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

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Message-ID: <20200403112428.GB20293@in.ibm.com>
Date:   Fri, 3 Apr 2020 16:54:28 +0530
From:   Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:     ego@...ux.vnet.ibm.com,
        Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Nathan Lynch <nathanl@...ux.ibm.com>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Tyrel Datwyler <tyreld@...ux.ibm.com>
Subject: Re: [PATCH v4 2/6] powerpc/idle: Add accessor function to always
 read latest idle PURR

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@...ux.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
>