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Thu, 6 Aug 2020 10:54:29 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 5274C220F91BB; Thu, 6 Aug 2020 10:54:29 +0200 (CEST) Date: Thu, 6 Aug 2020 10:54:29 +0200 From: peterz@infradead.org To: Michael Ellerman Subject: Re: [PATCH 1/2] sched/topology: Allow archs to override cpu_smt_mask Message-ID: <20200806085429.GX2674@hirez.programming.kicks-ass.net> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87ft90z6dy.fsf@mpe.ellerman.id.au> 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: Gautham R Shenoy , Michael Neuling , Vincent Guittot , Srikar Dronamraju , Rik van Riel , linuxppc-dev , LKML , Valentin Schneider , Thomas Gleixner , Mel Gorman , Ingo Molnar , Dietmar Eggemann Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" 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. > Yeah I agree the naming is confusing. > > Let's call them "SMT4 cores" and "SMT8 cores"? Works for me, thanks! > The problem is we are already lying to userspace, because firmware lies to us. > > ie. the firmware on these systems shows us an SMT8 core, and so current kernels > show SMT8 to userspace. I don't think we can realistically change that fact now, > as these systems are already out in the field. > > What this patch tries to do is undo some of the mess, and at least give the > scheduler the right information. What a mess... I think it depends on what you do with that P9 to P8 migration case. Does it make sense to have a "p8_compat" boot arg for the case where you want LPAR migration back onto P8 systems -- in which case it simply takes the firmware's word as gospel and doesn't untangle things, because it can actually land on a P8.