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 <[email protected]>
> Reported-by: Mike Galbraith <[email protected]>
> Cc: Chris Metcalf <[email protected]>
> Cc: Rik van Riel <[email protected]>
> Cc: Christoph Lameter <[email protected]>
> Cc: Mike Galbraith <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Dave Jones <[email protected]>
> Cc: Oleg Nesterov <[email protected]>
> Cc: Paul E. McKenney <[email protected]>
> Cc: Thomas Gleixner <[email protected]>
> Cc: Ingo Molnar <[email protected]>
> Cc: Alexey Dobriyan <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Signed-off-by: Frederic Weisbecker <[email protected]>
> ---
> 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
>
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