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@xxxxxxxxxx> > > > > From an RCU viewpoint: > > > > Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > > > [ 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@xxxxxxxxxx/ Ah, I should have looked at the context-tracking series again! And I have to ask... How much debugging capability are we really losing by continuing to use the raw versions of local_irq_{save,restore}()? Thanx, Paul > Thanks. > > > > > > --- > > > 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) > > > { > > > 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; > > > > > > -- > > > 2.35.3 > > >
