[tip:sched/core] nohz: Revert "nohz: Set isolcpus when nohz_full is set"

[tip-bot for Frederic Weisbecker <tipbot@zytor.com>]

Commit-ID:  0baabb385eb4bce699ddab0db015112be6cf1e6a
Gitweb:     http://git.kernel.org/tip/0baabb385eb4bce699ddab0db015112be6cf1e6a
Author:     Frederic Weisbecker <fweisbec@gmail.com>
AuthorDate: Mon, 12 Oct 2015 17:21:23 +0200
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 20 Oct 2015 10:13:36 +0200

nohz: Revert "nohz: Set isolcpus when nohz_full is set"

This reverts:

  8cb9764fc88b ("nohz: Set isolcpus when nohz_full is set")

We assumed that full-nohz users always want scheduler isolation on full
dynticks CPUs, therefore we included full-nohz 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 - which is true in a sense.

Some full-nohz users appear to be interested in running normal workloads
either before or after an isolation workload. Full-nohz 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>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul E . McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Link: http://lkml.kernel.org/r/1444663283-30068-1-git-send-email-fweisbec@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/core.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 10a8faa..5bd7d60 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);
-
 	sched_init_numa();
 
 	/*