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SubjectRe: [tip:sched/urgent] sched/rt: Up the root domain ref count when passing it around via IPIs

I see this was just applied to Linus's tree. This too probably should be
tagged for stable as well.

-- Steve


On Tue, 6 Feb 2018 03:54:42 -0800
"tip-bot for Steven Rostedt (VMware)" <tipbot@zytor.com> wrote:

> Commit-ID: 364f56653708ba8bcdefd4f0da2a42904baa8eeb
> Gitweb: https://git.kernel.org/tip/364f56653708ba8bcdefd4f0da2a42904baa8eeb
> Author: Steven Rostedt (VMware) <rostedt@goodmis.org>
> AuthorDate: Tue, 23 Jan 2018 20:45:38 -0500
> Committer: Ingo Molnar <mingo@kernel.org>
> CommitDate: Tue, 6 Feb 2018 10:20:33 +0100
>
> sched/rt: Up the root domain ref count when passing it around via IPIs
>
> When issuing an IPI RT push, where an IPI is sent to each CPU that has more
> than one RT task scheduled on it, it references the root domain's rto_mask,
> that contains all the CPUs within the root domain that has more than one RT
> task in the runable state. The problem is, after the IPIs are initiated, the
> rq->lock is released. This means that the root domain that is associated to
> the run queue could be freed while the IPIs are going around.
>
> Add a sched_get_rd() and a sched_put_rd() that will increment and decrement
> the root domain's ref count respectively. This way when initiating the IPIs,
> the scheduler will up the root domain's ref count before releasing the
> rq->lock, ensuring that the root domain does not go away until the IPI round
> is complete.
>
> Reported-by: Pavan Kondeti <pkondeti@codeaurora.org>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Mike Galbraith <efault@gmx.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Fixes: 4bdced5c9a292 ("sched/rt: Simplify the IPI based RT balancing logic")
> Link: http://lkml.kernel.org/r/CAEU1=PkiHO35Dzna8EQqNSKW1fr1y1zRQ5y66X117MG06sQtNA@mail.gmail.com
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
> kernel/sched/rt.c | 9 +++++++--
> kernel/sched/sched.h | 2 ++
> kernel/sched/topology.c | 13 +++++++++++++
> 3 files changed, 22 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 2fb627d..89a086e 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1990,8 +1990,11 @@ static void tell_cpu_to_push(struct rq *rq)
>
> rto_start_unlock(&rq->rd->rto_loop_start);
>
> - if (cpu >= 0)
> + if (cpu >= 0) {
> + /* Make sure the rd does not get freed while pushing */
> + sched_get_rd(rq->rd);
> irq_work_queue_on(&rq->rd->rto_push_work, cpu);
> + }
> }
>
> /* Called from hardirq context */
> @@ -2021,8 +2024,10 @@ void rto_push_irq_work_func(struct irq_work *work)
>
> raw_spin_unlock(&rd->rto_lock);
>
> - if (cpu < 0)
> + if (cpu < 0) {
> + sched_put_rd(rd);
> return;
> + }
>
> /* Try the next RT overloaded CPU */
> irq_work_queue_on(&rd->rto_push_work, cpu);
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 2e95505..fb5fc45 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -691,6 +691,8 @@ extern struct mutex sched_domains_mutex;
> extern void init_defrootdomain(void);
> extern int sched_init_domains(const struct cpumask *cpu_map);
> extern void rq_attach_root(struct rq *rq, struct root_domain *rd);
> +extern void sched_get_rd(struct root_domain *rd);
> +extern void sched_put_rd(struct root_domain *rd);
>
> #ifdef HAVE_RT_PUSH_IPI
> extern void rto_push_irq_work_func(struct irq_work *work);
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 034cbed..519b024 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -259,6 +259,19 @@ void rq_attach_root(struct rq *rq, struct root_domain *rd)
> call_rcu_sched(&old_rd->rcu, free_rootdomain);
> }
>
> +void sched_get_rd(struct root_domain *rd)
> +{
> + atomic_inc(&rd->refcount);
> +}
> +
> +void sched_put_rd(struct root_domain *rd)
> +{
> + if (!atomic_dec_and_test(&rd->refcount))
> + return;
> +
> + call_rcu_sched(&rd->rcu, free_rootdomain);
> +}
> +
> static int init_rootdomain(struct root_domain *rd)
> {
> if (!zalloc_cpumask_var(&rd->span, GFP_KERNEL))

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