Re: [PATCH v8] sched: fix init NOHZ_IDLE flag

From: Peter Zijlstra
Date: Thu Apr 25 2013 - 12:34:21 EST

On Tue, Apr 23, 2013 at 04:59:02PM +0200, Vincent Guittot wrote:
> On my smp platform which is made of 5 cores in 2 clusters, I have the
> nr_busy_cpu field of sched_group_power struct that is not null when the
> platform is fully idle. The root cause is:
> During the boot sequence, some CPUs reach the idle loop and set their
> NOHZ_IDLE flag while waiting for others CPUs to boot. But the nr_busy_cpus
> field is initialized later with the assumption that all CPUs are in the busy
> state whereas some CPUs have already set their NOHZ_IDLE flag.
> More generally, the NOHZ_IDLE flag must be initialized when new sched_domains
> are created in order to ensure that NOHZ_IDLE and nr_busy_cpus are aligned.
> This condition can be ensured by adding a synchronize_rcu between the
> destruction of old sched_domains and the creation of new ones so the NOHZ_IDLE
> flag will not be updated with old sched_domain once it has been initialized.
> But this solution introduces a additionnal latency in the rebuild sequence
> that is called during cpu hotplug.
> As suggested by Frederic Weisbecker, another solution is to have the same
> rcu lifecycle for both NOHZ_IDLE and sched_domain struct.
> A new nohz_idle field is added to sched_domain so both status and sched_domain
> will share the same RCU lifecycle and will be always synchronized.
> In addition, there is no more need to protect nohz_idle against concurrent
> access as it is only modified by 2 exclusive functions called by local cpu.
> This solution has been prefered to the creation of a new struct with an extra
> pointer indirection for sched_domain.

> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>

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