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

Naveen N. Rao
2020-04-03T10:35:07Z

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

Regards,
Naveen