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

Jiri Olsa
2022-05-17T10:18:06Z

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?

jirka