Subject: Re: [PATCH v2 3/5] powerpc/pseries: Account for SPURR ticks on idle CPUs

From: Gautham R Shenoy <[email-masked]>
Date: Mon, 24 Feb 2020 10:35:04 +0530

Hello Nathan,

On Fri, Feb 21, 2020 at 10:47:41AM -0600, Nathan Lynch wrote:
> "Gautham R. Shenoy" <[email-masked]> writes:
> > +static inline void snapshot_spurr_idle_entry(void)
> > +{
> > +	*this_cpu_ptr(&idle_entry_spurr_snap) = mfspr(SPRN_SPURR);
> > +}
> > +
> 
> [...]
> 
> > +static inline void update_idle_spurr_accounting(void)
> > +{
> > +	u64 *idle_spurr_cycles_ptr = this_cpu_ptr(&idle_spurr_cycles);
> > +	u64 in_spurr = *this_cpu_ptr(&idle_entry_spurr_snap);
> > +
> > +	*idle_spurr_cycles_ptr += mfspr(SPRN_SPURR) - in_spurr;
> > +}
> 
> [...]
> 
> > +static inline u64 read_this_idle_spurr(void)
> > +{
> > +	/*
> > +	 * If we are reading from an idle context, update the
> > +	 * idle-spurr cycles corresponding to the last idle period.
> > +	 * Since the idle context is not yet over, take a fresh
> > +	 * snapshot of the idle-spurr.
> > +	 */
> > +	if (get_lppaca()->idle == 1) {
> > +		update_idle_spurr_accounting();
> > +		snapshot_spurr_idle_entry();
> 
> This samples spurr twice when it could do with just one. I don't know
> the performance implications, but will the results be coherent?

We would have taken the snapshot in idle_loop_prolog() just before
entering idle. That fact that the "if" condition is true above in
read_this_idle_spurr() implies that we are reading the idle_spurr
value from an interrupt context and since get_lppaca()->idle == 1, we
haven't yet called idle_loop_epilog(), where we would have updated the
idle_spurr ticks for the last idle period.

Hence, in this function, we first update the idle_spurr accounting
from the time of the last snapshot to now. We update the snapshot to
the current SPURR value so that when we eventually call
idle_loop_epilog(), we will account for the remaining idle duration,
i.e from the read_this_idle_spurr() call to idle_loop_epilog()

The results are therefore coherant, in that we do not perform double
accounting the second time we invoke update_idle_spurr_accounting()
from idle_loop_epilog(), but only add the spurr ticks from
read_this_idle_spurr() to idle_loop_epilog().

--
Thanks and Regards
gautham.