[Fwd: sched_clock() has poor resolution]

[Fwd: sched_clock() has poor resolution]

[Pierre Ossman]

cc:ing some timer hackers.

-------- Original Message --------
Subject: sched_clock() has poor resolution
Date: Tue, 20 Sep 2005 12:02:34 +0200
From: Pierre Ossman <drzeus-list@drzeus.cx>
To: LKML <linux-kernel@vger.kernel.org>

I've been trying to hunt down a HZ bug and as part of that I thought
having printk timestamps would be nice. But too much dismay I noticed
that I couldn't get very good resolution out of these.

The problem turned out to be that sched_clock() uses jiffies as a time
source. It will only use the TSC if that's the primary time source of
the system. But since I have a PM timer that is given priority.

So I see two solutions here:

 * Let sched_clock() follow the time source. Don't have it coupled to
the TSC.

 * Init the TSC even though it isn't used for anything but sched_clock().

The first solution might have problems if one of the "better" timers are
slow as hell to read and the second if there is some assumed dependency
between sched_clock() and the primary timer. I'm blisfully ignorant of
this area so input is welcome.

Rgds
Pierre
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Re: [Fwd: sched_clock() has poor resolution]

[Dominik Brodowski]

Hi,

On Thu, Sep 22, 2005 at 09:28:49AM +0200, Pierre Ossman wrote:
>  * Let sched_clock() follow the time source. Don't have it coupled to
> the TSC.
> 
>  * Init the TSC even though it isn't used for anything but sched_clock().

As pmtmr requires somewhat working TSCs, you can even use TSC if pmtmr is
used as a time source:

static int init_pmtmr(char* override)
...
        /* we use the TSC for delay_pmtmr, so make sure it exists */
        if (!cpu_has_tsc)
                return -ENODEV;
...

So at least for this case, sched_clock() can and should use TSC, not
jiffies, AFAICS.

Thanks,
	Dominik