Re: vmstat: On demand vmstat workers V4

Thomas Gleixner
2014-05-08T22:18:32Z

On Thu, 8 May 2014, Andrew Morton wrote:
> On Thu, 8 May 2014 10:35:15 -0500 (CDT) Christoph Lameter <cl@linux.com> wrote:
> > --- linux.orig/kernel/time/tick-common.c 2014-05-06 10:51:19.711239813 -0500
> > +++ linux/kernel/time/tick-common.c 2014-05-06 10:51:19.711239813 -0500
> > @@ -222,6 +222,24 @@
> > tick_setup_oneshot(newdev, handler, next_event);
> > }
> >
> > +/*
> > + * Return a cpu number that may be used to run housekeeping
> > + * tasks. This is usually the timekeeping cpu unless that
> > + * is not available. Then we simply fall back to the current
> > + * cpu.
> > + */
>
> This comment is unusably vague. What the heck is a "housekeeping
> task"? Why would anyone call this and what is special about the CPU
> number it returns?
>
>
> > +int tick_get_housekeeping_cpu(void)
> > +{
> > + int cpu;
> > +
> > + if (system_state < SYSTEM_RUNNING || tick_do_timer_cpu < 0)
> > + cpu = raw_smp_processor_id();

That's completely bogus. The system state check is pointless and
tick_do_timer_cpu even more so because if you call that code from a
worker thread tick_do_timer_cpu should be assigned to some cpu.

Aside of that I'm having a hard time to understand why this stuff
wants to move around at all.

I think we agreed long ago, that for the whole HPC FULL_NOHZ stuff you
have to sacrify at least one CPU for housekeeping purposes of all
kinds, timekeeping, statistics and whatever.

So if you have a housekeeper, then it makes absolutely no sense at all
to move it around in circles.

Can you please enlighten me why we need this at all?

Thanks,

tglx