linux-kernel - Re: [PATCH V4] sched/isolation: isolate from handling managed interrupt

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Date:   Mon, 03 Feb 2020 23:15:50 +0000
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Xu <peterx@...hat.com>
Cc:     Ming Lei <ming.lei@...hat.com>, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Luiz Capitulino <lcapitulino@...hat.com>
Subject: Re: [PATCH V4] sched/isolation: isolate from handling managed interrupt

Peter Xu <peterx@...hat.com> writes:
> The new "managed_irq" works for us, thanks for both of your work!
>
> However I just noticed that this new sub-parameter might break users
> if applied incorrectly to old kernels, because iiuc "isolcpus="
> parameter will not apply at all when there's unknown sub-parameters:
>
> static int __init housekeeping_isolcpus_setup(char *str)
> {
> 	unsigned int flags = 0;
>
> 	while (isalpha(*str)) {
>                 ...
>                 pr_warn("isolcpus: Error, unknown flag\n");
>                 return 0;
>         }
>         ...
> }
>
> Then the same kernel parameter will break isolcpus= if the user
> reboots and switches to an older kernel.
>
> A solution to this could be that we introduce an isolated parameter
> for "managed_irq", then on the old kernels only the new parameter will
> be ignored rather than the whole "isolcpus=" parameter, so nothing
> will break.
>
> I'm not sure whether it's already too late for this, or if there's any
> better alternative.  Just raise this question up to see whether we
> still have chance to fix this up.

No, really. The basic guarantee is that your new kernel is going to work
fine with the previous command line, but making a guarantee that new
command line options still work on an old kernel are just creating a
horrible mess. So if that command line interface was not designed to
handle unknown arguments in the first place, you better fix that.

Thanks,

        tglx