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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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".

							Thanx, Paul

>  	sched_init_numa();
> 
>  	/*
> -- 
> 2.5.3
>