Re: [PATCH] nohz: Revert "nohz: Set isolcpus when nohz_full is set"

Paul E. McKenney
2015-10-12T16:53:50Z

On Mon, Oct 12, 2015 at 06:20:03PM +0200, Frederic Weisbecker wrote:
> On Mon, Oct 12, 2015 at 08:32:02AM -0700, Paul E. McKenney wrote:
> > On Mon, Oct 12, 2015 at 05:21:23PM +0200, Frederic Weisbecker wrote:
> > > This reverts commit 8cb9764fc88b41db11f251e8b2a0d006578b7eb4.
> > >
> > > We assumed that nohz full users always want scheduler isolation on full
> > > dynticks CPUs, therefore we included nohz full CPUs on cpu_isolated_map.
> > > This means that tasks run by default on CPUs outside the nohz_full range
> > > unless their affinity is explicity overwritten.
> > >
> > > This suits pure isolation workloads but when the machine is needed to
> > > run common workloads, the available sets of CPUs to run common tasks
> > > becomes reduced.
> > >
> > > We reach an extreme case when CONFIG_NO_HZ_FULL_ALL is enabled as it
> > > leaves only CPU 0 for non-isolation tasks, which makes people think that
> > > their supercomputer regressed to 90's UP.
> > >
> > > Some nohz full users appear to be interested in running normal workloads
> > > either before or after an isolation workload. Nohz full isn't optimized
> > > toward normal workloads but it's still better than UP performance.
> > >
> > > We are reaching a limitation in kernel presets here. Lets revert this
> > > cpu_isolated_map inclusion and let userspace do its own scheduler
> > > isolation using cpusets or explicit affinity settings.
> > >
> > > Reported-by: Ingo Molnar <mingo@kernel.org>
> > > Reported-by: Mike Galbraith <umgwanakikbuti@gmail.com>
> > > Cc: Chris Metcalf <cmetcalf@ezchip.com>
> > > Cc: Rik van Riel <riel@redhat.com>
> > > Cc: Christoph Lameter <cl@linux.com>
> > > Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
> > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > Cc: Dave Jones <davej@redhat.com>
> > > Cc: Oleg Nesterov <oleg@redhat.com>
> > > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > Cc: Ingo Molnar <mingo@kernel.org>
> > > Cc: Alexey Dobriyan <adobriyan@gmail.com>
> > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > > ---
> > > kernel/sched/core.c | 3 ---
> > > 1 file changed, 3 deletions(-)
> > >
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index 6159531..3c35b5f 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -7238,9 +7238,6 @@ void __init sched_init_smp(void)
> > > alloc_cpumask_var(&non_isolated_cpus, GFP_KERNEL);
> > > alloc_cpumask_var(&fallback_doms, GFP_KERNEL);
> > >
> > > - /* nohz_full won't take effect without isolating the cpus. */
> > > - tick_nohz_full_add_cpus_to(cpu_isolated_map);
> > > -
> >
> > Why not make this controlled by a boot parameter? That preserves
> > the ease of use for those needing it, but avoids problems from people
> > doing "make randconfig".
>
> Well it is already. As you pass nohz_full=1-32, you can pass as well isolcpus=1-32

True enough. Not sure that having to repeat the CPU list twice qualifies as
"easy to use", though. Why not a nohz_full_iso or some such that isolates
whatever CPUs you specified?

But we really need people using this to weigh in.

Thanx, Paul