Re: [PATCH bpf-next 1/2] cpuidle/rcu: Making arch_cpu_idle and rcu_idle_exit noinstr

Jiri Olsa
2022-05-23T13:13:27Z

On Thu, May 19, 2022 at 06:54:39AM -0700, Paul E. McKenney wrote:
> On Thu, May 19, 2022 at 01:33:16PM +0200, Jiri Olsa wrote:
> > On Wed, May 18, 2022 at 09:21:18AM -0700, Paul E. McKenney wrote:
> > > On Tue, May 17, 2022 at 12:13:45PM +0200, Jiri Olsa wrote:
> > > > On Mon, May 16, 2022 at 01:49:22PM +0200, Frederic Weisbecker wrote:
> > > > > On Sun, May 15, 2022 at 09:25:35PM -0700, Paul E. McKenney wrote:
> > > > > > On Sun, May 15, 2022 at 10:36:52PM +0200, Jiri Olsa wrote:
> > > > > > > Making arch_cpu_idle and rcu_idle_exit noinstr. Both functions run
> > > > > > > in rcu 'not watching' context and if there's tracer attached to
> > > > > > > them, which uses rcu (e.g. kprobe multi interface) it will hit RCU
> > > > > > > warning like:
> > > > > > >
> > > > > > > [ 3.017540] WARNING: suspicious RCU usage
> > > > > > > ...
> > > > > > > [ 3.018363] kprobe_multi_link_handler+0x68/0x1c0
> > > > > > > [ 3.018364] ? kprobe_multi_link_handler+0x3e/0x1c0
> > > > > > > [ 3.018366] ? arch_cpu_idle_dead+0x10/0x10
> > > > > > > [ 3.018367] ? arch_cpu_idle_dead+0x10/0x10
> > > > > > > [ 3.018371] fprobe_handler.part.0+0xab/0x150
> > > > > > > [ 3.018374] 0xffffffffa00080c8
> > > > > > > [ 3.018393] ? arch_cpu_idle+0x5/0x10
> > > > > > > [ 3.018398] arch_cpu_idle+0x5/0x10
> > > > > > > [ 3.018399] default_idle_call+0x59/0x90
> > > > > > > [ 3.018401] do_idle+0x1c3/0x1d0
> > > > > > >
> > > > > > > The call path is following:
> > > > > > >
> > > > > > > default_idle_call
> > > > > > > rcu_idle_enter
> > > > > > > arch_cpu_idle
> > > > > > > rcu_idle_exit
> > > > > > >
> > > > > > > The arch_cpu_idle and rcu_idle_exit are the only ones from above
> > > > > > > path that are traceble and cause this problem on my setup.
> > > > > > >
> > > > > > > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > > > > >
> > > > > > From an RCU viewpoint:
> > > > > >
> > > > > > Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
> > > > > >
> > > > > > [ I considered asking for an instrumentation_on() in rcu_idle_exit(),
> > > > > > but there is no point given that local_irq_restore() isn't something
> > > > > > you instrument anyway. ]
> > > > >
> > > > > So local_irq_save() in the beginning of rcu_idle_exit() is unsafe because
> > > > > it is instrumentable by the function (graph) tracers and the irqsoff tracer.
> > > > >
> > > > > Also it calls into lockdep that might make use of RCU.
> > > > >
> > > > > That's why rcu_idle_exit() is not noinstr yet. See this patch:
> > > > >
> > > > > https://lore.kernel.org/lkml/20220503100051.2799723-4-frederic@kernel.org/
> > > >
> > > > I see, could we mark it at least with notrace meanwhile?
> > >
> > > For the RCU part, how about as follows?
> > >
> > > If this approach is reasonable, my guess would be that Frederic will pull
> > > it into his context-tracking series, perhaps using a revert of this patch
> > > to maintain sanity in the near term.
> > >
> > > If this approach is unreasonable, well, that is Murphy for you!
> >
> > I checked and it works in my test ;-)
>
> Whew!!! One piece of the problem might be solved, then. ;-)
>
> > > For the x86 idle part, my feeling is still that the rcu_idle_enter()
> > > and rcu_idle_exit() need to be pushed deeper into the code. Perhaps
> > > an ongoing process as the idle loop continues to be dug deeper?
> >
> > for arch_cpu_idle with noinstr I'm getting this W=1 warning:
> >
> > vmlinux.o: warning: objtool: arch_cpu_idle()+0xb: call to {dynamic}() leaves .noinstr.text section
> >
> > we could have it with notrace if that's a problem
>
> I would be happy to queue the arch_cpu_idle() portion of your patch on
> -rcu, if that would move things forward. I suspect that additional
> x86_idle() surgery is required, but maybe I am just getting confused
> about what the x86_idle() function pointer can point to. But it looks
> to me like these need further help:
>
> o static void amd_e400_idle(void)
> Plus things it calls, like tick_broadcast_enter() and
> tick_broadcast_exit().
>
> o static __cpuidle void mwait_idle(void)
>
> So it might not be all that much additional work, even if I have avoided
> confusion about what the x86_idle() function pointer can point to. But
> I do not trust my ability to test this accurately.

same here ;-) you're right, there will be other places based
on x86_idle function pointer.. I'll check it, but perhaps we
could address that when someone reports that

jirka