linux-kernel - Re: [PATCH] i386 tsc: remove xtime_lock'ing around cpufreq notifier

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Message-Id: <20070411143357.e866b366.akpm@linux-foundation.org>
Date:	Wed, 11 Apr 2007 14:33:57 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Daniel Walker <dwalker@...sta.com>
Cc:	linux-kernel@...r.kernel.org, johnstul@...ibm.com,
	tglx@...utronix.de, Andi Kleen <ak@...e.de>
Subject: Re: [PATCH] i386 tsc: remove xtime_lock'ing around cpufreq notifier

On Wed, 11 Apr 2007 13:54:41 -0700
Daniel Walker <dwalker@...sta.com> wrote:

> On Wed, 2007-04-11 at 13:31 -0700, Andrew Morton wrote:
> > On Wed, 11 Apr 2007 09:29:04 -0700
> > Daniel Walker <dwalker@...sta.com> wrote:
> > 
> > > The locking of the xtime_lock around the cpu notifier is unessesary now. At one
> > > time the tsc was used after a frequency change for timekeeping, but the re-write
> > > of timekeeping no longer uses the TSC unless the frequency is constant. 
> > > 
> > > The variables that are changed in this section of code had also once been used
> > > for timekeeping, but not any longer ..
> > > 
> > > Signed-Off-By: Daniel Walker <dwalker@...sta.com>
> > > 
> > > ---
> > >  arch/i386/kernel/tsc.c |    8 +-------
> > >  1 file changed, 1 insertion(+), 7 deletions(-)
> > > 
> > > Index: linux-2.6.20/arch/i386/kernel/tsc.c
> > > ===================================================================
> > > --- linux-2.6.20.orig/arch/i386/kernel/tsc.c
> > > +++ linux-2.6.20/arch/i386/kernel/tsc.c
> > > @@ -200,13 +200,10 @@ time_cpufreq_notifier(struct notifier_bl
> > >  {
> > >  	struct cpufreq_freqs *freq = data;
> > >  
> > > -	if (val != CPUFREQ_RESUMECHANGE && val != CPUFREQ_SUSPENDCHANGE)
> > > -		write_seqlock_irq(&xtime_lock);
> > > -
> > >  	if (!ref_freq) {
> > >  		if (!freq->old){
> > >  			ref_freq = freq->new;
> > > -			goto end;
> > > +			return 0;
> > >  		}
> > >  		ref_freq = freq->old;
> > >  		loops_per_jiffy_ref = cpu_data[freq->cpu].loops_per_jiffy;
> > > @@ -237,9 +234,6 @@ time_cpufreq_notifier(struct notifier_bl
> > >  			}
> > >  		}
> > >  	}
> > > -end:
> > > -	if (val != CPUFREQ_RESUMECHANGE && val != CPUFREQ_SUSPENDCHANGE)
> > > -		write_sequnlock_irq(&xtime_lock);
> > >  
> > >  	return 0;
> > >  }
> > 
> > hm.
> > 
> > I've been permadropping Andi's
> > ftp://ftp.firstfloor.org/pub/ak/x86_64/quilt-current/patches/sched-clock-share
> > because it causes a lockup when initscripts start ondemand on my
> > single-CPU, CONFIG_SMP=n Vaio.
> > 
> > I don't know _why_ it locks up - I traced it down to the
> > write_seqlock_irq() which you have just removed.  But write_seqlock()
> > doesn't loop with CONFIG_SMP=n builds, so a hang there is quite mysterious.
> > 
> > Anyway, your patch might make that hang go away.  We'll see.
> 
> 
> I don't know to what extent this is relevant, but it's something I've
> noticed ..
> 
> >From the patch above ,
> 
> + */
> +unsigned long long sched_clock(void)
> +{
> +	int cpu = get_cpu();
> +	struct sc_data *sc = &per_cpu(sc_data, cpu);
> +	unsigned long long r;
> +
> +	if (sc->instable) {
> +		/* TBD find a cheaper fallback timer than this */
> +		r = ktime_to_ns(ktime_get());
> +	} else {
> +		get_scheduled_cycles(r);
> +		r = ((u64)sc->ns_base) + cycles_2_ns(cpu, r - sc->last_tsc);
> +	}
> +	put_cpu();
> +	return r;
> +}
> 
> Your VAIO is the "instable" case above I think .. So your using a case
> that needs to be implemented still , I guess .. ktime_get() has a
> peculiarity of recursively looping on the read seqlock on xtime_lock ..
> 
> Here is the call ordering ,
> 
> ktime_get()
>  ktime_get_ts() -> read_seqretry(&xtime_lock, seq)
>   getnstimeofday()
>    __get_realtime_clock_ts() -> read_seqretry(&xtime_lock, seq)
> 
> 
> I wonder if there is a weird case which case this to loop forever .. But
> as said , it's just something I noticed so I don't know if it's
> related .
> 

hm.

Bear in mind that printk calls sched_clock() for each line of output. 
(with the "time" kernel boot parameter).

If we're doing a read_seqretry() in sched_clock() then bascially any printk
inside the write_seqlock() will cause a lockup.

So in fact, this explains my hang: I was debugging it with printk and I
noticed that the printk before the write_seqlock() came out and the one
after it did not.  Presumably if I wasn't using "time", that hang wouldn't
have happened.

Which means that I still don't have a clue why Andi's patch is locking up
the Vaio.

It's a bad idea to make sched_clock() this complex - we've gone and
degraded kernel debuggability somewhat.

We have provision for fixing this: the architecture can provide its own
printk_clock().  We should do something quick-n-dirty in printk_clock()
which doesn't require any locks.

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