On Thu, May 27, 2021 at 01:34:41PM +0200, Frederic Weisbecker wrote:
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.
The above initially confused me into thinking that we were dealing with non-local CPUs, and the patch itself straightened me out. How about something like this?
Because the periodic tick is active during user/guest execution on non-nohz_full CPUs, only nohz_full CPUs need to check for RCU-nocb deferred wakeup on user/guest transitions.
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.
Reported-by: kernel test robot oliver.sang@intel.com 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) Cc: Ingo Molnar mingo@kernel.org Cc: Peter Zijlstra peterz@infradead.org Cc: stable@vger.kernel.org Cc: Paul E. McKenney paulmck@kernel.org Cc: Thomas Gleixner tglx@linutronix.de Signed-off-by: Frederic Weisbecker frederic@kernel.org
Reviewed-by: Paul E. McKenney paulmck@kernel.org
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; -- 2.25.1