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Date:   Fri, 03 Apr 2020 16:04:56 +0530
From:   "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Subject: Re: [PATCH v4 2/6] powerpc/idle: Add accessor function to always read
 latest idle PURR
To:     ego@linux.vnet.ibm.com
Cc:     Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        Michael Ellerman <mpe@ellerman.id.au>,
        Nathan Lynch <nathanl@linux.ibm.com>,
        Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>,
        Tyrel Datwyler <tyreld@linux.ibm.com>
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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