[PATCH] rcu: Guard deferred QS on kernel exit behind need_deferred_qs() check

["Mukesh Kumar Chaurasiya (IBM)" <mkchauras@gmail.com>]

ct_kernel_exit() unconditionally calls rcu_preempt_deferred_qs(current)
on every return to userspace.  On the common fast path nothing is
actually deferred, so this is a needless write to
current->rcu_read_unlock_special -- a word that lives on the task_struct
and is therefore subject to cross-CPU cache-line traffic.

On weakly-ordered architectures such as ppc64le, rcu_read_lock() and
rcu_read_unlock() already issue lwsync/isync barriers and touch that
same cache line in the syscall body.  Bouncing it again at syscall exit
adds measurable overhead, particularly on workloads with a high syscall
rate (e.g. SELinux-heavy workloads where every AVC check issues a
system call).

Introduce rcu_ct_kernel_exit_qs() which wraps the deferred-QS call with
a rcu_preempt_need_deferred_qs() guard, matching the pattern already
used in rcu_flavor_sched_clock_irq():

  notrace void rcu_ct_kernel_exit_qs(void)
  {
          if (rcu_preempt_need_deferred_qs(current))
                  rcu_preempt_deferred_qs(current);
  }

The declaration is added to <linux/rcutree.h> and a stub no-op is added
to <linux/rcutiny.h> so that TINY_RCU builds are unaffected.

ct_kernel_exit() is updated to call rcu_ct_kernel_exit_qs() in place of
the direct rcu_preempt_deferred_qs() call.  Semantics are identical when
a deferred QS is actually pending; only the unnecessary write on the
fast path is eliminated.

Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
---
 include/linux/rcutiny.h   |  1 +
 include/linux/rcutree.h   |  1 +
 kernel/context_tracking.c |  2 +-
 kernel/rcu/tree.c         | 19 +++++++++++++++++++
 4 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index e56ded733b1b..dcad641eb2c2 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -120,6 +120,7 @@ static inline bool rcu_preempt_need_deferred_qs(struct task_struct *t)
 	return false;
 }
 static inline void rcu_preempt_deferred_qs(struct task_struct *t) { }
+static inline void rcu_ct_kernel_exit_qs(void) { }
 void rcu_scheduler_starting(void);
 static inline void rcu_end_inkernel_boot(void) { }
 static inline bool rcu_inkernel_boot_has_ended(void) { return true; }
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 16a04202888b..d623f2a7d3fc 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -87,6 +87,7 @@ static inline void rcu_irq_exit_check_preempt(void) { }
 
 struct task_struct;
 void rcu_preempt_deferred_qs(struct task_struct *t);
+void rcu_ct_kernel_exit_qs(void);
 
 void exit_rcu(void);
 
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index a743e7ffa6c0..011018214c6d 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -118,7 +118,7 @@ static void noinstr ct_kernel_exit(bool user, int offset)
 	lockdep_assert_irqs_disabled();
 	trace_rcu_watching(TPS("End"), ct_nesting(), 0, ct_rcu_watching());
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
-	rcu_preempt_deferred_qs(current);
+	rcu_ct_kernel_exit_qs();
 
 	// instrumentation for the noinstr ct_kernel_exit_state()
 	instrument_atomic_write(&ct->state, sizeof(ct->state));
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 96848fc1f02b..c23478f70c17 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -368,6 +368,25 @@ notrace void rcu_momentary_eqs(void)
 }
 EXPORT_SYMBOL_GPL(rcu_momentary_eqs);
 
+/**
+ * rcu_ct_kernel_exit_qs - report deferred QS on syscall/exception exit if needed
+ *
+ * Called from ct_kernel_exit() on every return to userspace.  Guards the
+ * rcu_preempt_deferred_qs() call with rcu_preempt_need_deferred_qs() so that
+ * on the common fast path -- where nothing is deferred -- we avoid the
+ * cache-line traffic on current->rcu_read_unlock_special that the unconditional
+ * call causes.  This is particularly significant on weakly-ordered architectures
+ * (e.g. ppc64le) where rcu_read_lock/unlock issue lwsync/isync barriers and
+ * already touch that cache line in the syscall body.
+ *
+ * Follows the same pattern used by rcu_flavor_sched_clock_irq().
+ */
+notrace void rcu_ct_kernel_exit_qs(void)
+{
+	if (rcu_preempt_need_deferred_qs(current))
+		rcu_preempt_deferred_qs(current);
+}
+
 /**
  * rcu_is_cpu_rrupt_from_idle - see if 'interrupted' from idle
  *
-- 
2.55.0