linux-kernel - Re: [patch 1/2] tick/broadcast: Prevent deep idle states if no broadcast device available

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Date:	Mon, 6 Jul 2015 18:06:03 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Sudeep Holla <sudeep.holla@....com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Suzuki Poulose <Suzuki.Poulose@....com>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [patch 1/2] tick/broadcast: Prevent deep idle states if no
 broadcast device available

On Mon, 6 Jul 2015, Sudeep Holla wrote:
> On 06/07/15 16:35, Thomas Gleixner wrote:
> > Well, we should figure out what happens while we are at it before
> > everything gets paged out again.
> > 
> 
> True. I just wanted to mention that this patch works for all the
> practical purposes.
> 
> > In the case of CONFIG_NOHZ=n and CONFIG_HIGHRES=n the broadcast
> > hrtimer is not compiled as it depends on CONFIG_TICK_ONESHOT, so it
> > works via the bc.evtdev == NULL check.
> > 
> > With either option enabled CONFIG_TICK_ONESHOT gets set, so the
> > broadcast timer gets installed but somehow does not work proper if
> > nohz and highres are disabled on the kernel command line.
> > 
> 
> Let me know if you want to test something to help debug this configuration.

Can you try the following delta patch?

Thanks,

	tglx
---
Index: tip/kernel/time/tick-broadcast.c
===================================================================
--- tip.orig/kernel/time/tick-broadcast.c
+++ tip/kernel/time/tick-broadcast.c
@@ -360,14 +360,15 @@ void tick_broadcast_control(enum tick_br
 		cpumask_set_cpu(cpu, tick_broadcast_on);
 		if (!cpumask_test_and_set_cpu(cpu, tick_broadcast_mask)) {
 			/*
-			 * Only shutdown the cpu local device, if the
-			 * broadcast device exists and is in periodic
-			 * mode. The latter check prevents a hickup
-			 * during the switch from periodic to oneshot
-			 * mode.
+			 * Only shutdown the cpu local device, if:
+			 *
+			 * - the broadcast device exists
+			 * - the broadcast device is not a hrtimer based one
+			 * - the broadcast device is in periodic mode to
+			 *   avoid a hickup during switch to oneshot mode
 			 */
-			if (bc != NULL && tick_broadcast_device.mode ==
-			    TICKDEV_MODE_PERIODIC)
+			if (bc && !(bc->features & CLOCK_EVT_FEAT_HRTIMER) &&
+			    tick_broadcast_device.mode == TICKDEV_MODE_PERIODIC)
 				clockevents_shutdown(dev);
 		}
 		break;
@@ -697,14 +698,18 @@ int __tick_broadcast_oneshot_control(enu
 		 * shutdown.
 		 */
 		ret = broadcast_needs_cpu(bc, cpu);
+		if (ret)
+			goto out;
 
 		/*
-		 * If the hrtimer broadcast check tells us that the
-		 * cpu cannot go deep idle, or if the broadcast device
-		 * is in periodic mode, we just return.
+		 * If the broadcast device is in periodic mode, we
+		 * return.
 		 */
-		if (ret || tick_broadcast_device.mode == TICKDEV_MODE_PERIODIC)
-			goto out;
+		if (tick_broadcast_device.mode == TICKDEV_MODE_PERIODIC) {
+			/* If it is a hrtimer based broadcast, return busy */
+			if (bc->features & CLOCK_EVT_FEAT_HRTIMER)
+				ret = -EBUSY;
+		}
 
 		if (!cpumask_test_and_set_cpu(cpu, tick_broadcast_oneshot_mask)) {
 			WARN_ON_ONCE(cpumask_test_cpu(cpu, tick_broadcast_pending_mask));
--
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