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McKenney" Cc: josh@joshtriplett.org, linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org, peterz@infradead.org, bigeasy@linutronix.de, tglx@linutronix.de, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, joel@joelfernandes.org, rcu@vger.kernel.org Subject: Re: [PATCH] rcu: Make rcu_normal_after_boot writable on RT Message-ID: References: <20210805080123.16320-1-juri.lelli@redhat.com> <20210805160337.GI4397@paulmck-ThinkPad-P17-Gen-1> <20210805210824.GA1203206@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210805210824.GA1203206@paulmck-ThinkPad-P17-Gen-1> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, Thanks for reviewing Paul. On 05/08/21 14:08, Paul E. McKenney wrote: > On Thu, Aug 05, 2021 at 09:03:37AM -0700, Paul E. McKenney wrote: > > On Thu, Aug 05, 2021 at 10:01:23AM +0200, Juri Lelli wrote: > > > Certain configurations (e.g., systems that make heavy use of netns) > > > need to use synchronize_rcu_expedited() to service RCU grace periods > > > even after boot. > > > > > > Even though synchronize_rcu_expedited() has been traditionally > > > considered harmful for RT for the heavy use of IPIs, it is perfectly > > > usable under certain conditions (e.g. nohz_full). > > > > > > Make rcupdate.rcu_normal_after_boot= again writeable on RT, but keep > > > its default value to 1 (enabled) to avoid regressions. Users who need > > > synchronize_rcu_expedited() will boot with rcupdate.rcu_normal_after_ > > > boot=0 in the kernel cmdline. > > > > > > Signed-off-by: Juri Lelli > > > > Makes sense to me! > > > > But would another of the -rt people be willing to give an Acked-by? > > For example, maybe they would prefer this kernel boot parameter to be > > exposed only if (!PREEMPT_RT || NO_HZ_FULL). Or are there !NO_HZ_FULL > > situations where rcu_normal_after_boot makes sense? > > Ah, and this will also need to be reflected in the WARN_ON_ONCE() > in synchronize_rcu_expedited_wait() in kernel/rcu/tree_exp.h. Indeed. Will add the change as soon as I receive indication about your first point. Best, Juri