Messages in this thread | | | Subject | Re: [PATCH] nohz: Revert "nohz: Set isolcpus when nohz_full is set" | From | Chris Metcalf <> | Date | Mon, 12 Oct 2015 12:55:24 -0400 |
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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
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