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Subject[PATCH 7/9] sched: Add migrate_disable()
Add the base migrate_disable() support (under protest).

While migrate_disable() is (currently) required for PREEMPT_RT, it is
also one of the biggest flaws in the system.

Notably this is just the base implementation, it is broken vs
sched_setaffinity() and hotplug, both solved in additional patches for
ease of review.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/preempt.h | 60 ++++++++++++++++++++++++
include/linux/sched.h | 3 +
kernel/sched/core.c | 116 +++++++++++++++++++++++++++++++++++++++++++++---
kernel/sched/sched.h | 2
lib/smp_processor_id.c | 5 ++
5 files changed, 179 insertions(+), 7 deletions(-)

--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -322,6 +322,64 @@ static inline void preempt_notifier_init

#endif

+#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT)
+
+/*
+ * Migrate-Disable and why it is (strongly) undesired.
+ *
+ * The premise of the Real-Time schedulers we have on Linux
+ * (SCHED_FIFO/SCHED_DEADLINE) is that M CPUs can/will run M tasks
+ * concurrently, provided there are sufficient runnable tasks, also known as
+ * work-conserving. For instance SCHED_DEADLINE tries to schedule the M
+ * earliest deadline threads, and SCHED_FIFO the M highest priority threads.
+ *
+ * The correctness of various scheduling models depends on this, but is it
+ * broken by migrate_disable() that doesn't imply preempt_disable(). Where
+ * preempt_disable() implies an immediate priority ceiling, preemptible
+ * migrate_disable() allows nesting.
+ *
+ * The worst case is that all tasks preempt one another in a migrate_disable()
+ * region and stack on a single CPU. This then reduces the available bandwidth
+ * to a single CPU. And since Real-Time schedulability theory considers the
+ * Worst-Case only, all Real-Time analysis shall revert to single-CPU
+ * (instantly solving the SMP analysis problem).
+ *
+ *
+ * The reason we have it anyway.
+ *
+ * PREEMPT_RT breaks a number of assumptions traditionally held. By forcing a
+ * number of primitives into becoming preemptible, they would also allow
+ * migration. This turns out to break a bunch of per-cpu usage. To this end,
+ * all these primitives employ migirate_disable() to restore this implicit
+ * assumption.
+ *
+ * This is a 'temporary' work-around at best. The correct solution is getting
+ * rid of the above assumptions and reworking the code to employ explicit
+ * per-cpu locking or short preempt-disable regions.
+ *
+ * The end goal must be to get rid of migrate_disable(), alternatively we need
+ * a schedulability theory that does not depend on abritrary migration.
+ *
+ *
+ * Notes on the implementation.
+ *
+ * The implementation is particularly tricky since existing code patterns
+ * dictate neither migrate_disable() nor migrate_enable() is allowed to block.
+ * This means that it cannot use cpus_read_lock() to serialize against hotplug,
+ * nor can it easily migrate itself into a pending affinity mask change on
+ * migrate_enable().
+ *
+ *
+ * Note: even non-work-conserving schedulers like semi-partitioned depends on
+ * migration, so migrate_disable() is not only a problem for
+ * work-conserving schedulers.
+ *
+ */
+extern void migrate_disable(void);
+extern void migrate_enable(void);
+
+#else /* !(CONFIG_SMP && CONFIG_PREEMPT_RT) */
+
/**
* migrate_disable - Prevent migration of the current task
*
@@ -352,4 +410,6 @@ static __always_inline void migrate_enab
preempt_enable();
}

+#endif /* CONFIG_SMP && CONFIG_PREEMPT_RT */
+
#endif /* __LINUX_PREEMPT_H */
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -713,6 +713,9 @@ struct task_struct {
int nr_cpus_allowed;
const cpumask_t *cpus_ptr;
cpumask_t cpus_mask;
+#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT)
+ int migration_disabled;
+#endif

#ifdef CONFIG_PREEMPT_RCU
int rcu_read_lock_nesting;
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1700,6 +1700,68 @@ void check_preempt_curr(struct rq *rq, s

#ifdef CONFIG_SMP

+#ifdef CONFIG_PREEMPT_RT
+
+static void
+__do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask, u32 flags);
+
+static int __set_cpus_allowed_ptr(struct task_struct *p,
+ const struct cpumask *new_mask,
+ u32 flags);
+
+static void migrate_disable_switch(struct rq *rq, struct task_struct *p)
+{
+ if (likely(!p->migration_disabled))
+ return;
+
+ if (p->cpus_ptr != &p->cpus_mask)
+ return;
+
+ /*
+ * Violates locking rules! see comment in __do_set_cpus_allowed().
+ */
+ __do_set_cpus_allowed(p, cpumask_of(rq->cpu), SCA_MIGRATE_DISABLE);
+}
+
+void migrate_disable(void)
+{
+ if (current->migration_disabled++)
+ return;
+
+ barrier();
+}
+EXPORT_SYMBOL_GPL(migrate_disable);
+
+void migrate_enable(void)
+{
+ if (--current->migration_disabled)
+ return;
+
+ barrier();
+
+ if (p->cpus_ptr == &p->cpus_mask)
+ return;
+
+ __set_cpus_allowed_ptr(p, &p->cpus_mask, SCA_MIGRATE_ENABLE);
+}
+EXPORT_SYMBOL_GPL(migrate_enable);
+
+static inline bool is_migration_disabled(struct task_struct *p)
+{
+ return p->migration_disabled;
+}
+
+#else
+
+static inline void migrate_disable_switch(struct rq *rq, struct task_struct *p) { }
+
+static inline bool is_migration_disabled(struct task_struct *p)
+{
+ return false;
+}
+
+#endif
+
/*
* Per-CPU kthreads are allowed to run on !active && online CPUs, see
* __set_cpus_allowed_ptr() and select_fallback_rq().
@@ -1709,7 +1771,7 @@ static inline bool is_cpu_allowed(struct
if (!cpumask_test_cpu(cpu, p->cpus_ptr))
return false;

