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Date: Tue, 11 Jul 2017 11:41:57 +0200 From: Peter Zijlstra <peterz@...radead.org> To: "Li, Aubrey" <aubrey.li@...ux.intel.com> Cc: Andi Kleen <ak@...ux.intel.com>, Aubrey Li <aubrey.li@...el.com>, tglx@...utronix.de, len.brown@...el.com, rjw@...ysocki.net, tim.c.chen@...ux.intel.com, arjan@...ux.intel.com, paulmck@...ux.vnet.ibm.com, yang.zhang.wz@...il.com, x86@...nel.org, linux-kernel@...r.kernel.org Subject: Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods On Tue, Jul 11, 2017 at 12:40:06PM +0800, Li, Aubrey wrote: > > On Mon, Jul 10, 2017 at 06:42:06PM +0200, Peter Zijlstra wrote: > >> Data to indicate what hurts how much would be a very good addition to > >> the Changelogs. Clearly you have some, you really should have shared. > In the idle loop, > > - quiet_vmstat costs 5562ns - 6296ns Urgh, that thing is horrible, also I think its placed wrong. The comment near that function says it should be called when we enter NOHZ. Which suggests something like so: --- kernel/sched/idle.c | 1 - kernel/time/tick-sched.c | 1 + 2 files changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c index 6c23e30c0e5c..ef63adce0c9c 100644 --- a/kernel/sched/idle.c +++ b/kernel/sched/idle.c @@ -219,7 +219,6 @@ static void do_idle(void) */ __current_set_polling(); - quiet_vmstat(); tick_nohz_idle_enter(); while (!need_resched()) { diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index c7a899c5ce64..eb0e9753db8f 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -787,6 +787,7 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts, if (!ts->tick_stopped) { calc_load_nohz_start(); cpu_load_update_nohz_start(); + quiet_vmstat(); ts->last_tick = hrtimer_get_expires(&ts->sched_timer); ts->tick_stopped = 1; > - tick_nohz_idle_enter costs 7058ns - 10726ns > - tick_nohz_idle_exit costs 8372ns - 20850ns Right, those are horrible expensive, but skipping them isn't 'hard', the only tricky bit is finding a condition that makes sense. See Mike's patch: https://patchwork.kernel.org/patch/2839221/ Combined with the above, and possibly a better condition, that should get rid of most of this. > - totally from arch_cpu_idle_enter entry to arch_cpu_idle_exit return costs > 9122ns - 15318ns. > --In this period, rcu_idle_enter costs 1985ns - 2262ns, rcu_idle_exit costs > 1813ns - 3507ns Is that the POPF being painful? or something else?
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