Messages in this thread | | | Date | Mon, 24 Feb 2020 10:35:04 +0530 | From | Gautham R Shenoy <> | Subject | Re: [PATCH v2 3/5] powerpc/pseries: Account for SPURR ticks on idle CPUs |
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Hello Nathan,
On Fri, Feb 21, 2020 at 10:47:41AM -0600, Nathan Lynch wrote: > "Gautham R. Shenoy" <ego@linux.vnet.ibm.com> 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.
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