linux-kernel - Re: nohz and strange sleep latencies

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Message-Id: <200711250029.55461.rjw@sisk.pl>
Date:	Sun, 25 Nov 2007 00:29:54 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@....cz>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	kernel list <linux-kernel@...r.kernel.org>
Subject: Re: nohz and strange sleep latencies

On Saturday, 24 of November 2007, Pavel Machek wrote:
> Hi!
> 
> > > > but perhaps somehow we miss this fact and fail to turn off the lapic 
> > > > clockevents drivers?
> > > 
> > > Ok, I guess I'm lost. If I offline second CPU, I immediately get
> > > 1000Hz timer tick... is that expected?
> > 
> > Hmm. No. I have no idea why this is happening.
> > 
> > 34196 total events, 55.083 events/sec
> > echo 0 >/sys/devices/system/cpu/cpu1/online
> > 36073 total events, 54.679 events/sec
> 
> Digging into process_32|64.c...
> 
> 64:
>         while (1) {
>                 while (!need_resched()) {
>                         void (*idle)(void);
> 
>                         if (__get_cpu_var(cpu_idle_state))
>                                 __get_cpu_var(cpu_idle_state) = 0;
> 
>                         tick_nohz_stop_sched_tick();
> 
> 32:
>         while (1) {
>                 tick_nohz_stop_sched_tick();
>                 while (!need_resched()) {
>                         void (*idle)(void);
> 
>                         if (__get_cpu_var(cpu_idle_state))
>                                 __get_cpu_var(cpu_idle_state) = 0;
> 
> ...eek? Which one is wrong?

Hm, it looks like you should have quoted more lines ...

In the second case (32), the tick_nohz_stop_sched_tick() seems to be
redundant, so I bet it's this one.

Greetings,
Rafael
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