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authorPaul E. McKenney <paulmck@kernel.org>2020-08-07 00:12:50 +0200
committerPaul E. McKenney <paulmck@kernel.org>2020-08-25 03:40:25 +0200
commit44bad5b3cca2d452d17ef82841b20b42a2cf11a0 (patch)
tree511ed2b6da2d3d6996a64fbc3b37503c7f84cd6d /kernel/rcu
parentrcu: Always set .need_qs from __rcu_read_lock() for strict GPs (diff)
downloadlinux-44bad5b3cca2d452d17ef82841b20b42a2cf11a0.tar.xz
linux-44bad5b3cca2d452d17ef82841b20b42a2cf11a0.zip
rcu: Do full report for .need_qs for strict GPs
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>
Diffstat (limited to 'kernel/rcu')
-rw-r--r--kernel/rcu/tree_plugin.h8
1 files changed, 6 insertions, 2 deletions
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 668bbd2be807..dfdb9020f136 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