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 <[email protected]>
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