Messages in this thread Patch in this message | | | From | paulmck@kernel ... | Subject | [PATCH tip/core/rcu 06/13] rcu: Do full report for .need_qs for strict GPs | Date | Mon, 31 Aug 2020 11:11:13 -0700 |
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From: "Paul E. McKenney" <paulmck@kernel.org>
The rcu_preempt_deferred_qs_irqrestore() function is invoked at the end of an RCU read-side critical section (for example, directly from rcu_read_unlock()) and, if .need_qs is set, invokes rcu_qs() to report the new quiescent state. This works, except that rcu_qs() only updates per-CPU state, leaving reporting of the actual quiescent state to a later call to rcu_report_qs_rdp(), for example from within a later RCU_SOFTIRQ instance. Although this approach is exactly what you want if you are more concerned about efficiency than about short grace periods, in CONFIG_RCU_STRICT_GRACE_PERIOD=y kernels, short grace periods are the name of the game.
This commit therefore makes rcu_preempt_deferred_qs_irqrestore() directly invoke rcu_report_qs_rdp() in CONFIG_RCU_STRICT_GRACE_PERIOD=y, thus shortening grace periods.
Historical note: To the best of my knowledge, causing rcu_read_unlock() to directly report a quiescent state first appeared in Jim Houston's and Joe Korty's JRCU. This is the second instance of a Linux-kernel RCU feature being inspired by JRCU, the first being RCU callback offloading (as in the RCU_NOCB_CPU Kconfig option).
Reported-by Jann Horn <jannh@google.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> --- kernel/rcu/tree_plugin.h | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 668bbd2..dfdb902 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -459,8 +459,12 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags) return; } t->rcu_read_unlock_special.s = 0; - if (special.b.need_qs) - rcu_qs(); + if (special.b.need_qs) { + if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)) + rcu_report_qs_rdp(rdp->cpu, rdp); + else + rcu_qs(); + } /* * Respond to a request by an expedited grace period for a -- 2.9.5
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