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From:   Nathan Lynch <nathanl@linux.ibm.com>
To:     "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        Michael Ellerman <mpe@ellerman.id.au>,
        Nicholas Piggin <npiggin@gmail.com>,
        Tyrel Datwyler <tyreld@linux.ibm.com>,
        Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>,
        Kamalesh Babulal <kamaleshb@in.ibm.com>,
        "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Subject: Re: [PATCH v2 1/1] pseries/hotplug-cpu: Change default behaviour of cede_offline to "off"
In-Reply-To: <1571740391-3251-2-git-send-email-ego@linux.vnet.ibm.com>
References: <1571740391-3251-1-git-send-email-ego@linux.vnet.ibm.com> <1571740391-3251-2-git-send-email-ego@linux.vnet.ibm.com>
Date:   Thu, 31 Oct 2019 11:30:11 -0500
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"Gautham R. Shenoy" <ego@linux.vnet.ibm.com> writes:
> From: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>
>
> Currently on PSeries Linux guests, the offlined CPU can be put to one
> of the following two states:
>    - Long term processor cede (also called extended cede)
>    - Returned to the hypervisor via RTAS "stop-self" call.
>
> This is controlled by the kernel boot parameter "cede_offline=on/off".
>
> By default the offlined CPUs enter extended cede. The PHYP hypervisor
> considers CPUs in extended cede to be "active" since they are still
> under the control fo the Linux guests. Hence, when we change the SMT
> modes by offlining the secondary CPUs, the PURR and the RWMR SPRs will
> continue to count the values for offlined CPUs in extended cede as if
> they are online. This breaks the accounting in tools such as lparstat.
>
> To fix this, ensure that by default the offlined CPUs are returned to
> the hypervisor via RTAS "stop-self" call by changing the default value
> of "cede_offline_enabled" to false.
>
> Fixes: commit 3aa565f53c39 ("powerpc/pseries: Add hooks to put the CPU
> into an appropriate offline state")
>
> Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>

I'm OK with changing the default as a precursor to removing the code
that implements the cede offline mode.

Acked-by: Nathan Lynch <nathanl@linux.ibm.com>