diff options
Diffstat (limited to 'kernel/sched')
| -rw-r--r-- | kernel/sched/core.c | 523 | ||||
| -rw-r--r-- | kernel/sched/sched.h | 239 |
2 files changed, 714 insertions, 48 deletions
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 5a97ceb37c13..898fa3bc2765 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2101,6 +2101,8 @@ void activate_task(struct rq *rq, struct task_struct *p, int flags) { if (task_on_rq_migrating(p)) flags |= ENQUEUE_MIGRATED; + if (flags & ENQUEUE_MIGRATED) + sched_mm_cid_migrate_to(rq, p); enqueue_task(rq, p, flags); @@ -3210,6 +3212,7 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu) p->sched_class->migrate_task_rq(p, new_cpu); p->se.nr_migrations++; rseq_migrate(p); + sched_mm_cid_migrate_from(p); perf_event_task_migrate(p); } @@ -4483,6 +4486,7 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p) p->wake_entry.u_flags = CSD_TYPE_TTWU; p->migration_pending = NULL; #endif + init_sched_mm_cid(p); } DEFINE_STATIC_KEY_FALSE(sched_numa_balancing); @@ -5129,7 +5133,6 @@ prepare_task_switch(struct rq *rq, struct task_struct *prev, sched_info_switch(rq, prev, next); perf_event_task_sched_out(prev, next); rseq_preempt(prev); - switch_mm_cid(prev, next); fire_sched_out_preempt_notifiers(prev, next); kmap_local_sched_out(); prepare_task(next); @@ -5285,6 +5288,9 @@ context_switch(struct rq *rq, struct task_struct *prev, * * kernel -> user switch + mmdrop() active * user -> user switch + * + * switch_mm_cid() needs to be updated if the barriers provided + * by context_switch() are modified. */ if (!next->mm) { // to kernel enter_lazy_tlb(prev->active_mm, next); @@ -5314,6 +5320,9 @@ context_switch(struct rq *rq, struct task_struct *prev, } } + /* switch_mm_cid() requires the memory barriers above. */ + switch_mm_cid(rq, prev, next); + rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP); prepare_lock_switch(rq, next, rf); @@ -5602,6 +5611,7 @@ void scheduler_tick(void) resched_latency = cpu_resched_latency(rq); calc_global_load_tick(rq); sched_core_tick(rq); + task_tick_mm_cid(rq, curr); rq_unlock(rq, &rf); @@ -11469,45 +11479,524 @@ void call_trace_sched_update_nr_running(struct rq *rq, int count) } #ifdef CONFIG_SCHED_MM_CID -void sched_mm_cid_exit_signals(struct task_struct *t) + +/** + * @cid_lock: Guarantee forward-progress of cid allocation. + * + * Concurrency ID allocation within a bitmap is mostly lock-free. The cid_lock + * is only used when contention is detected by the lock-free allocation so + * forward progress can be guaranteed. + */ +DEFINE_RAW_SPINLOCK(cid_lock); + +/** + * @use_cid_lock: Select cid allocation behavior: lock-free vs spinlock. + * + * When @use_cid_lock is 0, the cid allocation is lock-free. When contention is + * detected, it is set to 1 to ensure that all newly coming allocations are + * serialized by @cid_lock until the allocation which detected contention + * completes and sets @use_cid_lock back to 0. This guarantees forward progress + * of a cid allocation. + */ +int use_cid_lock; + +/* + * mm_cid remote-clear implements a lock-free algorithm to clear per-mm/cpu cid + * concurrently with respect to the execution of the source runqueue context + * switch. + * + * There is one basic properties we want to guarantee here: + * + * (1) Remote-clear should _never_ mark a per-cpu cid UNSET when it is actively + * used by a task. That would lead to concurrent allocation of the cid and + * userspace corruption. + * + * Provide this guarantee by introducing a Dekker memory ordering to guarantee + * that a pair of loads observe at least one of a pair of stores, which can be + * shown as: + * + * X = Y = 0 + * + * w[X]=1 w[Y]=1 + * MB MB + * r[Y]=y r[X]=x + * + * Which guarantees that x==0 && y==0 is impossible. But rather than using + * values 0 and 1, this