sched: add minimalistic idle scheduler for free cpus

Instead of having a full blown scheduler running for the free cpus
add a very minimalistic scheduler for that purpose only ever scheduling
the related idle vcpu. This has the big advantage of not needing any
per-cpu, per-domain or per-scheduling unit data for free cpus and in
turn simplifying moving cpus to and from cpupools a lot.

Right now, CPUs that are not in any pool, still belong to Pool-0's
scheduler. This forces us to make, within the scheduler, extra effort
to avoid actually running vCPUs on those.

In the case of Credit1, this also cause issue to weights
(re)distribution, as the number of CPUs available to the scheduler is
wrong.

This is described in the changelog of commit e719192026 ("xen:
credit2: never consider CPUs outside of our cpupool").

This new scheduler will just use a common lock for all free cpus.

As this new scheduler is not user selectable don't register it as an
official scheduler, but just include it in schedule.c.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Dario Faggioli <dfaggioli@suse.com>

xen/common/sched_credit.c

@@ -617,15 +617,6 @@ csched_init_pdata(const struct scheduler *ops, void *pdata, int cpu)
 {
     unsigned long flags;
     struct csched_private *prv = CSCHED_PRIV(ops);
-    struct schedule_data *sd = &per_cpu(schedule_data, cpu);
-
-    /*
-     * This is called either during during boot, resume or hotplug, in
-     * case Credit1 is the scheduler chosen at boot. In such cases, the
-     * scheduler lock for cpu is already pointing to the default per-cpu
-     * spinlock, as Credit1 needs it, so there is no remapping to be done.
-     */
-    ASSERT(sd->schedule_lock == &sd->_lock && !spin_is_locked(&sd->_lock));
 
     spin_lock_irqsave(&prv->lock, flags);
     init_pdata(prv, pdata, cpu);

xen/common/sched_null.c

@@ -167,17 +167,10 @@ static void init_pdata(struct null_private *prv, unsigned int cpu)
 static void null_init_pdata(const struct scheduler *ops, void *pdata, int cpu)
 {
     struct null_private *prv = null_priv(ops);
-    struct schedule_data *sd = &per_cpu(schedule_data, cpu);
 
     /* alloc_pdata is not implemented, so we want this to be NULL. */
     ASSERT(!pdata);
 
-    /*
-     * The scheduler lock points already to the default per-cpu spinlock,
-     * so there is no remapping to be done.
-     */
-    ASSERT(sd->schedule_lock == &sd->_lock && !spin_is_locked(&sd->_lock));
-
     init_pdata(prv, cpu);
 }
 

xen/common/schedule.c

@@ -54,6 +54,10 @@ boolean_param("sched_smt_power_savings", sched_smt_power_savings);
  * */
 int sched_ratelimit_us = SCHED_DEFAULT_RATELIMIT_US;
 integer_param("sched_ratelimit_us", sched_ratelimit_us);
+
+/* Common lock for free cpus. */
+static DEFINE_SPINLOCK(sched_free_cpu_lock);
+
 /* Various timer handlers. */
 static void s_timer_fn(void *unused);
 static void vcpu_periodic_timer_fn(void *data);
@@ -73,6 +77,58 @@ extern const struct scheduler *__start_schedulers_array[], *__end_schedulers_arr
 
 static struct scheduler __read_mostly ops;
 
+static spinlock_t *
+sched_idle_switch_sched(struct scheduler *new_ops, unsigned int cpu,
+                        void *pdata, void *vdata)
+{
+    idle_vcpu[cpu]->sched_priv = NULL;
+
+    return &sched_free_cpu_lock;
+}
+
+static int
+sched_idle_cpu_pick(const struct scheduler *ops, struct vcpu *v)
+{
+    return v->processor;
+}
+
+static void *
+sched_idle_alloc_vdata(const struct scheduler *ops, struct vcpu *v,
+                       void *dd)
+{
+    /* Any non-NULL pointer is fine here. */
+    return ZERO_BLOCK_PTR;
+}
+
+static void
+sched_idle_free_vdata(const struct scheduler *ops, void *priv)
+{
+}
+
+static struct task_slice sched_idle_schedule(
+    const struct scheduler *ops, s_time_t now,
+    bool tasklet_work_scheduled)
+{
+    const unsigned int cpu = smp_processor_id();
+    struct task_slice ret = { .time = -1 };
+
+    ret.task = idle_vcpu[cpu];
+    return ret;
+}
+
+static struct scheduler sched_idle_ops = {
+    .name           = "Idle Scheduler",
+    .opt_name       = "idle",
+    .sched_data     = NULL,
+
+    .pick_cpu       = sched_idle_cpu_pick,
+    .do_schedule    = sched_idle_schedule,
+
+    .alloc_vdata    = sched_idle_alloc_vdata,
+    .free_vdata     = sched_idle_free_vdata,
+    .switch_sched   = sched_idle_switch_sched,
+};
+
 static inline struct scheduler *dom_scheduler(const struct domain *d)
 {
     if ( likely(d->cpupool != NULL) )
@@ -1587,12 +1643,10 @@ static void poll_timer_fn(void *data)
 static int cpu_schedule_up(unsigned int cpu)
 {
     struct schedule_data *sd = &per_cpu(schedule_data, cpu);
-    void *sched_priv;
 
