linux-kernel - Re: [PATCH] kernel/isolation: Asset that a housekeeping CPU comes up at boot time

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Message-Id: <1560911342.i8093fttpc.astroid@bobo.none>
Date:   Wed, 19 Jun 2019 12:40:42 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Frederic Weisbecker <frederic@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] kernel/isolation: Asset that a housekeeping CPU comes up
 at boot time

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 possible
>> > > to specify command line options that result in no housekeeping CPU
>> > > online at boot.
>> > > 
>> > > An 8 CPU system booted with "nohz_full=0-6 maxcpus=4", for example.
>> > > 
>> > > 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.
>> > > 
>> > > 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.
>> > 
>> > Ping on this one? This should resolve Frederic's remaining objection
>> > to the series (at least until he solves it more generally).
>> > 
>> > As the series has already been merged, should we get this upstream
>> > before release?
>> 
>> I was hoping for feedback from Frederic, lacking that, I've queued it
>> now.
>> 
> 
> Sorry I just came back from vacation. Any chance we can use a WARN() instead?
> 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