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

[Chris Metcalf <cmetcalf@ezchip.com>]

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@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?

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