1662] stressor bogo ops real time usr time sys time bogo ops/s bogo ops/s CPU used per RSS Max stress-ng: metrc: [776] msg 964459758 60.00 154.63 632.77 16073484.12 1224879.38 21.17 2132 After 2b6a3f061f11: stress-ng: metrc: [782] msg 1326214608 60.00 194.19 713.34 22102974.72 1461348.11 24.40 2140 And if I simply do: ipc/msgutil.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/ipc/msgutil.c b/ipc/msgutil.c index 7a03f6d03de3..df0d7a067bcf 100644 --- a/ipc/msgutil.c +++ b/ipc/msgutil.c @@ -26,7 +26,7 @@ DEFINE_SPINLOCK(mq_lock); * compiled when either CONFIG_SYSVIPC and CONFIG_POSIX_MQUEUE * and not CONFIG_IPC_NS. */ -struct ipc_namespace init_ipc_ns = { +struct ipc_namespace init_ipc_ns ____cacheline_aligned_in_smp = { .ns.__ns_ref = REFCOUNT_INIT(1), .user_ns = &init_user_ns, .ns.inum = ns_init_inum(&init_ipc_ns), -- 2.52.0 We observe: stress-ng: metrc: [764] msg 1321700290 60.00 196.73 723.82 22028700.93 1435779.72 24.75 2116 aligned So this really _does_ look like an alignment issue. So I think I should just submit the above patch right? Can you see how it behaves for you? Cheers, Lorenzo