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

[Chris Metcalf]

On 10/12/2015 12:53 PM, Paul E. McKenney wrote:
> 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@xxxxxxxxxx>
> > > > Reported-by: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
> > > > Cc: Chris Metcalf <cmetcalf@xxxxxxxxxx>
> > > > Cc: Rik van Riel <riel@xxxxxxxxxx>
> > > > Cc: Christoph Lameter <cl@xxxxxxxxx>
> > > > Cc: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
> > > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > > Cc: Dave Jones <davej@xxxxxxxxxx>
> > > > Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> > > > Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > > > Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> > > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > > Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> > > > ---
> > > >   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?

Is it worth starting to think about grouping things under the
"task isolation" model somehow?  "task_isolation_cpus=1-31"
or some such for this, and then that just sets up the nohz_full
and isolcpus options under the hood?

--
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

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