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

[Kees Cook]

On Wed, Nov 08, 2023 at 01:03:33AM +0100, Mateusz Guzik wrote:
> On 11/8/23, Kees Cook <kees@xxxxxxxxxx> wrote:
> >
> >
> > 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...
> >
> 
> I'm getting around 3.4k execs/s. However, if I "taskset -c 3
> ./static-doexec 1" the number goes up to about 9.5k and lock
> contention disappears from the profile. So off hand looks like the
> task is walking around the box when it perhaps could be avoided -- it
> is idle apart from running the test. Again this is going to require a
> serious look instead of ad hoc pokes.

Peter, is this something you can speak to? It seems like execve() forces
a change in running CPU. Is this really something we want to be doing?
Or is there some better way to keep it on the same CPU unless there is
contention?

-Kees

-- 
Kees Cook