- if (is_per_cpu_kthread(p))
+ if (is_per_cpu_kthread(p) || is_migration_disabled(p))
return cpu_online(cpu);

return cpu_active(cpu);
@@ -1830,8 +1892,19 @@ static int migration_cpu_stop(void *data
*/
void set_cpus_allowed_common(struct task_struct *p, const struct cpumask *new_mask, u32 flags)
{
- cpumask_copy(&p->cpus_mask, new_mask);
- p->nr_cpus_allowed = cpumask_weight(new_mask);
+ if (flags & SCA_MIGRATE_DISABLE) {
+ p->cpus_ptr = new_mask;
+ p->nr_cpus_allowed = 1;
+ return;
+ }
+
+ if (flags & SCA_MIGRATE_ENABLE)
+ p->cpus_ptr = &p->cpus_mask;
+ else
+ cpumask_copy(&p->cpus_mask, new_mask);
+
+ if (p->cpus_ptr == &p->cpus_mask)
+ p->nr_cpus_allowed = cpumask_weight(p->cpus_ptr);
}

static void
@@ -1840,7 +1913,22 @@ __do_set_cpus_allowed(struct task_struct
struct rq *rq = task_rq(p);
bool queued, running;

- lockdep_assert_held(&p->pi_lock);
+ /*
+ * This here violates the locking rules for affinity, since we're only
+ * supposed to change these variables while holding both rq->lock and
+ * p->pi_lock.
+ *
+ * HOWEVER, it magically works, because ttwu() is the only code that
+ * accesses these variables under p->pi_lock and only does so after
+ * smp_cond_load_acquire(&p->on_cpu, !VAL), and we're in __schedule()
+ * before finish_task().
+ *
+ * XXX do further audits, this smells like something putrid.
+ */
+ if (flags & SCA_MIGRATE_DISABLE)
+ SCHED_WARN_ON(!p->on_cpu);
+ else
+ lockdep_assert_held(&p->pi_lock);

queued = task_on_rq_queued(p);
running = task_current(rq, p);
@@ -1891,9 +1979,14 @@ static int __set_cpus_allowed_ptr(struct
rq = task_rq_lock(p, &rf);
update_rq_clock(rq);

- if (p->flags & PF_KTHREAD) {
+ if (p->flags & PF_KTHREAD || is_migration_disabled(p)) {
/*
- * Kernel threads are allowed on online && !active CPUs
+ * Kernel threads are allowed on online && !active CPUs.
+ *
+ * Specifically, migration_disabled() tasks must not fail the
+ * cpumask_and_and_distribute() pick below, esp. so on
+ * SCA_MIGRATE_ENABLE, otherwise we'll not call
+ * set_cpus_allowed_common() and actually reset p->cpus_ptr.
*/
cpu_valid_mask = cpu_online_mask;
}
@@ -1907,7 +2000,7 @@ static int __set_cpus_allowed_ptr(struct
goto out;
}

- if (cpumask_equal(&p->cpus_mask, new_mask))
+ if (!(flags & SCA_MIGRATE_ENABLE) && cpumask_equal(&p->cpus_mask, new_mask))
goto out;

/*
@@ -1999,6 +2092,8 @@ void set_task_cpu(struct task_struct *p,
* Clearly, migrating tasks to offline CPUs is a fairly daft thing.
*/
WARN_ON_ONCE(!cpu_online(new_cpu));
+
+ WARN_ON_ONCE(is_migration_disabled(p));
#endif

trace_sched_migrate_task(p, new_cpu);
@@ -2329,6 +2424,12 @@ static int select_fallback_rq(int cpu, s
}
fallthrough;
case possible:
+ /*
+ * XXX When called from select_task_rq() we only
+ * hold p->pi_lock and again violate locking order.
+ *
+ * More yuck to audit.
+ */
do_set_cpus_allowed(p, cpu_possible_mask);
state = fail;
break;
@@ -4589,6 +4690,7 @@ static void __sched notrace __schedule(b
*/
++*switch_count;

+ migrate_disable_switch(rq, prev);
psi_sched_switch(prev, next, !task_on_rq_queued(prev));

trace_sched_switch(preempt, prev, next);
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1901,6 +1901,8 @@ extern void update_group_capacity(struct
extern void trigger_load_balance(struct rq *rq);

#define SCA_CHECK 0x01
+#define SCA_MIGRATE_DISABLE 0x02
+#define SCA_MIGRATE_ENABLE 0x04

extern void set_cpus_allowed_common(struct task_struct *p, const struct cpumask *new_mask, u32 flags);

--- a/lib/smp_processor_id.c
+++ b/lib/smp_processor_id.c
@@ -26,6 +26,11 @@ unsigned int check_preemption_disabled(c
if (current->nr_cpus_allowed == 1)
goto out;

+#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT)
+ if (current->migration_disabled)
+ goto out;
+#endif
+
/*
* It is valid to assume CPU-locality during early bootup:
*/

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