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

Thomas Gleixner
2015-10-12T19:52:51Z

On Mon, 12 Oct 2015, 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.

Acked-by: Thomas Gleixner <tglx@linutronix.de>