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Tue, 17 Sep 2019 17:36:35 +0000 (GMT) Received: from b03ledav005.gho.boulder.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 8C0FEBE054; Tue, 17 Sep 2019 17:36:35 +0000 (GMT) Received: from localhost (unknown [9.41.179.186]) by b03ledav005.gho.boulder.ibm.com (Postfix) with ESMTP; Tue, 17 Sep 2019 17:36:35 +0000 (GMT) From: Nathan Lynch To: Gautham R Shenoy Subject: Re: [PATCH 0/2] pseries/hotplug: Change the default behaviour of cede_offline In-Reply-To: <20190915074217.GA943@in.ibm.com> References: <1568284541-15169-1-git-send-email-ego@linux.vnet.ibm.com> <87impxr0am.fsf@linux.ibm.com> <20190915074217.GA943@in.ibm.com> Date: Tue, 17 Sep 2019 12:36:35 -0500 Message-ID: <87a7b2rfj0.fsf@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:, , definitions=2019-09-17_08:, , signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=1 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=892 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1908290000 definitions=main-1909170168 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Tyrel Datwyler , "Aneesh Kumar K.V" , Kamalesh Babulal , linux-kernel@vger.kernel.org, Nicholas Piggin , "Naveen N . Rao" , Vaidyanathan Srinivasan , linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Gautham R Shenoy writes: > On Thu, Sep 12, 2019 at 10:39:45AM -0500, Nathan Lynch wrote: >> "Gautham R. Shenoy" writes: >> > The patchset also defines a new sysfs attribute >> > "/sys/device/system/cpu/cede_offline_enabled" on PSeries Linux guests >> > to allow userspace programs to change the state into which the >> > offlined CPU need to be put to at runtime. >> >> A boolean sysfs interface will become awkward if we need to add another >> mode in the future. >> >> What do you think about naming the attribute something like >> 'offline_mode', with the possible values 'extended-cede' and >> 'rtas-stopped'? > > We can do that. However, IMHO in the longer term, on PSeries guests, > we should have only one offline state - rtas-stopped. The reason for > this being, that on Linux, SMT switch is brought into effect through > the CPU Hotplug interface. The only state in which the SMT switch will > recognized by the hypervisors such as PHYP is rtas-stopped. OK. Why "longer term" though, instead of doing it now? > All other states (such as extended-cede) should in the long-term be > exposed via the cpuidle interface. > > With this in mind, I made the sysfs interface boolean to mirror the > current "cede_offline" commandline parameter. Eventually when we have > only one offline-state, we can deprecate the commandline parameter as > well as the sysfs interface. I don't care for adding a sysfs interface that is intended from the beginning to become vestigial... This strikes me as unnecessarily incremental if you're changing the default offline state. Any user space programs depending on the current behavior will have to change anyway (and why is it OK to break them?) Why isn't the plan: 1. Add extended cede support to the pseries cpuidle driver 2. Make stop-self the only cpu offline state for pseries (no sysfs interface necessary) ?