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Date:   Tue, 19 Apr 2022 16:56:26 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        rcu <rcu@vger.kernel.org>, Steven Rostedt <rostedt@goodmis.org>,
        Rushikesh S Kadam <rushikesh.s.kadam@intel.com>,
        Vineeth Pillai <vineethrp@gmail.com>,
        "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Subject: Re: [PATCH v4 rcu/dev] rcu/nocb: Add an option to offload all CPUs
 on boot
Message-ID: <20220419235626.GW4285@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20220419200823.2790490-1-joel@joelfernandes.org>
 <CAEXW_YS7cdrfP58Qvbt_WS2QRNkV=wVrwY6iTWmEH5qBxyStxQ@mail.gmail.com>
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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.  ;-)

							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
> >