linux-kernel - Re: [PATCH] sched/isolation: Allow "isolcpus=" to skip unknown sub-parameters

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Date:   Fri, 14 Feb 2020 21:28:25 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Ming Lei <ming.lei@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Luiz Capitulino <lcapitulino@...hat.com>
Subject: Re: [PATCH] sched/isolation: Allow "isolcpus=" to skip unknown sub-parameters

Peter Xu <peterx@...hat.com> writes:

> On Tue, Feb 04, 2020 at 11:16:39AM -0500, Peter Xu wrote:
>> The "isolcpus=" parameter allows sub-parameters to exist before the
>> cpulist is specified, and if it sees unknown sub-parameters the whole
>> parameter will be ignored.  This design is incompatible with itself
>> when we add more sub-parameters to "isolcpus=", because the old
>> kernels will not recognize the new "isolcpus=" sub-parameters, then it
>> will invalidate the whole parameter so the CPU isolation will not
>> really take effect if we start to use the new sub-parameters while
>> later we reboot into an old kernel. Instead we will see this when
>> booting the old kernel:
>> 
>>     isolcpus: Error, unknown flag
>> 
>> The better and compatible way is to allow "isolcpus=" to skip unknown
>> sub-parameters, so that even if we add new sub-parameters to it the
>> old kernel will still be able to behave as usual even if with the new
>> sub-parameter is specified.
>> 
>> Ideally this patch should be there when we introduce the first
>> sub-parameter for "isolcpus=", so it's already a bit late.  However
>> late is better than nothing.
>
> Ping - Hi, Thomas, do you have any further comment on this patch?

Fine with me.

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>