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McKenney" Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, Juri Lelli Subject: Re: [PATCH rcu 11/14] rcu: Make rcu_normal_after_boot writable again Message-ID: References: <20210915233305.GA3906641@paulmck-ThinkPad-P17-Gen-1> <20210915233343.3906738-11-paulmck@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210915233343.3906738-11-paulmck@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 15, 2021 at 04:33:40PM -0700, Paul E. McKenney wrote: > From: Juri Lelli > > 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 (if NO_HZ_ > FULL is defined), 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. > > Reflect the change in synchronize_rcu_expedited_wait() by removing the > WARN related to CONFIG_PREEMPT_RT. > > Signed-off-by: Juri Lelli > Signed-off-by: Paul E. McKenney > --- > kernel/rcu/tree_exp.h | 1 - > kernel/rcu/update.c | 2 +- > 2 files changed, 1 insertion(+), 2 deletions(-) > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > index 2796084ef85a..d9e4f8eb9ae2 100644 > --- a/kernel/rcu/tree_exp.h > +++ b/kernel/rcu/tree_exp.h > @@ -512,7 +512,6 @@ static void synchronize_rcu_expedited_wait(void) > j = READ_ONCE(jiffies_till_first_fqs); > if (synchronize_rcu_expedited_wait_once(j + HZ)) > return; > - WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT)); > } > > for (;;) { Urgh... I really don't like this. That would force RT applications to always use NOHZ_FULL which isn't always possible or desired. WTH does that netns crud need expedited so much?