Re: [PATCH] fs/exec.c: Add fast path for ENOENT on PATH search before allocating mm

Mateusz Guzik
2023-11-07T23:08:55Z

On 11/7/23, Kees Cook <keescook@chromium.org> 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
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.

--
Mateusz Guzik <mjguzik gmail.com>