-    per_cpu(scheduler, cpu) = &ops;
+    per_cpu(scheduler, cpu) = &sched_idle_ops;
     spin_lock_init(&sd->_lock);
-    sd->schedule_lock = &sd->_lock;
+    sd->schedule_lock = &sched_free_cpu_lock;
-    sd->curr = idle_vcpu[cpu];
     init_timer(&sd->s_timer, s_timer_fn, NULL, cpu);
     atomic_set(&sd->urgent_count, 0);
 
@@ -1602,40 +1656,19 @@ static int cpu_schedule_up(unsigned int cpu)
 
     if ( idle_vcpu[cpu] == NULL )
         vcpu_create(idle_vcpu[0]->domain, cpu, cpu);
-    else
-    {
-        struct vcpu *idle = idle_vcpu[cpu];
 
-        /*
-         * During (ACPI?) suspend the idle vCPU for this pCPU is not freed,
-         * while its scheduler specific data (what is pointed by sched_priv)
-         * is. Also, at this stage of the resume path, we attach the pCPU
-         * to the default scheduler, no matter in what cpupool it was before
-         * suspend. To avoid inconsistency, let's allocate default scheduler
-         * data for the idle vCPU here. If the pCPU was in a different pool
-         * with a different scheduler, it is schedule_cpu_switch(), invoked
-         * later, that will set things up as appropriate.
-         */
-        ASSERT(idle->sched_priv == NULL);
-
-        idle->sched_priv = sched_alloc_vdata(&ops, idle,
-                                             idle->domain->sched_priv);
-        if ( idle->sched_priv == NULL )
-            return -ENOMEM;
-    }
     if ( idle_vcpu[cpu] == NULL )
         return -ENOMEM;
 
     /*
-     * We don't want to risk calling xfree() on an sd->sched_priv
+     * No need to allocate any scheduler data, as cpus coming online are
-     * (e.g., inside free_pdata, from cpu_schedule_down() called
+     * free initially and the idle scheduler doesn't need any data areas
-     * during CPU_UP_CANCELLED) that contains an IS_ERR value.
+     * allocated.
      */
-    sched_priv = sched_alloc_pdata(&ops, cpu);
-    if ( IS_ERR(sched_priv) )
-        return PTR_ERR(sched_priv);
 
-    sd->sched_priv = sched_priv;
+    sd->curr = idle_vcpu[cpu];
+
+    sd->sched_priv = NULL;
 
     return 0;
 }
@@ -1643,13 +1676,6 @@ static int cpu_schedule_up(unsigned int cpu)
 static void cpu_schedule_down(unsigned int cpu)
 {
     struct schedule_data *sd = &per_cpu(schedule_data, cpu);
-    struct scheduler *sched = per_cpu(scheduler, cpu);
-
-    sched_free_pdata(sched, sd->sched_priv, cpu);
-    sched_free_vdata(sched, idle_vcpu[cpu]->sched_priv);
-
-    idle_vcpu[cpu]->sched_priv = NULL;
-    sd->sched_priv = NULL;
 
     kill_timer(&sd->s_timer);
 }
@@ -1657,14 +1683,11 @@ static void cpu_schedule_down(unsigned int cpu)
 void sched_rm_cpu(unsigned int cpu)
 {
     int rc;
-    struct schedule_data *sd = &per_cpu(schedule_data, cpu);
-    struct scheduler *sched = per_cpu(scheduler, cpu);
 
     rcu_read_lock(&domlist_read_lock);
     rc = cpu_disable_scheduler(cpu);
     BUG_ON(rc);
     rcu_read_unlock(&domlist_read_lock);
-    sched_deinit_pdata(sched, sd->sched_priv, cpu);
     cpu_schedule_down(cpu);
 }
 
@@ -1672,8 +1695,6 @@ static int cpu_schedule_callback(
     struct notifier_block *nfb, unsigned long action, void *hcpu)
 {
     unsigned int cpu = (unsigned long)hcpu;
-    struct scheduler *sched = per_cpu(scheduler, cpu);
-    struct schedule_data *sd = &per_cpu(schedule_data, cpu);
     int rc = 0;
 
