Messages in this thread Patch in this message | | | From | Frederic Weisbecker <> | Subject | [PATCH 09/10] nohz: Switch nohz full timekeeper to dynticks idle on top of sysidle detection | Date | Sat, 19 Jul 2014 02:44:20 +0200 |
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In full dynticks, the CPU 0 carries the timekeeping duty on behalf of all other CPUs in the system. This way full dynticks are left undisturbed on this regard.
Of course this prevents CPU 0 from entering in dynticks idle mode because any CPU may need uptodate timekeeping at any time.
Theoretically though, we could put CPU 0 in dynticks idle mode once we are sure that all other CPUs are dynticks idle as well. Then when a CPU wakes up and finds the timekeeper idle, it would send an IPI to wake it up on its duty.
Such a machine state needs to take care of all the races in the way, make sure that CPU 0 is neither stuck accidentally to sleep for ever, nor stuck in periodic mode when it could sleep. Also given the amount of shared data this involves and their access frequency, this must be built on top of lockless low-overhead state machine.
This is what sysidle provides. The feature is ready for a while, we were just waiting for the nohz susbsystem to support it. And we just reached that state.
So lets defer the last call for CPU 0 to enter in dynticks idle to when we find a full system idle state. And lets wake it up when its duty is needed.
Cc: Ingo Molnar <mingo@kernel.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Viresh Kumar <viresh.kumar@linaro.org> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> --- kernel/time/tick-sched.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index bcba79d..845aaff 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -547,8 +547,10 @@ static u64 timekeeping_deferment(struct tick_sched *ts, int cpu) if (tick_do_timer_cpu == cpu) { time_delta = timekeeping_max_deferment(); - tick_do_timer_cpu = TICK_DO_TIMER_NONE; ts->do_timer_last = 1; + /* In full dynticks mode, CPU 0 always keeps the duty */ + if (!tick_nohz_full_enabled()) + tick_do_timer_cpu = TICK_DO_TIMER_NONE; } else if (ts->do_timer_last) { if (tick_do_timer_cpu == TICK_DO_TIMER_NONE) time_delta = timekeeping_max_deferment(); @@ -745,7 +747,7 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts) * if there are full dynticks CPUs around */ if (tick_do_timer_cpu == cpu) - return false; + return rcu_sys_is_idle(); } return true; -- 1.8.3.1
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