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From: Tejun Heo on 13 May 2010 06:50
Concurrency managed workqueue needs to know when workers are going to
sleep and waking up, and, when a worker goes to sleep, be able to wake
up another worker to maintain adequate concurrency. This patch
introduces PF_WQ_WORKER to identify workqueue workers and adds the
following two hooks.

* wq_worker_waking_up(): called when a worker is woken up.

* wq_worker_sleeping(): called when a worker is going to sleep and may
return a pointer to a local task which should be woken up. The
returned task is woken up using try_to_wake_up_local() which is
simplified ttwu which is called under rq lock and can only wake up
local tasks.

Both hooks are currently defined as noop in kernel/workqueue_sched.h.
Later cmwq implementation will replace them with proper
implementation.

These hooks are hard coded as they'll always be enabled.

Signed-off-by: Tejun Heo <tj(a)kernel.org>
Cc: Peter Zijlstra <peterz(a)infradead.org>
Cc: Mike Galbraith <efault(a)gmx.de>
Cc: Ingo Molnar <mingo(a)elte.hu>
---
include/linux/sched.h | 1 +
kernel/fork.c | 2 +-
kernel/sched.c | 53 ++++++++++++++++++++++++++++++++++++++++++++-
kernel/workqueue_sched.h | 16 +++++++++++++
4 files changed, 69 insertions(+), 3 deletions(-)
create mode 100644 kernel/workqueue_sched.h

diff --git a/include/linux/sched.h b/include/linux/sched.h
index ef6067c..553ffae 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1702,6 +1702,7 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t *
#define PF_EXITING 0x00000004 /* getting shut down */
#define PF_EXITPIDONE 0x00000008 /* pi exit done on shut down */
#define PF_VCPU 0x00000010 /* I'm a virtual CPU */
+#define PF_WQ_WORKER 0x00000020 /* I'm a workqueue worker */
#define PF_FORKNOEXEC 0x00000040 /* forked but didn't exec */
#define PF_MCE_PROCESS 0x00000080 /* process policy on mce errors */
#define PF_SUPERPRIV 0x00000100 /* used super-user privileges */
diff --git a/kernel/fork.c b/kernel/fork.c
index 44b0791..1440f90 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -900,7 +900,7 @@ static void copy_flags(unsigned long clone_flags, struct task_struct *p)
{
unsigned long new_flags = p->flags;

- new_flags &= ~PF_SUPERPRIV;
+ new_flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER);
new_flags |= PF_FORKNOEXEC;
new_flags |= PF_STARTING;
p->flags = new_flags;
diff --git a/kernel/sched.c b/kernel/sched.c
index 0b6da84..460b176 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -77,6 +77,7 @@
#include <asm/irq_regs.h>

#include "sched_cpupri.h"
+#include "workqueue_sched.h"

#define CREATE_TRACE_POINTS
#include <trace/events/sched.h>
@@ -2361,6 +2362,9 @@ static inline void ttwu_post_activation(struct task_struct *p, struct rq *rq,
rq->idle_stamp = 0;
}
#endif
+ /* if a worker is waking up, notify workqueue */
+ if ((p->flags & PF_WQ_WORKER) && success)
+ wq_worker_waking_up(p, cpu_of(rq));
}

/**
@@ -2469,6 +2473,37 @@ out:
}

/**
+ * try_to_wake_up_local - try to wake up a local task with rq lock held
+ * @p: the thread to be awakened
+ *
+ * Put @p on the run-queue if it's not alredy there. The caller must
+ * ensure that this_rq() is locked, @p is bound to this_rq() and not
+ * the current task. this_rq() stays locked over invocation.
+ */
+static void try_to_wake_up_local(struct task_struct *p)
+{
+ struct rq *rq = task_rq(p);
+ bool success = false;
+
+ BUG_ON(rq != this_rq());
+ BUG_ON(p == current);
+ lockdep_assert_held(&rq->lock);
+
+ if (!(p->state & TASK_NORMAL))
+ return;
+
+ if (!p->se.on_rq) {
+ if (likely(!task_running(rq, p))) {
+ schedstat_inc(rq, ttwu_count);
+ schedstat_inc(rq, ttwu_local);
+ }
+ ttwu_activate(p, rq, false, false, true, ENQUEUE_WAKEUP);
+ success = true;
+ }
+ ttwu_post_activation(p, rq, 0, success);
+}
+
+/**
* wake_up_process - Wake up a specific process
* @p: The process to be woken up.
*
@@ -3673,10 +3708,24 @@ need_resched_nonpreemptible:
clear_tsk_need_resched(prev);

if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
- if (unlikely(signal_pending_state(prev->state, prev)))
+ if (unlikely(signal_pending_state(prev->state, prev))) {
prev->state = TASK_RUNNING;
- else
+ } else {
+ /*
+ * If a worker is going to sleep, notify and
+ * ask workqueue whether it wants to wake up a
+ * task to maintain concurrency. If so, wake
+ * up the task.
+ */
+ if (prev->flags & PF_WQ_WORKER) {
+ struct task_struct *to_wakeup;
+
+ to_wakeup = wq_worker_sleeping(prev, cpu);
+ if (to_wakeup)
+ try_to_wake_up_local(to_wakeup);
+ }
deactivate_task(rq, prev, DEQUEUE_SLEEP);
+ }
switch_count = &prev->nvcsw;
}

diff --git a/kernel/workqueue_sched.h b/kernel/workqueue_sched.h
new file mode 100644
index 0000000..af040ba
--- /dev/null
+++ b/kernel/workqueue_sched.h
@@ -0,0 +1,16 @@
+/*
+ * kernel/workqueue_sched.h
+ *
+ * Scheduler hooks for concurrency managed workqueue. Only to be
+ * included from sched.c and workqueue.c.
+ */
+static inline void wq_worker_waking_up(struct task_struct *task,
+ unsigned int cpu)
+{
+}
+
+static inline struct task_struct *wq_worker_sleeping(struct task_struct *task,
+ unsigned int cpu)
+{
+ return NULL;
+}
--
1.6.4.2

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Prev: [PATCH 2/4] sched: implement __set_cpus_allowed()
Next: [PATCH] vfs: Fix O_NOFOLLOW behavior for paths with trailing slashes