algorithm cares about specific state transitions of the + * runqueue current task (as updated by the scheduler context switch), and the + * per-mm/cpu cid value. + * + * Let's introduce task (Y) which has task->mm == mm and task (N) which has + * task->mm != mm for the rest of the discussion. There are two scheduler state + * transitions on context switch we care about: + * + * (TSA) Store to rq->curr with transition from (N) to (Y) + * + * (TSB) Store to rq->curr with transition from (Y) to (N) + * + * On the remote-clear side, there is one transition we care about: + * + * (TMA) cmpxchg to *pcpu_cid to set the LAZY flag + * + * There is also a transition to UNSET state which can be performed from all + * sides (scheduler, remote-clear). It is always performed with a cmpxchg which + * guarantees that only a single thread will succeed: + * + * (TMB) cmpxchg to *pcpu_cid to mark UNSET + * + * Just to be clear, what we do _not_ want to happen is a transition to UNSET + * when a thread is actively using the cid (property (1)). + * + * Let's looks at the relevant combinations of TSA/TSB, and TMA transitions. + * + * Scenario A) (TSA)+(TMA) (from next task perspective) + * + * CPU0 CPU1 + * + * Context switch CS-1 Remote-clear + * - store to rq->curr: (N)->(Y) (TSA) - cmpxchg to *pcpu_id to LAZY (TMA) + * (implied barrier after cmpxchg) + * - switch_mm_cid() + * - memory barrier (see switch_mm_cid() + * comment explaining how this barrier + * is combined with other scheduler + * barriers) + * - mm_cid_get (next) + * - READ_ONCE(*pcpu_cid) - rcu_dereference(src_rq->curr) + * + * This Dekker ensures that either task (Y) is observed by the + * rcu_dereference() or the LAZY flag is observed by READ_ONCE(), or both are + * observed. + * + * If task (Y) store is observed by rcu_dereference(), it means that there is + * still an active task on the cpu. Remote-clear will therefore not transition + * to UNSET, which fulfills property (1). + * + * If task (Y) is not observed, but the lazy flag is observed by READ_ONCE(), + * it will move its state to UNSET, which clears the percpu cid perhaps + * uselessly (which is not an issue for correctness). Because task (Y) is not + * observed, CPU1 can move ahead to set the state to UNSET. Because moving + * state to UNSET is done with a cmpxchg expecting that the old state has the + * LAZY flag set, only one thread will successfully UNSET. + * + * If both states (LAZY flag and task (Y)) are observed, the thread on CPU0 + * will observe the LAZY flag and transition to UNSET (perhaps uselessly), and + * CPU1 will observe task (Y) and do nothing more, which is fine. + * + * What we are effectively preventing with this Dekker is a scenario where + * neither LAZY flag nor store (Y) are observed, which would fail property (1) + * because this would UNSET a cid which is actively used. + */ + +void sched_mm_cid_migrate_from(struct task_struct *t) +{ + t->migrate_from_cpu = task_cpu(t); +} + +static +int __sched_mm_cid_migrate_from_fetch_cid(struct rq *src_rq, + struct task_struct *t, + struct mm_cid *src_pcpu_cid) { struct mm_struct *mm = t->mm; - unsigned long flags; + struct task_struct *src_task; + int src_cid, last_mm_cid; if (!mm) + return -1; + + last_mm_cid = t->last_mm_cid; + /* + * If the migrated task has no last cid, or if the current + * task on src rq uses the cid, it means the source cid does not need + * to be moved to the destination cpu. + */ + if (last_mm_cid == -1) + return -1; + src_cid = READ_ONCE(src_pcpu_cid->cid); + if (!mm_cid_is_valid(src_cid) || last_mm_cid != src_cid) + return -1; + + /* + * If we observe an active task using the mm on this rq, it means we + * are not the last task to be migrated from this cpu for this mm, so + * there is no need to move src_cid to the