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

From: Peter Xu <peterx@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Ming Lei <ming.lei@redhat.com>,
	linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Luiz Capitulino <lcapitulino@redhat.com>
Subject: Re: [PATCH V4] sched/isolation: isolate from handling managed interrupt
Date: Mon, 3 Feb 2020 18:57:43 -0500	[thread overview]
Message-ID: <20200203235743.GH155875@xz-x1> (raw)
In-Reply-To: <87a75zz2hl.fsf@nanos.tec.linutronix.de>

On Mon, Feb 03, 2020 at 11:15:50PM +0000, Thomas Gleixner wrote:
> Peter Xu <peterx@redhat.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.

Hi, Thomas,

Just to make sure I understand it right: are you suggesting that we
fix up housekeeping_isolcpus_setup() to be able to skip unknown sub
parameters?

Thanks,

-- 
Peter Xu