Re: [PATCH v4 rcu/dev] rcu/nocb: Add an option to offload all CPUs on boot

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Apr 19, 2022 at 04:56:26PM -0700, Paul E. McKenney wrote:
> On Tue, Apr 19, 2022 at 04:10:33PM -0400, Joel Fernandes wrote:
> > On Tue, Apr 19, 2022 at 4:08 PM Joel Fernandes (Google)
> > <joel@joelfernandes.org> wrote:
> > >
> > > From: Joel Fernandes <joel@joelfernandes.org>
> > >
> > > On systems with CONFIG_RCU_NOCB_CPU=y, there is no default mask provided
> > > which ends up not offloading any CPU. This patch removes a dependency
> > > from the bootloader having to know about RCU and about how to provide
> > > the mask.
> > >
> > > With the new option enabled, all CPUs will be offloaded on boot unless
> > > rcu_nocbs= or rcu_nohz_full= kernel parameters provide a CPU list.
> > >
> > > Signed-off-by: Joel Fernandes <joel@joelfernandes.org>
> > 
> > Oops, forgot to add Reviewed-by tags:
> > Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > Reviewed-by: Kalesh Singh <kaleshsingh@google.com>
> > 
> > Paul, would you mind doing so, so I don't have to resend? Apologies.
> 
> OK, but only because it passed an allmodconfig build this time.  ;-)

But the rcutorture TASKS03 scenario failed to build.  Kernel test robot
saw similar issues here:

https://lore.kernel.org/all/202204201732.oTzYh0Xo-lkp@intel.com/

Next version!  ;-)

							Thanx, Paul

> > Thanks,
> > 
> > Joel
> > 
> > 
> > > ---
> > > v4: Rebased on rcu/dev, fixed conflict with Frederick's patch changing
> > >     location of nocb_is_setup variable.
> > >
> > >  Documentation/admin-guide/kernel-parameters.txt |  6 ++++++
> > >  kernel/rcu/Kconfig                              | 13 +++++++++++++
> > >  kernel/rcu/tree_nocb.h                          | 13 +++++++++++++
> > >  3 files changed, 32 insertions(+)
> > >
> > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > > index 789ef586009b..1e82ecb7a649 100644
> > > --- a/Documentation/admin-guide/kernel-parameters.txt
> > > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > > @@ -3572,6 +3572,9 @@
> > >                         just as if they had also been called out in the
> > >                         rcu_nocbs= boot parameter.
> > >
> > > +                       Note that this argument takes precedence over
> > > +                       the CONFIG_RCU_NOCB_CPU_DEFAULT_ALL option.
> > > +
> > >         noiotrap        [SH] Disables trapped I/O port accesses.
> > >
> > >         noirqdebug      [X86-32] Disables the code which attempts to detect and
> > > @@ -4475,6 +4478,9 @@
> > >                         no-callback mode from boot but the mode may be
> > >                         toggled at runtime via cpusets.
> > >
> > > +                       Note that this argument takes precedence over
> > > +                       the CONFIG_RCU_NOCB_CPU_DEFAULT_ALL option.
> > > +
> > >         rcu_nocb_poll   [KNL]
> > >                         Rather than requiring that offloaded CPUs
> > >                         (specified by rcu_nocbs= above) explicitly
> > > diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> > > index 1c630e573548..27aab870ae4c 100644
> > > --- a/kernel/rcu/Kconfig
> > > +++ b/kernel/rcu/Kconfig
> > > @@ -262,6 +262,19 @@ config RCU_NOCB_CPU
> > >           Say Y here if you need reduced OS jitter, despite added overhead.
> > >           Say N here if you are unsure.
> > >
> > > +config RCU_NOCB_CPU_DEFAULT_ALL
> > > +       bool "Offload RCU callback processing from all CPUs by default"
> > > +       depends on RCU_NOCB_CPU
> > > +       default n
> > > +       help
> > > +         Use this option to offload callback processing from all CPUs
> > > +         by default, in the absence of the rcu_nocbs or nohz_full boot
> > > +         parameter. This also avoids the need to use any boot parameters
> > > +         to achieve the effect of offloading all CPUs on boot.
> > > +
> > > +         Say Y here if you want offload all CPUs by default on boot.
> > > +         Say N here if you are unsure.
> > > +
> > >  config TASKS_TRACE_RCU_READ_MB
> > >         bool "Tasks Trace RCU readers use memory barriers in user and idle"
> > >         depends on RCU_EXPERT && TASKS_TRACE_RCU
> > > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > > index 4cf9a29bba79..9f29f25a0cbc 100644
> > > --- a/kernel/rcu/tree_nocb.h
> > > +++ b/kernel/rcu/tree_nocb.h
> > > @@ -1197,11 +1197,21 @@ void __init rcu_init_nohz(void)
> > >  {
> > >         int cpu;
> > >         bool need_rcu_nocb_mask = false;
> > > +       bool offload_all = false;
> > >         struct rcu_data *rdp;
> > >
> > > +#if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL)
> > > +       if (!rcu_state.nocb_is_setup) {
> > > +               need_rcu_nocb_mask = true;
> > > +               offload_all = true;
> > > +       }
> > > +#endif /* #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) */
> > > +
> > >  #if defined(CONFIG_NO_HZ_FULL)
> > >         if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> > >                 need_rcu_nocb_mask = true;
> > > +               offload_all = false; /* NO_HZ_FULL has its own mask. */
> > > +       }
> > >  #endif /* #if defined(CONFIG_NO_HZ_FULL) */
> > >
> > >         if (need_rcu_nocb_mask) {
> > > @@ -1222,6 +1232,9 @@ void __init rcu_init_nohz(void)
> > >                 cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
> > >  #endif /* #if defined(CONFIG_NO_HZ_FULL) */
> > >
> > > +       if (offload_all)
> > > +               cpumask_setall(rcu_nocb_mask);
> > > +
> > >         if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
> > >                 pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
> > >                 cpumask_and(rcu_nocb_mask, cpu_possible_mask,
> > > --
> > > 2.36.0.rc0.470.gd361397f0d-goog
> > >