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

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Message-ID: <20200331205759.GA648829@xz-x1>
Date:   Tue, 31 Mar 2020 16:57:59 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, 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

On Mon, Mar 09, 2020 at 11:19:17AM -0400, Peter Xu wrote:
> On Fri, Feb 14, 2020 at 09:28:25PM +0100, Thomas Gleixner wrote:
> > 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>
> 
> Thanks Thomas!
> 
> Does anyone like to pick this up, or does this patch needs more
> review?

Another gentle ping with the hope that this patch can be picked up.

It's a very simple patch, but I really hope it can be in asap because
the latter means the more kernel versions will be affected by this
isolcpus incompatibility defect (and imo should consider stable too).

Thanks.

-- 
Peter Xu