[RFC][PATCH 1/7] sched,rcu: Rework try_invoke_on_locked_down_task()

Peter Zijlstra
2021-09-22T11:16:30Z

Give try_invoke_on_locked_down_task() a saner name and have it return
an int so that the caller might distinguish between different reasons
of failure.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/wait.h | 3 ++-
kernel/rcu/tasks.h | 12 ++++++------
kernel/rcu/tree_stall.h | 8 ++++----
kernel/sched/core.c | 11 +++++------
4 files changed, 17 insertions(+), 17 deletions(-)

--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -1160,6 +1160,7 @@ int autoremove_wake_function(struct wait
(wait)->flags = 0; \
} while (0)

-bool try_invoke_on_locked_down_task(struct task_struct *p, bool (*func)(struct task_struct *t, void *arg), void *arg);
+typedef int (*task_try_f)(struct task_struct *p, void *arg);
+extern int task_try_func(struct task_struct *p, task_try_f func, void *arg);

#endif /* _LINUX_WAIT_H */
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -928,7 +928,7 @@ static void trc_read_check_handler(void
}

/* Callback function for scheduler to check locked-down task. */
-static bool trc_inspect_reader(struct task_struct *t, void *arg)
+static int trc_inspect_reader(struct task_struct *t, void *arg)
{
int cpu = task_cpu(t);
bool in_qs = false;
@@ -939,7 +939,7 @@ static bool trc_inspect_reader(struct ta

// If no chance of heavyweight readers, do it the hard way.
if (!ofl && !IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
- return false;
+ return -EINVAL;

// If heavyweight readers are enabled on the remote task,
// we can inspect its state despite its currently running.
@@ -947,7 +947,7 @@ static bool trc_inspect_reader(struct ta
n_heavy_reader_attempts++;
if (!ofl && // Check for "running" idle tasks on offline CPUs.
!rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting))
- return false; // No quiescent state, do it the hard way.
+ return -EINVAL; // No quiescent state, do it the hard way.
n_heavy_reader_updates++;
if (ofl)
n_heavy_reader_ofl_updates++;
@@ -962,7 +962,7 @@ static bool trc_inspect_reader(struct ta
t->trc_reader_checked = true;

if (in_qs)
- return true; // Already in quiescent state, done!!!
+ return 0; // Already in quiescent state, done!!!

// The task is in a read-side critical section, so set up its
// state so that it will awaken the grace-period kthread upon exit
@@ -970,7 +970,7 @@ static bool trc_inspect_reader(struct ta
atomic_inc(&trc_n_readers_need_end); // One more to wait on.
WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs));
WRITE_ONCE(t->trc_reader_special.b.need_qs, true);
- return true;
+ return 0;
}

/* Attempt to extract the state for the specified task. */
@@ -992,7 +992,7 @@ static void trc_wait_for_one_reader(stru

// Attempt to nail down the task for inspection.
get_task_struct(t);
- if (try_invoke_on_locked_down_task(t, trc_inspect_reader, NULL)) {
+ if (!task_try_func(t, trc_inspect_reader, NULL)) {
put_task_struct(t);
return;
}
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -240,16 +240,16 @@ struct rcu_stall_chk_rdr {
* Report out the state of a not-running task that is stalling the
* current RCU grace period.
*/
-static bool check_slow_task(struct task_struct *t, void *arg)
+static int check_slow_task(struct task_struct *t, void *arg)
{
struct rcu_stall_chk_rdr *rscrp = arg;

if (task_curr(t))
- return false; // It is running, so decline to inspect it.
+ return -EBUSY; // It is running, so decline to inspect it.
rscrp->nesting = t->rcu_read_lock_nesting;
rscrp->rs = t->rcu_read_unlock_special;
rscrp->on_blkd_list = !list_empty(&t->rcu_node_entry);
- return true;
+ return 0;
}

/*
@@ -283,7 +283,7 @@ static int rcu_print_task_stall(struct r
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
while (i) {
t = ts[--i];
- if (!try_invoke_on_locked_down_task(t, check_slow_task, &rscr))
+ if (task_try_func(t, check_slow_task, &rscr))
pr_cont(" P%d", t->pid);
else
pr_cont(" P%d/%d:%c%c%c%c",
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4106,7 +4106,7 @@ try_to_wake_up(struct task_struct *p, un
}

/**
- * try_invoke_on_locked_down_task - Invoke a function on task in fixed state
+ * task_try_func - Invoke a function on task in fixed state
* @p: Process for which the function is to be invoked, can be @current.
* @func: Function to invoke.
* @arg: Argument to function.
@@ -4119,14 +4119,13 @@ try_to_wake_up(struct task_struct *p, un
* lightweight.
*
* Returns:
- * @false if the task slipped out from under the locks.
- * @true if the task was locked onto a runqueue or is sleeping.
- * However, @func can override this by returning @false.
+ * -EAGAIN: we raced against task movement/state
+ * *: as returned by @func
*/
-bool try_invoke_on_locked_down_task(struct task_struct *p, bool (*func)(struct task_struct *t, void *arg), void *arg)
+int task_try_func(struct task_struct *p, task_try_f func, void *arg)
{
struct rq_flags rf;
- bool ret = false;
+ int ret = -EAGAIN; /* raced, try again later */
struct rq *rq;

raw_spin_lock_irqsave(&p->pi_lock, rf.flags);