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Message-ID: <87y2a9b4xw.ffs@nanos.tec.linutronix.de>
Date: Wed, 14 Jul 2021 11:49:47 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Peter Zijlstra <peterz@...radead.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...nel.org>,
Juri Lelli <juri.lelli@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Will Deacon <will@...nel.org>,
Waiman Long <longman@...hat.com>,
Boqun Feng <boqun.feng@...il.com>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [patch 05/50] sched: Provide schedule point for RT locks
On Wed, Jul 14 2021 at 10:28, Peter Zijlstra wrote:
> On Tue, Jul 13, 2021 at 05:10:59PM +0200, Thomas Gleixner wrote:
>
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -5832,8 +5832,14 @@ pick_next_task(struct rq *rq, struct tas
>> */
>> #define SM_NONE 0x0
>> #define SM_PREEMPT 0x1
>> -#define SM_MASK_PREEMPT UINT_MAX
>> -#define SM_MASK_STATE SM_MASK_PREEMPT
>> +#ifndef CONFIG_PREEMPT_RT
>> +# define SM_MASK_PREEMPT UINT_MAX
>> +# define SM_MASK_STATE SM_MASK_PREEMPT
>> +#else
>> +# define SM_RTLOCK_WAIT 0x2
>> +# define SM_MASK_PREEMPT SM_PREEMPT
>> +# define SM_MASK_STATE (SM_PREEMPT | SM_RTLOCK_WAIT)
>> +#endif
>
> Wouldn't something like this:
>
>
> #ifndef CONFIG_PREEMPT_RT
> # define SM_MASK_PREEMPT (~0U)
> #else
> # define SM_RTLOCK_WAIT 0x2
> # define SM_MASK_PREEMPT SM_PREEMPT
> #endif
>
> #define SM_MASK_STATE (~0U)
>
> Be even better?
SM_MASK_STATE is overengineered. See combo patch 4+5 below
Thanks,
tglx
---
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5822,6 +5822,24 @@ 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 is UINT_MAX which allows the compiler to
+ * optimize the AND operation out and just check for zero.
+ */
+#define SM_NONE 0x0
+#define SM_PREEMPT 0x1
+
+#ifndef CONFIG_PREEMPT_RT
+# define SM_MASK_PREEMPT UINT_MAX
+#else
+# define SM_RTLOCK_WAIT 0x2
+# define SM_MASK_PREEMPT SM_PREEMPT
+#endif
+
+/*
* __schedule() is the main scheduler function.
*
* The main means of driving the scheduler and thus entering this function are:
@@ -5860,7 +5878,7 @@ 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;
@@ -5873,13 +5891,13 @@ 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
@@ -5913,7 +5931,7 @@ 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 {
@@ -5979,7 +5997,7 @@ 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);
@@ -6000,7 +6018,7 @@ 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: */
@@ -6061,7 +6079,7 @@ 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);
@@ -6089,7 +6107,7 @@ void __sched schedule_idle(void)
*/
WARN_ON_ONCE(current->__state);
do {
- __schedule(false);
+ __schedule(SM_NONE);
} while (need_resched());
}
@@ -6124,6 +6142,18 @@ void __sched schedule_preempt_disabled(v
preempt_disable();
}
+#ifdef CONFIG_PREEMPT_RT
+void __sched notrace schedule_rtlock(void)
+{
+ do {
+ preempt_disable();
+ __schedule(SM_RTLOCK_WAIT);
+ sched_preempt_enable_no_resched();
+ } while (need_resched());
+}
+NOKPROBE_SYMBOL(schedule_rtlock);
+#endif
+
static void __sched notrace preempt_schedule_common(void)
{
do {
@@ -6142,7 +6172,7 @@ 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();
@@ -6221,7 +6251,7 @@ 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);
@@ -6370,7 +6400,7 @@ 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());
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -294,6 +294,9 @@ extern long schedule_timeout_idle(long t
asmlinkage void schedule(void);
extern void schedule_preempt_disabled(void);
asmlinkage void preempt_schedule_irq(void);
+#ifdef CONFIG_PREEMPT_RT
+ extern void schedule_rtlock(void);
+#endif
extern int __must_check io_schedule_prepare(void);
extern void io_schedule_finish(int token);
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