linux-kernel - Re: [PATCH Resend v5] sched: fix init NOHZ

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Date:	Thu, 4 Apr 2013 16:24:23 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	linaro-kernel@...ts.linaro.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH Resend v5] sched: fix init NOHZ_IDLE flag

2013/4/3 Vincent Guittot <vincent.guittot@...aro.org>:
> 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. I have introduce
> a new sched_domain_rq struct that is the entry point for both sched_domains
> and objects that must follow the same lifecycle like NOHZ_IDLE flags. They
> will share the same RCU lifecycle and will be always synchronized.
>
> The synchronization is done at the cost of :
>  - an additional indirection for accessing the first sched_domain level
>  - an additional indirection and a rcu_dereference before accessing to the
>    NOHZ_IDLE flag.
>
> Change since v4:
>  - link both sched_domain and NOHZ_IDLE flag in one RCU object so
>    their states are always synchronized.
>
> Change since V3;
>  - NOHZ flag is not cleared if a NULL domain is attached to the CPU
>  - Remove patch 2/2 which becomes useless with latest modifications
>
> Change since V2:
>  - change the initialization to idle state instead of busy state so a CPU that
>    enters idle during the build of the sched_domain will not corrupt the
>    initialization state
>
> Change since V1:
>  - remove the patch for SCHED softirq on an idle core use case as it was
>    a side effect of the other use cases.
>
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  include/linux/sched.h |    6 +++
>  kernel/sched/core.c   |  105 ++++++++++++++++++++++++++++++++++++++++++++-----
>  kernel/sched/fair.c   |   35 +++++++++++------
>  kernel/sched/sched.h  |   24 +++++++++--
>  4 files changed, 145 insertions(+), 25 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index d35d2b6..2a52188 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -959,6 +959,12 @@ struct sched_domain {
>         unsigned long span[0];
>  };
>
> +struct sched_domain_rq {
> +       struct sched_domain *sd;

Why not make it part of the structure content instead of pointing to it?

> +       unsigned long flags;
> +       struct rcu_head rcu;    /* used during destruction */
> +};
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