destination cpu. + */ + rcu_read_lock(); + src_task = rcu_dereference(src_rq->curr); + if (READ_ONCE(src_task->mm_cid_active) && src_task->mm == mm) { + rcu_read_unlock(); + t->last_mm_cid = -1; + return -1; + } + rcu_read_unlock(); + + return src_cid; +} + +static +int __sched_mm_cid_migrate_from_try_steal_cid(struct rq *src_rq, + struct task_struct *t, + struct mm_cid *src_pcpu_cid, + int src_cid) +{ + struct task_struct *src_task; + struct mm_struct *mm = t->mm; + int lazy_cid; + + if (src_cid == -1) + return -1; + + /* + * Attempt to clear the source cpu cid to move it to the destination + * cpu. + */ + lazy_cid = mm_cid_set_lazy_put(src_cid); + if (!try_cmpxchg(&src_pcpu_cid->cid, &src_cid, lazy_cid)) + return -1; + + /* + * The implicit barrier after cmpxchg per-mm/cpu cid before loading + * rq->curr->mm matches the scheduler barrier in context_switch() + * between store to rq->curr and load of prev and next task's + * per-mm/cpu cid. + * + * The implicit barrier after cmpxchg per-mm/cpu cid before loading + * rq->curr->mm_cid_active matches the barrier in + * sched_mm_cid_exit_signals(), sched_mm_cid_before_execve(), and + * sched_mm_cid_after_execve() between store to t->mm_cid_active and + * load of per-mm/cpu cid. + */ + + /* + * If we observe an active task using the mm on this rq after setting + * the lazy-put flag, this task will be responsible for transitioning + * from lazy-put flag set to MM_CID_UNSET. + */ + rcu_read_lock(); + src_task = rcu_dereference(src_rq->curr); + if (READ_ONCE(src_task->mm_cid_active) && src_task->mm == mm) { + rcu_read_unlock(); + /* + * We observed an active task for this mm, there is therefore + * no point in moving this cid to the destination cpu. + */ + t->last_mm_cid = -1; + return -1; + } + rcu_read_unlock(); + + /* + * The src_cid is unused, so it can be unset. + */ + if (!try_cmpxchg(&src_pcpu_cid->cid, &lazy_cid, MM_CID_UNSET)) + return -1; + return src_cid; +} + +/* + * Migration to dst cpu. Called with dst_rq lock held. + * Interrupts are disabled, which keeps the window of cid ownership without the + * source rq lock held small. + */ +void sched_mm_cid_migrate_to(struct rq *dst_rq, struct task_struct *t) +{ + struct mm_cid *src_pcpu_cid, *dst_pcpu_cid; + struct mm_struct *mm = t->mm; + int src_cid, dst_cid, src_cpu; + struct rq *src_rq; + + lockdep_assert_rq_held(dst_rq); + + if (!mm) + return; + src_cpu = t->migrate_from_cpu; + if (src_cpu == -1) { + t->last_mm_cid = -1; + return; + } + /* + * Move the src cid if the dst cid is unset. This keeps id + * allocation closest to 0 in cases where few threads migrate around + * many cpus. + * + * If destination cid is already set, we may have to just clear + * the src cid to ensure compactness in frequent migrations + * scenarios. + * + * It is not useful to clear the src cid when the number of threads is + * greater or equal to the number of allowed cpus, because user-space + * can expect that the number of allowed cids can reach the number of + * allowed cpus. + */ + dst_pcpu_cid = per_cpu_ptr(mm->pcpu_cid, cpu_of(dst_rq)); + dst_cid = READ_ONCE(dst_pcpu_cid->cid); + if (!mm_cid_is_unset(dst_cid) && + atomic_read(&mm->mm_users) >= t->nr_cpus_allowed) + return; + src_pcpu_cid = per_cpu_ptr(mm->pcpu_cid, src_cpu); + src_rq = cpu_rq(src_cpu); + src_cid = __sched_mm_cid_migrate_from_fetch_cid(src_rq, t, src_pcpu_cid); + if (src_cid == -1) + return; + src_cid = __sched_mm_cid_migrate_from_try_steal_cid(src_rq, t, src_pcpu_cid, + src_cid); + if (src_cid == -1) + return; + if (!mm_cid_is_unset(dst_cid)) { + __mm_cid_put(mm, src_cid); + return; + } + /* Move src_cid to dst cpu. */ + mm_cid_snapshot_time(dst_rq, mm); + WRITE_ONCE(dst_pcpu_cid->cid, src_cid); +} + +static void sched_mm_cid_remote_clear(struct mm_struct *mm, struct