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[193.116.92.108]) by smtp.gmail.com with ESMTPSA id 27sm13036087pgt.6.2019.06.18.19.45.44 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Tue, 18 Jun 2019 19:45:45 -0700 (PDT) Date: Wed, 19 Jun 2019 12:40:42 +1000 From: Nicholas Piggin Subject: Re: [PATCH] kernel/isolation: Asset that a housekeeping CPU comes up at boot time To: Frederic Weisbecker , Peter Zijlstra Cc: linux-kernel@vger.kernel.org, Ingo Molnar References: <20190601113919.2678-1-npiggin@gmail.com> <1560151344.y4aukciain.astroid@bobo.none> <20190617155931.GK3436@hirez.programming.kicks-ass.net> <20190617190552.GA10264@lerouge> In-Reply-To: <20190617190552.GA10264@lerouge> MIME-Version: 1.0 User-Agent: astroid/0.14.0 (https://github.com/astroidmail/astroid) Message-Id: <1560911342.i8093fttpc.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 June 18, 2019 5:05 am: > On Mon, Jun 17, 2019 at 05:59:31PM +0200, Peter Zijlstra wrote: >> On Mon, Jun 10, 2019 at 05:24:32PM +1000, Nicholas Piggin wrote: >> > Nicholas Piggin's on June 1, 2019 9:39 pm: >> > > With the change to allow the boot CPU0 to be isolated, it is possibl= e >> > > to specify command line options that result in no housekeeping CPU >> > > online at boot. >> > >=20 >> > > An 8 CPU system booted with "nohz_full=3D0-6 maxcpus=3D4", for examp= le. >> > >=20 >> > > It is not easily possible at housekeeping init time to know all the >> > > various SMP options that will result in an invalid configuration, so >> > > this patch adds a sanity check after SMP init, to ensure that a >> > > housekeeping CPU has been onlined. >> > >=20 >> > > The panic is undesirable, but it's better than the alternative of an >> > > obscure non deterministic failure. The panic will reliably happen >> > > when advanced parameters are used incorrectly. >> >=20 >> > Ping on this one? This should resolve Frederic's remaining objection >> > to the series (at least until he solves it more generally). >> >=20 >> > As the series has already been merged, should we get this upstream >> > before release? >>=20 >> I was hoping for feedback from Frederic, lacking that, I've queued it >> now. >>=20 >=20 > Sorry I just came back from vacation. Any chance we can use a WARN() inst= ead? > I prefer to use panic() only when data is really threatened or such. I thought it was decided to panic here, because we don't assign a house keeping CPU so the system is unlikely to behave properly. A warn might scroll off the screen by the time things grind to a halt. This is a one-time boot parameter misconfiguration, many cases of which can cause a panic and boot stop. No question if we can make this more dynamic that would be better, but for near term at least can we go with this? Thanks, Nick =