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Date:   Wed, 19 Jun 2019 12:40:42 +1000
From:   Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH] kernel/isolation: Asset that a housekeeping CPU comes up
 at boot time
To:     Frederic Weisbecker <frederic@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>
References: <20190601113919.2678-1-npiggin@gmail.com>
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        <20190617190552.GA10264@lerouge>
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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
=