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Date:   Sun, 8 Mar 2020 07:28:22 +0000
From:   Alex Belits <abelits@...vell.com>
To:     "frederic@...nel.org" <frederic@...nel.org>
CC:     "mingo@...nel.org" <mingo@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "will@...nel.org" <will@...nel.org>
Subject: Re: [EXT] Re: [PATCH 08/12] task_isolation: don't interrupt CPUs with
 tick_nohz_full_kick_cpu()

On Fri, 2020-03-06 at 17:03 +0100, Frederic Weisbecker wrote:
> On Wed, Mar 04, 2020 at 04:12:40PM +0000, Alex Belits wrote:
> > From: Yuri Norov <ynorov@...vell.com>
> > 
> > For nohz_full CPUs the desirable behavior is to receive interrupts
> > generated by tick_nohz_full_kick_cpu(). But for hard isolation it's
> > obviously not desirable because it breaks isolation.
> > 
> > This patch adds check for it.
> > 
> > Signed-off-by: Alex Belits <abelits@...vell.com>
> > ---
> >  kernel/time/tick-sched.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 1d4dec9d3ee7..fe4503ba1316 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -20,6 +20,7 @@
> >  #include <linux/sched/clock.h>
> >  #include <linux/sched/stat.h>
> >  #include <linux/sched/nohz.h>
> > +#include <linux/isolation.h>
> >  #include <linux/module.h>
> >  #include <linux/irq_work.h>
> >  #include <linux/posix-timers.h>
> > @@ -262,7 +263,7 @@ static void tick_nohz_full_kick(void)
> >   */
> >  void tick_nohz_full_kick_cpu(int cpu)
> >  {
> > -	if (!tick_nohz_full_cpu(cpu))
> > +	if (!tick_nohz_full_cpu(cpu) || task_isolation_on_cpu(cpu))
> >  		return;
> 
> I fear you can't do that. A nohz full CPU is kicked for a reason.
> As for the other cases, you need to fix the callers.
> 
> In the general case, randomly ignoring an interrupt is a correctness
> issue.

Not ignoring, just delaying until we are back from userspace. We know
that everything was done on this CPU when we successfully entered
userspace in isolated mode -- otherwise we would be kicked out. We
restart timers when we are back in kernel again on cleanup, so things
will be back to normal at that point. Between those moments we can just
as well remain in userspace and forget about the timers until we are
back in kernel.

> 
> Thanks.

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