[patch V4 05/68] sched: Rework the __schedule() preempt argument

Thomas Gleixner
2021-08-11T12:23:17Z

PREEMPT_RT needs to hand a special state into __schedule() when a task
blocks on a 'sleeping' spin/rwlock. This is required to handle
rcu_note_context_switch() correctly without having special casing in the
RCU code. From an RCU point of view the blocking on the sleeping spinlock
is equivalent to preemption because the task might be in a read side
critical section.

schedule_debug() also has a check which would trigger with the !preempt
case, but that could be handled differently.

To avoid adding another argument and extra checks which cannot be optimized
out by the compiler the following solution has been chosen:

- Replace the boolean 'preempt' argument with an unsigned integer
'sched_mode' argument and define constants to hand in:
(0 == No preemption, 1 = preemption).

- Add two masks to apply on that mode one for the debug/rcu invocations
and one for the actual scheduling decision.

For a non RT kernel these masks are UINT_MAX, i.e. all bits are set
which allows the compiler to optimize the AND operation out because it is
not masking out anything. IOW, it's not different from the boolean.

RT enabled kernels will define these masks separately.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
V2: Simplify the masking logic
---
kernel/sched/core.c | 34 +++++++++++++++++++++++-----------
1 file changed, 23 insertions(+), 11 deletions(-)
---
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5820,6 +5820,18 @@ pick_next_task(struct rq *rq, struct tas
#endif /* CONFIG_SCHED_CORE */

/*
+ * Constants for the sched_mode argument of __schedule().
+ *
+ * The mode argument allows RT enabled kernels to differentiate a
+ * preemption from blocking on an 'sleeping' spin/rwlock. Note, that
+ * SM_MASK_PREEMPT for !RT has all bits set which allows the compiler to
+ * optimize the AND operation out and just check for zero.
+ */
+#define SM_NONE 0x0
+#define SM_PREEMPT 0x1
+#define SM_MASK_PREEMPT (~0U)
+
+/*
* __schedule() is the main scheduler function.
*
* The main means of driving the scheduler and thus entering this function are:
@@ -5858,7 +5870,7 @@ pick_next_task(struct rq *rq, struct tas
*
* WARNING: must be called with preemption disabled!
*/
-static void __sched notrace __schedule(bool preempt)
+static void __sched notrace __schedule(unsigned int sched_mode)
{
struct task_struct *prev, *next;
unsigned long *switch_count;
@@ -5871,13 +5883,13 @@ static void __sched notrace __schedule(b
rq = cpu_rq(cpu);
prev = rq->curr;

- schedule_debug(prev, preempt);
+ schedule_debug(prev, !!sched_mode);

if (sched_feat(HRTICK) || sched_feat(HRTICK_DL))
hrtick_clear(rq);

local_irq_disable();
- rcu_note_context_switch(preempt);
+ rcu_note_context_switch(!!sched_mode);

/*
* Make sure that signal_pending_state()->signal_pending() below
@@ -5911,7 +5923,7 @@ static void __sched notrace __schedule(b
* - ptrace_{,un}freeze_traced() can change ->state underneath us.
*/
prev_state = READ_ONCE(prev->__state);
- if (!preempt && prev_state) {
+ if (!(sched_mode & SM_MASK_PREEMPT) && prev_state) {
if (signal_pending_state(prev_state, prev)) {
WRITE_ONCE(prev->__state, TASK_RUNNING);
} else {
@@ -5977,7 +5989,7 @@ static void __sched notrace __schedule(b
migrate_disable_switch(rq, prev);
psi_sched_switch(prev, next, !task_on_rq_queued(prev));

- trace_sched_switch(preempt, prev, next);
+ trace_sched_switch(sched_mode & SM_MASK_PREEMPT, prev, next);

/* Also unlocks the rq: */
rq = context_switch(rq, prev, next, &rf);
@@ -5998,7 +6010,7 @@ void __noreturn do_task_dead(void)
/* Tell freezer to ignore us: */
current->flags |= PF_NOFREEZE;

- __schedule(false);
+ __schedule(SM_NONE);
BUG();

/* Avoid "noreturn function does return" - but don't continue if BUG() is a NOP: */
@@ -6059,7 +6071,7 @@ asmlinkage __visible void __sched schedu
sched_submit_work(tsk);
do {
preempt_disable();
- __schedule(false);
+ __schedule(SM_NONE);
sched_preempt_enable_no_resched();
} while (need_resched());
sched_update_worker(tsk);
@@ -6087,7 +6099,7 @@ void __sched schedule_idle(void)
*/
WARN_ON_ONCE(current->__state);
do {
- __schedule(false);
+ __schedule(SM_NONE);
} while (need_resched());
}

@@ -6140,7 +6152,7 @@ static void __sched notrace preempt_sche
*/
preempt_disable_notrace();
preempt_latency_start(1);
- __schedule(true);
+ __schedule(SM_PREEMPT);
preempt_latency_stop(1);
preempt_enable_no_resched_notrace();

@@ -6219,7 +6231,7 @@ asmlinkage __visible void __sched notrac
* an infinite recursion.
*/
prev_ctx = exception_enter();
- __schedule(true);
+ __schedule(SM_PREEMPT);
exception_exit(prev_ctx);

preempt_latency_stop(1);
@@ -6368,7 +6380,7 @@ asmlinkage __visible void __sched preemp
do {
preempt_disable();
local_irq_enable();
- __schedule(true);
+ __schedule(SM_PREEMPT);
local_irq_disable();
sched_preempt_enable_no_resched();
} while (need_resched());