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Date: Thu, 21 Feb 2013 12:02:56 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Alexander Gordeev <agordeev@...hat.com>
Cc: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2 -tip] sched/rt: Fix locality of threaded interrupt
handlers
On Wed, 2013-02-20 at 10:19 +0100, Alexander Gordeev wrote:
> When a interrupt affinity mask targets multiple CPUs, the
> RT scheduler selects a runqueue for RT task corresponding
> to a threaded interrupt handler without consideration of
> where the interrupt is actually gets delivered. It leads
> to a suboptimal condition when a hardware interrupt handler
> executes on one CPU while the threaded interrupt handler
> executes on another CPU.
>
> This fix alters the behaviour of threaded handler wake-ups
> by getting priority to a CPU where the hardware interrupt
> handler is executing. As result, most of the time both
> halves of interrupt handling are kept local.
This fails to explain the ramifications for the 'global' FIFO push-pull
stuff.
Left patch in-place for Steven.
> Signed-off-by: Alexander Gordeev <agordeev@...hat.com>
> ---
> include/linux/sched.h | 2 ++
> kernel/irq/handle.c | 2 +-
> kernel/sched/core.c | 5 +++++
> kernel/sched/rt.c | 27 +++++++++++++++++++--------
> 4 files changed, 27 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 206bb08..1d59efa 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1061,6 +1061,7 @@ struct sched_domain;
> #define WF_SYNC 0x01 /* waker goes to sleep after wakup */
> #define WF_FORK 0x02 /* child wakeup after fork */
> #define WF_MIGRATED 0x04 /* internal use, task got migrated */
> +#define WF_LOCAL 0x08 /* try to wake up locally */
>
> #define ENQUEUE_WAKEUP 1
> #define ENQUEUE_HEAD 2
> @@ -2207,6 +2208,7 @@ extern void xtime_update(unsigned long ticks);
>
> extern int wake_up_state(struct task_struct *tsk, unsigned int state);
> extern int wake_up_process(struct task_struct *tsk);
> +extern int wake_up_local(struct task_struct *tsk);
> extern void wake_up_new_task(struct task_struct *tsk);
> #ifdef CONFIG_SMP
> extern void kick_process(struct task_struct *tsk);
> diff --git a/kernel/irq/handle.c b/kernel/irq/handle.c
> index 131ca17..fe97d0c 100644
> --- a/kernel/irq/handle.c
> +++ b/kernel/irq/handle.c
> @@ -126,7 +126,7 @@ static void irq_wake_thread(struct irq_desc *desc, struct irqaction *action)
> */
> atomic_inc(&desc->threads_active);
>
> - wake_up_process(action->thread);
> + wake_up_local(action->thread);
> }
>
> irqreturn_t
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 257002c..38413f6 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1527,6 +1527,11 @@ int wake_up_process(struct task_struct *p)
> }
> EXPORT_SYMBOL(wake_up_process);
>
> +int wake_up_local(struct task_struct *p)
> +{
> + return try_to_wake_up(p, TASK_ALL, WF_LOCAL);
> +}
> +
> int wake_up_state(struct task_struct *p, unsigned int state)
> {
> return try_to_wake_up(p, state, 0);
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 418feb0..de16e16 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1234,7 +1234,7 @@ static void yield_task_rt(struct rq *rq)
> }
>
> #ifdef CONFIG_SMP
> -static int find_lowest_rq(struct task_struct *task);
> +static int find_lowest_rq(struct task_struct *task, bool prefer_this_cpu);
>
> static int
> select_task_rq_rt(struct task_struct *p, int sd_flag, int flags)
> @@ -1242,6 +1242,7 @@ select_task_rq_rt(struct task_struct *p, int sd_flag, int flags)
> struct task_struct *curr;
> struct rq *rq;
> int cpu;
> + bool prefer_this_cpu = flags & WF_LOCAL;
>
> cpu = task_cpu(p);
>
> @@ -1258,6 +1259,11 @@ select_task_rq_rt(struct task_struct *p, int sd_flag, int flags)
> curr = ACCESS_ONCE(rq->curr); /* unlocked access */
>
> /*
> + * If this RT task is a threaded interrupt handler, then
> + * it is being awaken from the hardware interrupt handler.
> + * In this case try to keep hardware and threaded interrupt
> + * handlers as close as possible and wake it up on this CPU.
> + *
> * If the current task on @p's runqueue is an RT task, then
> * try to see if we can wake this RT task up on another
> * runqueue. Otherwise simply start this RT task
> @@ -1279,11 +1285,12 @@ select_task_rq_rt(struct task_struct *p, int sd_flag, int flags)
> * This test is optimistic, if we get it wrong the load-balancer
> * will have to sort it out.
> */
> - if (curr && unlikely(rt_task(curr)) &&
> - (curr->nr_cpus_allowed < 2 ||
> - curr->prio <= p->prio) &&
> - (p->nr_cpus_allowed > 1)) {
> - int target = find_lowest_rq(p);
> + if (prefer_this_cpu ||
> + (curr && unlikely(rt_task(curr)) &&
> + (curr->nr_cpus_allowed < 2 ||
> + curr->prio <= p->prio) &&
> + (p->nr_cpus_allowed > 1))) {
> + int target = find_lowest_rq(p, prefer_this_cpu);
>
> if (target != -1)
> cpu = target;
> @@ -1473,7 +1480,7 @@ next_idx:
>
> static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask);
>
> -static int find_lowest_rq(struct task_struct *task)
> +static int find_lowest_rq(struct task_struct *task, bool prefer_this_cpu)
> {
> struct sched_domain *sd;
> struct cpumask *lowest_mask = __get_cpu_var(local_cpu_mask);
> @@ -1495,9 +1502,13 @@ static int find_lowest_rq(struct task_struct *task)
> * lowest priority tasks in the system. Now we want to elect
> * the best one based on our affinity and topology.
> *
> + * If asked explicitly, try to pick up this cpu.
> + *
> * We prioritize the last cpu that the task executed on since
> * it is most likely cache-hot in that location.
> */
> + if (prefer_this_cpu && cpumask_test_cpu(this_cpu, lowest_mask))
> + return this_cpu;
> if (cpumask_test_cpu(cpu, lowest_mask))
> return cpu;
>
> @@ -1555,7 +1566,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
> int cpu;
>
> for (tries = 0; tries < RT_MAX_TRIES; tries++) {
> - cpu = find_lowest_rq(task);
> + cpu = find_lowest_rq(task, false);
>
> if ((cpu == -1) || (cpu == rq->cpu))
> break;
> --
> 1.7.7.6
>
>
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