linux-kernel - Re: x86: improve default idle

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Message-ID: <20080421142737.GU9554@elte.hu>
Date:	Mon, 21 Apr 2008 16:27:38 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: x86: improve default idle


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Fri, 18 Apr 2008 17:37:02 GMT
> Linux Kernel Mailing List <linux-kernel@...r.kernel.org> wrote:
> 
> > Gitweb:     http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=13af4836b3914b23946f6a8982934e2c828c183f
> > Commit:     13af4836b3914b23946f6a8982934e2c828c183f
> > Parent:     f5149a49f994e5c469ac398af7cdeb8eb612d3a4
> > Author:     Ingo Molnar <mingo@...e.hu>
> > AuthorDate: Wed Apr 2 13:23:22 2008 +0200
> > Committer:  Ingo Molnar <mingo@...e.hu>
> > CommitDate: Thu Apr 17 17:41:34 2008 +0200
> > 
> >     x86: improve default idle
> >     
> >     Signed-off-by: Ingo Molnar <mingo@...e.hu>
> > ---
> >  arch/x86/kernel/process_32.c |    8 --------
> >  arch/x86/kernel/process_64.c |    8 --------
> >  2 files changed, 0 insertions(+), 16 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
> > index 08c41ed..3903a8f 100644
> > --- a/arch/x86/kernel/process_32.c
> > +++ b/arch/x86/kernel/process_32.c
> > @@ -113,16 +113,8 @@ void default_idle(void)
> >  
> >  		local_irq_disable();
> >  		if (!need_resched()) {
> > -			ktime_t t0, t1;
> > -			u64 t0n, t1n;
> > -
> > -			t0 = ktime_get();
> > -			t0n = ktime_to_ns(t0);
> >  			safe_halt();	/* enables interrupts racelessly */
> >  			local_irq_disable();
> > -			t1 = ktime_get();
> > -			t1n = ktime_to_ns(t1);
> > -			sched_clock_idle_wakeup_event(t1n - t0n);
> >  		}
> >  		local_irq_enable();
> >  		current_thread_info()->status |= TS_POLLING;
> > diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> > index 4f40272..e75ccc8 100644
> > --- a/arch/x86/kernel/process_64.c
> > +++ b/arch/x86/kernel/process_64.c
> > @@ -107,16 +107,8 @@ void default_idle(void)
> >  	smp_mb();
> >  	local_irq_disable();
> >  	if (!need_resched()) {
> > -		ktime_t t0, t1;
> > -		u64 t0n, t1n;
> > -
> > -		t0 = ktime_get();
> > -		t0n = ktime_to_ns(t0);
> >  		safe_halt();	/* enables interrupts racelessly */
> >  		local_irq_disable();
> > -		t1 = ktime_get();
> > -		t1n = ktime_to_ns(t1);
> > -		sched_clock_idle_wakeup_event(t1n - t0n);
> >  	}
> >  	local_irq_enable();
> >  	current_thread_info()->status |= TS_POLLING;
> 
> Ingo, what's going on?  There is no way that this patch is so obvious 
> that it doesn't even need a changelog.

sorry, the changelog should be: if a system uses acpi_pm clocksource but 
default idle method then the overhead of entry/exit measurement is quite 
noticeable in profiles. The measurement is redundant anyway, as in HLT 
sched_clock() is still supposed to work fine. (the worst i know of is a 
slight skew on certain CPUs in HLT)

	Ingo
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