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

Gautham R Shenoy
2020-04-06T04:57:20Z

On Fri, Apr 03, 2020 at 04:04:56PM +0530, Naveen N. Rao wrote:
> 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@linux.vnet.ibm.com>
> >>>
> >>>+
> >>>+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
>

Fair point. Will move those functions to Patch 4.

>
> Regards,
> Naveen
>