Re: [PATCH v4 2/6] powerpc/idle: Add accessor function to always read latest idle PURR

[Naveen N. Rao]

Gautham R Shenoy wrote:
> On Wed, Apr 01, 2020 at 03:12:53PM +0530, Naveen N. Rao wrote:
> > Hi Gautham,
> >
> > Gautham R. Shenoy wrote:
> > >From: "Gautham R. Shenoy" <ego@xxxxxxxxxxxxxxxxxx>
> > >
> > >+
> > >+static inline u64 read_this_idle_purr(void)
> > >+{
> > >+	/*
> > >+	 * If we are reading from an idle context, update the
> > >+	 * idle-purr cycles corresponding to the last idle period.
> > >+	 * Since the idle context is not yet over, take a fresh
> > >+	 * snapshot of the idle-purr.
> > >+	 */
> > >+	if (unlikely(get_lppaca()->idle == 1)) {
> > >+		update_idle_purr_accounting();
> > >+		snapshot_purr_idle_entry();
> > >+	}
> > >+
> > >+	return be64_to_cpu(get_lppaca()->wait_state_cycles);
> > >+}
> > >+
> >
> > I think this and read_this_idle_spurr() from the next patch should be moved
> > to Patch 4/6, where they are actually used.
>
> The reason I included this function in this patch was to justify why
> we were introducing snapshotting the purr values in a global per-cpu
> variable instead of on a stack variable. The reason being that someone
> might want to read the PURR value from an interrupt context which had
> woken up the CPU from idle. At this point, since epilog() function
> wasn't called, the idle PURR count corresponding to this latest idle
> period would have been accumulated in lppaca->wait_cycles. Thus, this
> helper function safely reads the value by
>    1) First updating the lppaca->wait_cycles with the latest idle_purr
>    count.
>    2) Take a fresh snapshot, since the time from now to the epilog()
>    call is also counted under idle CPU. So the PURR cycle increment
>    during this short period should also be accumulated in lppaca->wait_cycles.
>
>
> prolog()
> |	snapshot PURR
> |
> |
> |
> Idle
> |
> | <----- Interrupt . Read idle PURR ---- update idle PURR;
> |                              	         snapshot PURR;
> |                                   	 Read idle PURR.       
> |
> epilog()
> 	update idle PURR

Yes, I understand. It makes sense.

> However, if you feel that moving this function to Patch 4 where it is
> actually used makes it more readable, I can do that.

My suggestion was from a bisectability standpoint though. This is a 
fairly simple function, but it is generally recommended to ensure that 
newly added code gets exercized in the patch that it is introduced in:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/5.Posting.rst#n119


Regards,
Naveen