Re: clocksource_watchdog causing scheduling of timers every second (was [v13] support "task_isolation" mode)

From: Chris Metcalf
Date: Wed Jul 27 2016 - 16:04:53 EST

On 7/27/2016 2:56 PM, Christoph Lameter wrote:
On Wed, 27 Jul 2016, Chris Metcalf wrote:

How about using cpumask_next_and(raw_smp_processor_id(), cpu_online_mask,
housekeeping_cpumask()), likewise cpumask_first_and()? Does that work?
Ok here is V2:

Subject: clocksource: Do not schedule watchdog on isolated or NOHZ cpus V2

watchdog checks can only run on housekeeping capable cpus. Otherwise
we will be generating noise that we would like to avoid on the isolated

Signed-off-by: Christoph Lameter <cl@xxxxxxxxx>

Index: linux/kernel/time/clocksource.c
--- linux.orig/kernel/time/clocksource.c
+++ linux/kernel/time/clocksource.c
@@ -269,9 +269,10 @@ static void clocksource_watchdog(unsigne
* Cycle through CPUs to check if the CPUs stay synchronized
* to each other.
- next_cpu = cpumask_next(raw_smp_processor_id(), cpu_online_mask);
+ next_cpu = cpumask_next_and(raw_smp_processor_id(), cpu_online_mask, housekeeping_cpumask());
if (next_cpu >= nr_cpu_ids)
- next_cpu = cpumask_first(cpu_online_mask);
+ next_cpu = cpumask_first_and(cpu_online_mask, housekeeping_cpumask());
watchdog_timer.expires += WATCHDOG_INTERVAL;
add_timer_on(&watchdog_timer, next_cpu);

Looks good. Did you omit the equivalent fix in clocksource_start_watchdog()
on purpose? For now I just took your change, but tweaked it to add the
equivalent diff with cpumask_first_and() there.

Chris Metcalf, Mellanox Technologies