Re: [PATCH v15 04/13] task_isolation: add initial support

[Chris Metcalf]

On 8/29/2016 12:48 PM, Peter Zijlstra wrote:
> On Mon, Aug 29, 2016 at 12:40:32PM -0400, Chris Metcalf wrote:
> > On 8/29/2016 12:33 PM, Peter Zijlstra wrote:
> > > On Tue, Aug 16, 2016 at 05:19:27PM -0400, Chris Metcalf wrote:
> > > > +	/*
> > > > +	 * Request rescheduling unless we are in full dynticks mode.
> > > > +	 * We would eventually get pre-empted without this, and if
> > > > +	 * there's another task waiting, it would run; but by
> > > > +	 * explicitly requesting the reschedule, we may reduce the
> > > > +	 * latency.  We could directly call schedule() here as well,
> > > > +	 * but since our caller is the standard place where schedule()
> > > > +	 * is called, we defer to the caller.
> > > > +	 *
> > > > +	 * A more substantive approach here would be to use a struct
> > > > +	 * completion here explicitly, and complete it when we shut
> > > > +	 * down dynticks, but since we presumably have nothing better
> > > > +	 * to do on this core anyway, just spinning seems plausible.
> > > > +	 */
> > > > +	if (!tick_nohz_tick_stopped())
> > > > +		set_tsk_need_resched(current);
> > > This is broken.. and it would be really good if you don't actually need
> > > to do this.
> > Can you elaborate?
> Naked use of TIF_NEED_RESCHED like this is busted. There is more state
> that needs to be poked to keep things consistent / working.

Would it be cleaner to just replace the set_tsk_need_resched() call
with something like:

    set_current_state(TASK_INTERRUPTIBLE);
    schedule();
    __set_current_state(TASK_RUNNING);

or what would you recommend?

Or, as I said, just doing a busy loop here while testing to see
if need_resched or signal had been set?

--
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com