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

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 17 May 2022 09:49:33 +0200
Jiri Olsa <olsajiri@gmail.com> wrote:

> On Mon, May 16, 2022 at 07:54:37PM -0700, Yonghong Song wrote:
> > 
> > 
> > On 5/15/22 1:36 PM, 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>
> > > ---
> > >   arch/x86/kernel/process.c | 2 +-
> > >   kernel/rcu/tree.c         | 2 +-
> > >   2 files changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> > > index b370767f5b19..1345cb0124a6 100644
> > > --- a/arch/x86/kernel/process.c
> > > +++ b/arch/x86/kernel/process.c
> > > @@ -720,7 +720,7 @@ void arch_cpu_idle_dead(void)
> > >   /*
> > >    * Called from the generic idle code.
> > >    */
> > > -void arch_cpu_idle(void)
> > > +void noinstr arch_cpu_idle(void)
> > 
> > noinstr includes a lot of attributes:
> > 
> > #define noinstr                                                         \
> >         noinline notrace __attribute((__section__(".noinstr.text")))    \
> >         __no_kcsan __no_sanitize_address __no_profile __no_sanitize_coverage
> > 
> > should we use notrace here?
> 
> hm right, so notrace should be enough for our case (kprobe_multi)
> which is based on ftrace/fprobe jump
> 
> noinstr (among other things) adds the function also the kprobes
> blacklist, which will prevent standard kprobes to attach
> 
> ASAICS standard kprobes use rcu in probe path as well, like in
> opt_pre_handler function
> 
> so I think we should go with noinstr

Yes, I agree that noinstr is preferrable for these functions.

Thank you!

> 
> jirka
> 
> > 
> > >   {
> > >   	x86_idle();
> > >   }
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index a4b8189455d5..20d529722f51 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -896,7 +896,7 @@ static void noinstr rcu_eqs_exit(bool user)
> > >    * If you add or remove a call to rcu_idle_exit(), be sure to test with
> > >    * CONFIG_RCU_EQS_DEBUG=y.
> > >    */
> > > -void rcu_idle_exit(void)
> > > +void noinstr rcu_idle_exit(void)
> > >   {
> > >   	unsigned long flags;


-- 
Masami Hiramatsu <mhiramat@kernel.org>