Re: [PATCH v5 7/9] task_isolation: don't interrupt CPUs with tick_nohz_full_kick_cpu()

[Alex Belits <abelits@marvell.com>]

On Mon, 2020-11-23 at 23:13 +0100, Frederic Weisbecker wrote:
> External Email
> 
> -------------------------------------------------------------------
> ---
> Hi Alex,
> 
> On Mon, Nov 23, 2020 at 05:58:22PM +0000, Alex Belits wrote:
> > From: Yuri Norov <ynorov@marvell.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: Yuri Norov <ynorov@marvell.com>
> > [abelits@marvell.com: updated, only exclude CPUs running isolated
> > tasks]
> > Signed-off-by: Alex Belits <abelits@marvell.com>
> > ---
> >  kernel/time/tick-sched.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index a213952541db..6c8679e200f0 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>
> > @@ -268,7 +269,8 @@ static void tick_nohz_full_kick(void)
> >   */
> >  void tick_nohz_full_kick_cpu(int cpu)
> >  {
> > -	if (!tick_nohz_full_cpu(cpu))
> > +	smp_rmb();
> > +	if (!tick_nohz_full_cpu(cpu) || task_isolation_on_cpu(cpu))
> >  		return;
> 
> Like I said in subsequent reviews, we are not going to ignore IPIs.
> We must fix the sources of these IPIs instead.

This is what I am working on right now. This is made with an assumption
that CPU running isolated task has no reason to be kicked because
nothing else is supposed to be there. Usually this is true and when not
true is still safe when everything else is behaving right. For this
version I have kept the original implementation with minimal changes to
make it possible to use task isolation at all.

I agree that it's a much better idea is to determine if the CPU should
be kicked. If it really should, that will be a legitimate cause to
break isolation there, because CPU running isolated task has no
legitimate reason to have timers running. Right now I am trying to
determine the origin of timers that _still_ show up as running in the
current kernel version, so I think, this is a rather large chunk of
work that I have to do separately.

-- 
Alex