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Thu, 6 Aug 2020 12:53:46 +0000 (GMT) Received: from linux.vnet.ibm.com (unknown [9.126.150.29]) by b06wcsmtp001.portsmouth.uk.ibm.com (Postfix) with SMTP; Thu, 6 Aug 2020 12:53:45 +0000 (GMT) Date: Thu, 6 Aug 2020 18:23:45 +0530 From: Srikar Dronamraju To: peterz@infradead.org Cc: Michael Ellerman , Ingo Molnar , LKML , Mel Gorman , Rik van Riel , Thomas Gleixner , Valentin Schneider , Vincent Guittot , Dietmar Eggemann , linuxppc-dev , Michael Neuling , Gautham R Shenoy , Vaidyanathan Srinivasan Subject: Re: [PATCH 1/2] sched/topology: Allow archs to override cpu_smt_mask Message-ID: <20200806125345.GB31068@linux.vnet.ibm.com> Reply-To: Srikar Dronamraju References: <20200804033307.76111-1-srikar@linux.vnet.ibm.com> <20200804104520.GB2657@hirez.programming.kicks-ass.net> <20200804121007.GJ24375@linux.vnet.ibm.com> <20200804124755.GJ2674@hirez.programming.kicks-ass.net> <87ft90z6dy.fsf@mpe.ellerman.id.au> <20200806085429.GX2674@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20200806085429.GX2674@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.235,18.0.687 definitions=2020-08-06_06:2020-08-06,2020-08-06 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 priorityscore=1501 mlxlogscore=999 malwarescore=0 adultscore=0 mlxscore=0 phishscore=0 suspectscore=1 clxscore=1015 impostorscore=0 bulkscore=0 lowpriorityscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2008060091 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * peterz@infradead.org [2020-08-06 10:54:29]: > On Thu, Aug 06, 2020 at 03:32:25PM +1000, Michael Ellerman wrote: > > > That brings with it a bunch of problems, such as existing software that > > has been developed/configured for Power8 and expects to see SMT8. > > > > We also allow LPARs to be live migrated from Power8 to Power9 (and back), so > > maintaining the illusion of SMT8 is considered a requirement to make that work. > > So how does that work if the kernel booted on P9 and demuxed the SMT8 > into 2xSMT4? If you migrate that state onto a P8 with actual SMT8 you're > toast again. > To add to what Michael already said, the reason we don't expose the demux of SMT8 into 2xSMT4 to userspace, is to make the userspace believe they are on a SMT8. When the kernel is live migrated from P8 to P9, till the time of reboot they would only have the older P8 topology. After reboot the kernel topology would change, but the userspace is made to believe that they are running on SMT8 core by way of keeping the sibling_cpumask at SMT8 core level. -- Thanks and Regards Srikar Dronamraju