On Wed, Jun 05, 2013 at 10:13:11AM +0200, Vincent Guittot wrote:
I have faced a sequence where the Idle Load Balance was sometime not triggered for a while on my platform.
CPU 0 and CPU 1 are running tasks and CPU 2 is idle
CPU 1 kicks the Idle Load Balance CPU 1 selects CPU 2 as the new Idle Load Balancer CPU 2 sets NOHZ_BALANCE_KICK for CPU 2 CPU 2 sends a reschedule IPI to CPU 2 While CPU 3 wakes up, CPU 0 or CPU 1 migrates a waking up task A on CPU 2 CPU 2 finally wakes up, runs task A and discards the Idle Load Balance task A quickly goes back to sleep (before a tick occurs on CPU 2) CPU 2 goes back to idle with NOHZ_BALANCE_KICK set
Whenever CPU 2 will be selected as the ILB, no reschedule IPI will be sent because NOHZ_BALANCE_KICK is already set and no Idle Load Balance will be performed.
We must wait for the sched softirq to be raised on CPU 2 thanks to another part the kernel to come back to clear NOHZ_BALANCE_KICK.
The proposed solution clears NOHZ_BALANCE_KICK in schedule_ipi if we can't raise the sched_softirq for the Idle Load Balance.
Change since V1:
- move the clear of NOHZ_BALANCE_KICK in got_nohz_idle_kick if the ILB
can't run on this CPU (as suggested by Peter)
Signed-off-by: Vincent Guittot vincent.guittot@linaro.org
Thanks!