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

Jiri Olsa
2022-05-19T11:33:26Z

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 ;-)

>
> 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

thanks,
jirka

>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit cd338be719a0a692e0d50e1a8438e1f6c7165d9c
> Author: Paul E. McKenney <paulmck@kernel.org>
> Date: Tue May 17 21:00:04 2022 -0700
>
> rcu: Apply noinstr to rcu_idle_enter() and rcu_idle_exit()
>
> This commit applies the "noinstr" tag to the rcu_idle_enter() and
> rcu_idle_exit() functions, which are invoked from portions of the idle
> loop that cannot be instrumented. These tags require reworking the
> rcu_eqs_enter() and rcu_eqs_exit() functions that these two functions
> invoke in order to cause them to use normal assertions rather than
> lockdep. In addition, within rcu_idle_exit(), the raw versions of
> local_irq_save() and local_irq_restore() are used, again to avoid issues
> with lockdep in uninstrumented code.
>
> This patch is based in part on an earlier patch by Jiri Olsa, discussions
> with Peter Zijlstra and Frederic Weisbecker, earlier changes by Thomas
> Gleixner, and off-list discussions with Yonghong Song.
>
> Link: https://lore.kernel.org/lkml/20220515203653.4039075-1-jolsa@kernel.org/
> Reported-by: Jiri Olsa <jolsa@kernel.org>
> Reported-by: Alexei Starovoitov <ast@kernel.org>
> Reported-by: Andrii Nakryiko <andrii@kernel.org>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Reviewed-by: Yonghong Song <yhs@fb.com>
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 222d59299a2af..02233b17cce0e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -635,8 +635,8 @@ static noinstr void rcu_eqs_enter(bool user)
> return;
> }
>
> - lockdep_assert_irqs_disabled();
> instrumentation_begin();
> + lockdep_assert_irqs_disabled();
> trace_rcu_dyntick(TPS("Start"), rdp->dynticks_nesting, 0, atomic_read(&rdp->dynticks));
> WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
> rcu_preempt_deferred_qs(current);
> @@ -663,9 +663,9 @@ static noinstr void rcu_eqs_enter(bool user)
> * If you add or remove a call to rcu_idle_enter(), be sure to test with
> * CONFIG_RCU_EQS_DEBUG=y.
> */
> -void rcu_idle_enter(void)
> +void noinstr rcu_idle_enter(void)
> {
> - lockdep_assert_irqs_disabled();
> + WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !raw_irqs_disabled());
> rcu_eqs_enter(false);
> }
> EXPORT_SYMBOL_GPL(rcu_idle_enter);
> @@ -865,7 +865,7 @@ static void noinstr rcu_eqs_exit(bool user)
> struct rcu_data *rdp;
> long oldval;
>
> - lockdep_assert_irqs_disabled();
> + WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !raw_irqs_disabled());
> rdp = this_cpu_ptr(&rcu_data);
> oldval = rdp->dynticks_nesting;
> WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && oldval < 0);
> @@ -900,13 +900,13 @@ 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;
>
> - local_irq_save(flags);
> + raw_local_irq_save(flags);
> rcu_eqs_exit(false);
> - local_irq_restore(flags);
> + raw_local_irq_restore(flags);
> }
> EXPORT_SYMBOL_GPL(rcu_idle_exit);
>