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[193.116.98.44]) by smtp.gmail.com with ESMTPSA id q80sm9913053pfa.66.2019.05.03.23.59.18 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Fri, 03 May 2019 23:59:19 -0700 (PDT) Date: Sat, 04 May 2019 16:59:12 +1000 From: Nicholas Piggin Subject: Re: [tip:sched/core] sched/isolation: Require a present CPU in housekeeping mask To: Frederic Weisbecker , fweisbec@gmail.com, hpa@zytor.com, linux-kernel@vger.kernel.org, mingo@kernel.org, peterz@infradead.org, rafael.j.wysocki@intel.com, tglx@linutronix.de, torvalds@linux-foundation.org Cc: linux-tip-commits@vger.kernel.org References: <20190411033448.20842-5-npiggin@gmail.com> <20190504002733.GB19076@lenoir> In-Reply-To: <20190504002733.GB19076@lenoir> MIME-Version: 1.0 User-Agent: astroid/0.14.0 (https://github.com/astroidmail/astroid) Message-Id: <1556952021.2xpa7joi2y.astroid@bobo.none> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Frederic Weisbecker's on May 4, 2019 10:27 am: > On Fri, May 03, 2019 at 10:47:37AM -0700, tip-bot for Nicholas Piggin wro= te: >> Commit-ID: 9219565aa89033a9cfdae788c1940473a1253d6c >> Gitweb: https://git.kernel.org/tip/9219565aa89033a9cfdae788c1940473a= 1253d6c >> Author: Nicholas Piggin >> AuthorDate: Thu, 11 Apr 2019 13:34:47 +1000 >> Committer: Ingo Molnar >> CommitDate: Fri, 3 May 2019 19:42:58 +0200 >>=20 >> sched/isolation: Require a present CPU in housekeeping mask >>=20 >> During housekeeping mask setup, currently a possible CPU is required. >> That does not guarantee the CPU would be available at boot time, so >> check to ensure that at least one present CPU is in the mask. >=20 > I have a doubt about the requirements and semantics of cpu_present_mask. > IIUC a present CPU means that it is physically plugged in (from ACPI > perspective) but might not be logically plugged in (set on cpu_online_mas= k). Right, a superset of cpu_possible_mask, subset of cpu_online_mask. It=20 means that CPU can be brought online at any time. > But do we have the guarantee that a present CPU _will_ be online at least= once > right after the boot? After all, kernel parameters such as "maxcpus=3D" c= an prevent > from turning some CPUs on. I guess there are even more creative ways to a= chieve > that. >=20 > In any case we really require the housekeeper to be forced online. Perhap= s > I missed that enforcement somewhere in the patchset? No I think you're right, that may be able to boot without anything in the housekeeping mask. Maybe we can just cpu_up() a CPU in the=20 housekeeping mask with a warning that it has overidden their SMP command line option. I'll take a look at it. Thanks, Nick =