     /*
@@ -1681,39 +1702,25 @@ static int cpu_schedule_callback(
      * allocating and initializing the per-pCPU scheduler specific data,
      * as well as "registering" this pCPU to the scheduler (which may
      * involve modifying some scheduler wide data structures).
-     * This happens by calling the alloc_pdata and init_pdata hooks, in
+     * As new pCPUs always start as "free" cpus with the minimal idle
-     * this order. A scheduler that does not need to allocate any per-pCPU
+     * scheduler being in charge, we don't need any of that.
-     * data can avoid implementing alloc_pdata. init_pdata may, however, be
-     * necessary/useful in this case too (e.g., it can contain the "register
-     * the pCPU to the scheduler" part). alloc_pdata (if present) is called
-     * during CPU_UP_PREPARE. init_pdata (if present) is called during
-     * CPU_STARTING.
      *
      * On the other hand, at teardown, we need to reverse what has been done
-     * during initialization, and then free the per-pCPU specific data. This
+     * during initialization, and then free the per-pCPU specific data. A
-     * happens by calling the deinit_pdata and free_pdata hooks, in this
+     * pCPU brought down is not forced through "free" cpus, so here we need to
+     * use the appropriate hooks.
+     *
+     * This happens by calling the deinit_pdata and free_pdata hooks, in this
      * order. If no per-pCPU memory was allocated, there is no need to
      * provide an implementation of free_pdata. deinit_pdata may, however,
      * be necessary/useful in this case too (e.g., it can undo something done
      * on scheduler wide data structure during init_pdata). Both deinit_pdata
      * and free_pdata are called during CPU_DEAD.
      *
-     * If someting goes wrong during bringup, we go to CPU_UP_CANCELLED
+     * If someting goes wrong during bringup, we go to CPU_UP_CANCELLED.
-     * *before* having called init_pdata. In this case, as there is no
-     * initialization needing undoing, only free_pdata should be called.
-     * This means it is possible to call free_pdata just after alloc_pdata,
-     * without a init_pdata/deinit_pdata "cycle" in between the two.
-     *
-     * So, in summary, the usage pattern should look either
-     *  - alloc_pdata-->init_pdata-->deinit_pdata-->free_pdata, or
-     *  - alloc_pdata-->free_pdata.
      */
     switch ( action )
     {
-    case CPU_STARTING:
-        if ( system_state != SYS_STATE_resume )
-            sched_init_pdata(sched, sd->sched_priv, cpu);
-        break;
     case CPU_UP_PREPARE:
         if ( system_state != SYS_STATE_resume )
             rc = cpu_schedule_up(cpu);
@@ -1824,9 +1831,7 @@ void __init scheduler_init(void)
     idle_domain->max_vcpus = nr_cpu_ids;
     if ( vcpu_create(idle_domain, 0, 0) == NULL )
         BUG();
-    this_cpu(schedule_data).sched_priv = sched_alloc_pdata(&ops, 0);
+    this_cpu(schedule_data).curr = idle_vcpu[0];
-    BUG_ON(IS_ERR(this_cpu(schedule_data).sched_priv));
-    sched_init_pdata(&ops, this_cpu(schedule_data).sched_priv, 0);
 }
 
 /*
@@ -1834,18 +1839,14 @@ void __init scheduler_init(void)
  * cpupool, or subject it to the scheduler of a new cpupool.
  *
  * For the pCPUs that are removed from their cpupool, their scheduler becomes
- * &ops (the default scheduler, selected at boot, which also services the
+ * &sched_idle_ops (the idle scheduler).
- * default cpupool). However, as these pCPUs are not really part of any pool,
- * there won't be any scheduling event on them, not even from the default
- * scheduler. Basically, they will just sit idle until they are explicitly
- * added back to a cpupool.
  */
 int schedule_cpu_switch(unsigned int cpu, struct cpupool *c)
 {
     struct vcpu *idle;
     void *ppriv, *ppriv_old, *vpriv, *vpriv_old;
     struct scheduler *old_ops = per_cpu(scheduler, cpu);
-    struct scheduler *new_ops = (c == NULL) ? &ops : c->sched;
+    struct scheduler *new_ops = (c == NULL) ? &sched_idle_ops : c->sched;
     struct cpupool *old_pool = per_cpu(cpupool, cpu);
     struct schedule_data *sd = &per_cpu(schedule_data, cpu);
     spinlock_t *old_lock, *new_lock;
@@ -1865,9 +1866,6 @@ int schedule_cpu_switch(unsigned int cpu, struct cpupool *c)
     ASSERT((c == NULL && !cpumask_test_cpu(cpu, old_pool->cpu_valid)) ||
            (c != NULL && !cpumask_test_cpu(cpu, c->cpu_valid)));
 
-    if ( old_ops == new_ops )
-        goto out;
-
     /*
      * To setup the cpu for the new scheduler we need:
      *  - a valid instance of per-CPU scheduler specific data, as it is
@@ -1931,7 +1929,6 @@ int schedule_cpu_switch(unsigned int cpu, struct cpupool *c)
     sched_free_vdata(old_ops, vpriv_old);
     sched_free_pdata(old_ops, ppriv_old, cpu);
 
- out:
     per_cpu(cpupool, cpu) = c;
     /* When a cpu is added to a pool, trigger it to go pick up some work */
     if ( c != NULL )