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Date: Thu, 19 May 2022 06:54:39 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Jiri Olsa <olsajiri@...il.com>
Cc: Frederic Weisbecker <frederic@...nel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Masami Hiramatsu <mhiramat@...nel.org>, netdev@...r.kernel.org,
bpf@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>,
Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...omium.org>,
Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH bpf-next 1/2] cpuidle/rcu: Making arch_cpu_idle and
rcu_idle_exit noinstr
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@...nel.org>
> > > > >
> > > > > From an RCU viewpoint:
> > > > >
> > > > > Reviewed-by: Paul E. McKenney <paulmck@...nel.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.
Thoughts?
Thanx, Paul
> thanks,
> jirka
>
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit cd338be719a0a692e0d50e1a8438e1f6c7165d9c
> > Author: Paul E. McKenney <paulmck@...nel.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@...nel.org>
> > Reported-by: Alexei Starovoitov <ast@...nel.org>
> > Reported-by: Andrii Nakryiko <andrii@...nel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > Reviewed-by: Yonghong Song <yhs@...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);
> >
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