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SubjectRe: [PATCH] i386 tsc: remove xtime_lock'ing around cpufreq notifier
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On Wed, 2007-04-11 at 13:31 -0700, Andrew Morton wrote:
> On Wed, 11 Apr 2007 09:29:04 -0700
> Daniel Walker <dwalker@mvista.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@mvista.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 .

Daniel

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