Re: [PATCH v7 06/11] nohz: task_isolation: allow tick to be fully disabled

Thomas Gleixner
2015-10-01T14:14:56Z

On Thu, 1 Oct 2015, Frederic Weisbecker wrote:
> On Mon, Sep 28, 2015 at 04:40:56PM -0400, Andy Lutomirski wrote:
> > On Mon, Sep 28, 2015 at 11:17 AM, Chris Metcalf <cmetcalf@ezchip.com> wrote:
> > > While the current fallback to 1-second tick is still helpful for
> > > maintaining completely correct kernel semantics, processes using
> > > prctl(PR_SET_TASK_ISOLATION) semantics place a higher priority on
> > > running completely tickless, so don't bound the time_delta for such
> > > processes. In addition, due to the way such processes quiesce by
> > > waiting for the timer tick to stop prior to returning to userspace,
> > > without this commit it won't be possible to use the task_isolation
> > > mode at all.
> > >
> > > Removing the 1-second cap was previously discussed (see link
> > > below) and Thomas Gleixner observed that vruntime, load balancing
> > > data, load accounting, and other things might be impacted.
> > > Frederic Weisbecker similarly observed that allowing the tick to
> > > be indefinitely deferred just meant that no one would ever fix the
> > > underlying bugs. However it's at least true that the mode proposed
> > > in this patch can only be enabled on a nohz_full core by a process
> > > requesting task_isolation mode, which may limit how important it is
> > > to maintain scheduler data correctly, for example.
> >
> > What goes wrong when a task enables this? Presumably either tasks
> > that enable it experience problems or performance issues or it should
> > always be enabled.
>
> We need to make the scheduler resilient to 0Hz tick. Currently it doesn't
> even correctly support 1Hz or any dynticks behaviour that isn't idle.

Rik has started to work on this. No idea what the status of that is.

Thanks,

tglx