On November 7, 2023 3:08:47 PM PST, Mateusz Guzik <mjguzik@xxxxxxxxx> wrote: >On 11/7/23, Kees Cook <keescook@xxxxxxxxxxxx> wrote: >> On Tue, Nov 07, 2023 at 10:23:16PM +0100, Mateusz Guzik wrote: >>> If the patch which dodges second lookup still somehow appears slower a >>> flamegraph or other profile would be nice. I can volunteer to take a >>> look at what's going on provided above measurements will be done and >>> show funkyness. >> >> When I looked at this last, it seemed like all the work done in >> do_filp_open() (my patch, which moved the lookup earlier) was heavier >> than the duplicate filename_lookup(). >> >> What I didn't test was moving the sched_exec() before the mm creation, >> which Peter confirmed shouldn't be a problem, but I think that might be >> only a tiny benefit, if at all. >> >> If you can do some comparisons, that would be great; it always takes me >> a fair bit of time to get set up for flame graph generation, etc. :) >> > >So I spawned *one* process executing one statocally linked binary in a >loop, test case from http://apollo.backplane.com/DFlyMisc/doexec.c . > >The profile is definitely not what I expected: > 5.85% [kernel] [k] asm_exc_page_fault > 5.84% [kernel] [k] __pv_queued_spin_lock_slowpath >[snip] > >I'm going to have to recompile with lock profiling, meanwhile >according to bpftrace >(bpftrace -e 'kprobe:__pv_queued_spin_lock_slowpath { @[kstack()] = count(); }') >top hits would be: > >@[ > __pv_queued_spin_lock_slowpath+1 > _raw_spin_lock+37 > __schedule+192 > schedule_idle+38 > do_idle+366 > cpu_startup_entry+38 > start_secondary+282 > secondary_startup_64_no_verify+381 >]: 181 >@[ > __pv_queued_spin_lock_slowpath+1 > _raw_spin_lock_irq+43 > wait_for_completion+141 > stop_one_cpu+127 > sched_exec+165 There's the suspicious sched_exec() I was talking about! :) I think it needs to be moved, and perhaps _later_ instead of earlier? Hmm... -Kees > bprm_execve+328 > do_execveat_common.isra.0+429 > __x64_sys_execve+50 > do_syscall_64+46 > entry_SYSCALL_64_after_hwframe+110 >]: 206 > >I did not see this coming for sure. I'll poke around maybe this weekend. > -- Kees Cook
