Messages in this thread | | | Date | Mon, 12 Oct 2015 08:32:02 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] nohz: Revert "nohz: Set isolcpus when nohz_full is set" |
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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 >
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