Messages in this thread | | | Date | Tue, 16 Aug 2011 13:44:15 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 31/32] rcu: Switch to extended quiescent state in userspace from nohz cpuset |
| |
On Mon, Aug 15, 2011 at 05:52:28PM +0200, Frederic Weisbecker wrote: > When we switch to adaptive nohz mode and we run in userspace, > we can still receive IPIs from the RCU core if a grace period > has been started by another CPU because we need to take part > of its completion. > > However running in userspace is similar to that of running in > idle because we don't make use of RCU there, thus we can be > considered as running in RCU extended quiescent state. The > benefit when running into that mode is that we are not > anymore disturbed by needless IPIs coming from the RCU core. > > To perform this, we just to use the RCU extended quiescent state > APIs on the following points: > > - kernel exit or tick stop in userspace: here we switch to extended > quiescent state because we run in userspace without the tick. > > - kernel entry or tick restart: here we exit the extended quiescent > state because either we enter the kernel and we may make use of RCU > read side critical section anytime, or we need the timer tick for some > reason and that takes care of RCU grace period in a traditional way. > > TODO: hook into do_notify_resume() because we may have called > rcu_enter_nohz() from syscall exit hook, but we might call > do_notify_resume() right after, which may use RCU.
I don't see exactly how the exception path works, but this does reassure me a bit on the syscall path.
Thanx, Paul
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: Anton Blanchard <anton@au1.ibm.com> > Cc: Avi Kivity <avi@redhat.com> > Cc: Ingo Molnar <mingo@elte.hu> > Cc: Lai Jiangshan <laijs@cn.fujitsu.com> > Cc: Paul E . McKenney <paulmck@linux.vnet.ibm.com> > Cc: Paul Menage <menage@google.com> > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> > Cc: Stephen Hemminger <shemminger@vyatta.com> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Tim Pepper <lnxninja@linux.vnet.ibm.com> > --- > include/linux/tick.h | 2 ++ > kernel/sched.c | 1 + > kernel/time/tick-sched.c | 21 +++++++++++++++++++++ > 3 files changed, 24 insertions(+), 0 deletions(-) > > diff --git a/include/linux/tick.h b/include/linux/tick.h > index 9d0270e..4e7555f 100644 > --- a/include/linux/tick.h > +++ b/include/linux/tick.h > @@ -138,12 +138,14 @@ extern u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time); > > #ifdef CONFIG_CPUSETS_NO_HZ > DECLARE_PER_CPU(int, task_nohz_mode); > +DECLARE_PER_CPU(int, nohz_task_ext_qs); > > extern void tick_nohz_enter_kernel(void); > extern void tick_nohz_exit_kernel(void); > extern void tick_nohz_enter_exception(struct pt_regs *regs); > extern void tick_nohz_exit_exception(struct pt_regs *regs); > extern int tick_nohz_adaptive_mode(void); > +extern void tick_nohz_cpu_exit_qs(void); > extern bool tick_nohz_account_tick(void); > extern void tick_nohz_flush_current_times(bool restart_tick); > #else /* !CPUSETS_NO_HZ */ > diff --git a/kernel/sched.c b/kernel/sched.c > index 2bcd456..576d0bf 100644 > --- a/kernel/sched.c > +++ b/kernel/sched.c > @@ -2504,6 +2504,7 @@ static void cpuset_nohz_restart_tick(void) > __get_cpu_var(task_nohz_mode) = 0; > tick_nohz_restart_sched_tick(); > clear_thread_flag(TIF_NOHZ); > + tick_nohz_cpu_exit_qs(); > } > > void cpuset_update_nohz(void) > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > index 9a2ba5b..b611b77 100644 > --- a/kernel/time/tick-sched.c > +++ b/kernel/time/tick-sched.c > @@ -757,6 +757,7 @@ void tick_check_idle(int cpu) > } > > #ifdef CONFIG_CPUSETS_NO_HZ > +DEFINE_PER_CPU(int, nohz_task_ext_qs); > > void tick_nohz_exit_kernel(void) > { > @@ -783,6 +784,9 @@ void tick_nohz_exit_kernel(void) > ts->saved_jiffies = jiffies; > ts->saved_jiffies_whence = JIFFIES_SAVED_USER; > > + __get_cpu_var(nohz_task_ext_qs) = 1; > + rcu_enter_nohz();
OK, I was wondering how this was going to work if RCU didn't know about kernel entry/exit. Whew!!! ;-)
> + > local_irq_restore(flags); > } > > @@ -799,6 +803,11 @@ void tick_nohz_enter_kernel(void) > return; > } > > + if (__get_cpu_var(nohz_task_ext_qs) == 1) { > + __get_cpu_var(nohz_task_ext_qs) = 0; > + rcu_exit_nohz(); > + } > + > ts = &__get_cpu_var(tick_cpu_sched); > > WARN_ON_ONCE(ts->saved_jiffies_whence == JIFFIES_SAVED_SYS); > @@ -814,6 +823,16 @@ void tick_nohz_enter_kernel(void) > local_irq_restore(flags); > } > > +void tick_nohz_cpu_exit_qs(void) > +{ > + struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched); > + > + if (__get_cpu_var(nohz_task_ext_qs)) { > + rcu_exit_nohz(); > + __get_cpu_var(nohz_task_ext_qs) = 0; > + } > +} > + > void tick_nohz_enter_exception(struct pt_regs *regs) > { > if (user_mode(regs)) > @@ -858,6 +877,8 @@ static void tick_nohz_cpuset_stop_tick(int user) > if (user) { > ts->saved_jiffies_whence = JIFFIES_SAVED_USER; > ts->saved_jiffies = jiffies; > + __get_cpu_var(nohz_task_ext_qs) = 1; > + rcu_enter_nohz();
When entering an exception, shouldn't we call rcu_exit_nohz() rather than rcu_exit_nohz()? Or is this a "didn't really mean an exception" code path?
> } else if (!current->mm) { > ts->saved_jiffies_whence = JIFFIES_SAVED_SYS; > ts->saved_jiffies = jiffies; > -- > 1.7.5.4 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
| |