[PATCH 5.12 070/173] tick/nohz: Only check for RCU deferred wakeup on user/guest entry when needed

From: Greg Kroah-Hartman
Date: Mon Jun 14 2021 - 07:30:44 EST


From: Frederic Weisbecker <frederic@xxxxxxxxxx>

commit f268c3737ecaefcfeecfb4cb5e44958a8976f067 upstream.

Checking for and processing RCU-nocb deferred wakeup upon user/guest
entry is only relevant when nohz_full runs on the local CPU, otherwise
the periodic tick should take care of it.

Make sure we don't needlessly pollute these fast-paths as a -3%
performance regression on a will-it-scale.per_process_ops has been
reported so far.

Fixes: 47b8ff194c1f (entry: Explicitly flush pending rcuog wakeup before last rescheduling point)
Fixes: 4ae7dc97f726 (entry/kvm: Explicitly flush pending rcuog wakeup before last rescheduling point)
Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20210527113441.465489-1-frederic@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
include/linux/entry-kvm.h | 3 ++-
include/linux/tick.h | 7 +++++++
kernel/entry/common.c | 5 +++--
kernel/time/tick-sched.c | 1 +
4 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/include/linux/entry-kvm.h b/include/linux/entry-kvm.h
index 8b2b1d68b954..136b8d97d8c0 100644
--- a/include/linux/entry-kvm.h
+++ b/include/linux/entry-kvm.h
@@ -3,6 +3,7 @@
#define __LINUX_ENTRYKVM_H

#include <linux/entry-common.h>
+#include <linux/tick.h>

/* Transfer to guest mode work */
#ifdef CONFIG_KVM_XFER_TO_GUEST_WORK
@@ -57,7 +58,7 @@ int xfer_to_guest_mode_handle_work(struct kvm_vcpu *vcpu);
static inline void xfer_to_guest_mode_prepare(void)
{
lockdep_assert_irqs_disabled();
- rcu_nocb_flush_deferred_wakeup();
+ tick_nohz_user_enter_prepare();
}

/**
diff --git a/include/linux/tick.h b/include/linux/tick.h
index 7340613c7eff..1a0ff88fa107 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -11,6 +11,7 @@
#include <linux/context_tracking_state.h>
#include <linux/cpumask.h>
#include <linux/sched.h>
+#include <linux/rcupdate.h>

#ifdef CONFIG_GENERIC_CLOCKEVENTS
extern void __init tick_init(void);
@@ -300,4 +301,10 @@ static inline void tick_nohz_task_switch(void)
__tick_nohz_task_switch();
}

+static inline void tick_nohz_user_enter_prepare(void)
+{
+ if (tick_nohz_full_cpu(smp_processor_id()))
+ rcu_nocb_flush_deferred_wakeup();
+}
+
#endif
diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index a0b3b04fb596..bf16395b9e13 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -5,6 +5,7 @@
#include <linux/highmem.h>
#include <linux/livepatch.h>
#include <linux/audit.h>
+#include <linux/tick.h>

#include "common.h"

@@ -186,7 +187,7 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
local_irq_disable_exit_to_user();

/* Check if any of the above work has queued a deferred wakeup */
- rcu_nocb_flush_deferred_wakeup();
+ tick_nohz_user_enter_prepare();

ti_work = READ_ONCE(current_thread_info()->flags);
}
@@ -202,7 +203,7 @@ static void exit_to_user_mode_prepare(struct pt_regs *regs)
lockdep_assert_irqs_disabled();

/* Flush pending rcuog wakeup before the last need_resched() check */
- rcu_nocb_flush_deferred_wakeup();
+ tick_nohz_user_enter_prepare();

if (unlikely(ti_work & EXIT_TO_USER_MODE_WORK))
ti_work = exit_to_user_mode_loop(regs, ti_work);
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 828b091501ca..6784f27a3099 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -230,6 +230,7 @@ static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs)

#ifdef CONFIG_NO_HZ_FULL
cpumask_var_t tick_nohz_full_mask;
+EXPORT_SYMBOL_GPL(tick_nohz_full_mask);
bool tick_nohz_full_running;
EXPORT_SYMBOL_GPL(tick_nohz_full_running);
static atomic_t tick_dep_mask;
--
2.32.0