mm_cid *pcpu_cid, + int cpu) +{ + struct rq *rq = cpu_rq(cpu); + struct task_struct *t; + unsigned long flags; + int cid, lazy_cid; + + cid = READ_ONCE(pcpu_cid->cid); + if (!mm_cid_is_valid(cid)) return; + + /* + * Clear the cpu cid if it is set to keep cid allocation compact. If + * there happens to be other tasks left on the source cpu using this + * mm, the next task using this mm will reallocate its cid on context + * switch. + */ + lazy_cid = mm_cid_set_lazy_put(cid); + if (!try_cmpxchg(&pcpu_cid->cid, &cid, lazy_cid)) + return; + + /* + * The implicit barrier after cmpxchg per-mm/cpu cid before loading + * rq->curr->mm matches the scheduler barrier in context_switch() + * between store to rq->curr and load of prev and next task's + * per-mm/cpu cid. + * + * The implicit barrier after cmpxchg per-mm/cpu cid before loading + * rq->curr->mm_cid_active matches the barrier in + * sched_mm_cid_exit_signals(), sched_mm_cid_before_execve(), and + * sched_mm_cid_after_execve() between store to t->mm_cid_active and + * load of per-mm/cpu cid. + */ + + /* + * If we observe an active task using the mm on this rq after setting + * the lazy-put flag, that task will be responsible for transitioning + * from lazy-put flag set to MM_CID_UNSET. + */ + rcu_read_lock(); + t = rcu_dereference(rq->curr); + if (READ_ONCE(t->mm_cid_active) && t->mm == mm) { + rcu_read_unlock(); + return; + } + rcu_read_unlock(); + + /* + * The cid is unused, so it can be unset. + * Disable interrupts to keep the window of cid ownership without rq + * lock small. + */ local_irq_save(flags); - mm_cid_put(mm, t->mm_cid); - t->mm_cid = -1; - t->mm_cid_active = 0; + if (try_cmpxchg(&pcpu_cid->cid, &lazy_cid, MM_CID_UNSET)) + __mm_cid_put(mm, cid); local_irq_restore(flags); } +static void sched_mm_cid_remote_clear_old(struct mm_struct *mm, int cpu) +{ + struct rq *rq = cpu_rq(cpu); + struct mm_cid *pcpu_cid; + struct task_struct *curr; + u64 rq_clock; + + /* + * rq->clock load is racy on 32-bit but one spurious clear once in a + * while is irrelevant. + */ + rq_clock = READ_ONCE(rq->clock); + pcpu_cid = per_cpu_ptr(mm->pcpu_cid, cpu); + + /* + * In order to take care of infrequently scheduled tasks, bump the time + * snapshot associated with this cid if an active task using the mm is + * observed on this rq. + */ + rcu_read_lock(); + curr = rcu_dereference(rq->curr); + if (READ_ONCE(curr->mm_cid_active) && curr->mm == mm) { + WRITE_ONCE(pcpu_cid->time, rq_clock); + rcu_read_unlock(); + return; + } + rcu_read_unlock(); + + if (rq_clock < pcpu_cid->time + SCHED_MM_CID_PERIOD_NS) + return; + sched_mm_cid_remote_clear(mm, pcpu_cid, cpu); +} + +static void sched_mm_cid_remote_clear_weight(struct mm_struct *mm, int cpu, + int weight) +{ + struct mm_cid *pcpu_cid; + int cid; + + pcpu_cid = per_cpu_ptr(mm->pcpu_cid, cpu); + cid = READ_ONCE(pcpu_cid->cid); + if (!mm_cid_is_valid(cid) || cid < weight) + return; + sched_mm_cid_remote_clear(mm, pcpu_cid, cpu); +} + +static void task_mm_cid_work(struct callback_head *work) +{ + unsigned long now = jiffies, old_scan, next_scan; + struct task_struct *t = current; + struct cpumask *cidmask; + struct mm_struct *mm; + int weight, cpu; + + SCHED_WARN_ON(t != container_of(work, struct task_struct, cid_work)); + + work->next = work; /* Prevent double-add */ + if (t->flags & PF_EXITING) + return; + mm = t->mm; + if (!mm) + return; + old_scan = READ_ONCE(mm->mm_cid_next_scan); + next_scan = now + msecs_to_jiffies(MM_CID_SCAN_DELAY); + if (!old_scan) { + unsigned long res; + + res = cmpxchg(&mm->mm_cid_next_scan, old_scan, next_scan); + if (res != old_scan) + old_scan = res; + else + old_scan = next_scan; + } + if (time_before(now, old_scan)) + return; + if (!try_cmpxchg(&mm->mm_cid_next_scan, &old_scan, next_scan)) + return; + cidmask = mm_cidmask(mm); + /* Clear cids that were not recently used. */ + for_each_possible_cpu(cpu) + sched_mm_cid_remote_clear_old(mm, cpu); + weight = cpumask_weight(cidmask); + /* + * Clear cids that are greater or equal to the cidmask weight to + * recompact it. + */ + for_each_possible_cpu(cpu) + sched_mm_cid_remote_clear_weight(mm, cpu, weight); +} + +void init_sched_mm_cid(struct task_struct *t) +{ + struct mm_struct *mm = t->mm; + int mm_users = 0; + + if (mm) { + mm_users = atomic_read(&mm->mm_users); + if (mm_users == 1) + mm->mm_cid_next_scan = jiffies + msecs_to_jiffies(MM_CID_SCAN_DELAY); + } + t->cid_work.next = &t->cid_work; /* Protect against double add */ + init_task_work(&t->cid_work, task_mm_cid_work); +} + +void task_tick_mm_cid(struct rq *rq, struct task_struct *curr) +{ + struct callback_head *work = &curr->cid_work; + unsigned long now = jiffies; + + if (!curr->mm || (curr->flags & (PF_EXITING | PF_KTHREAD)) || + work->next != work) + return; + if (time_before(now, READ_ONCE(curr->mm->mm_cid_next_scan))) + return; + task_work_add(curr, work, TWA_RESUME); +} + +void sched_mm_cid_exit_signals(struct task_struct *t) +{ + struct mm_struct *mm = t->mm; + struct rq_flags rf; + struct rq *rq; + + if (!mm) + return; + + preempt_disable(); + rq = this_rq(); + rq_lock_irqsave(rq, &rf); + preempt_enable_no_resched(); /* holding spinlock */ + WRITE_ONCE(t->mm_cid_active, 0); + /* + * Store t->mm_cid_active before loading per-mm/cpu cid. + * Matches barrier in sched_mm_cid_remote_clear_old(). + */ + smp_mb(); + mm_cid_put(mm); + t->last_mm_cid = t->mm_cid = -1; + rq_unlock_irqrestore(rq, &rf); +} + void sched_mm_cid_before_execve(struct task_struct *t) { struct mm_struct *mm = t->mm; - unsigned long flags; + struct rq_flags rf; + struct rq *rq; if (!mm) return; - local_irq_save(flags); - mm_cid_put(mm, t->mm_cid); - t->mm_cid = -1; - t->mm_cid_active = 0; - local_irq_restore(flags); + + preempt_disable(); + rq = this_rq(); + rq_lock_irqsave(rq, &rf); + preempt_enable_no_resched(); /* holding spinlock */ + WRITE_ONCE(t->mm_cid_active, 0); + /* + * Store t->mm_cid_active before loading per-mm/cpu cid. + * Matches barrier in sched_mm_cid_remote_clear_old(). + */ + smp_mb(); + mm_cid_put(mm); + t->last_mm_cid = t->mm_cid = -1; + rq_unlock_irqrestore(rq, &rf); } void sched_mm_cid_after_execve(struct task_struct *t) { struct mm_struct *mm = t->mm; - unsigned long flags; + struct rq_flags rf; + struct rq *rq; if (!mm) return; - local_irq_save(flags); - t->mm_cid = mm_cid_get(mm); - t->mm_cid_active = 1; - local_irq_restore(flags); + + preempt_disable(); + rq = this_rq(); + rq_lock_irqsave(rq, &rf); + preempt_enable_no_resched(); /* holding spinlock */ + WRITE_ONCE(t->mm_cid_active, 1); + /* + * Store t->mm_cid_active before loading per-mm/cpu cid. + * Matches barrier in sched_mm_cid_remote_clear_old(). + */ + smp_mb(); + t->last_mm_cid = t->mm_cid = mm_cid_get(rq, mm); + rq_unlock_irqrestore(rq, &rf); rseq_set_notify_resume(t); } diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 060616944d7a..ec7b3e0a2b20 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -3253,61 +3253,238 @@ static inline void update_current_exec_runtime(struct task_struct *curr, } #ifdef CONFIG_SCHED_MM_CID -static inline int __mm_cid_get(struct mm_struct *mm) + +#define SCHED_MM_CID_PERIOD_NS (100ULL * 1000000) /* 100ms */ +#define MM_CID_SCAN_DELAY 100 /* 100ms */ + +extern raw_spinlock_t cid_lock; +extern int use_cid_lock; + +extern void sched_mm_cid_migrate_from(struct task_struct *t); +extern void sched_mm_cid_migrate_to(struct rq *dst_rq, struct task_struct *t); +extern void task_tick_mm_cid(struct rq *rq, struct task_struct *curr); +extern void init_sched_mm_cid(struct task_struct *t); + +static inline void __mm_cid_put(struct mm_struct *mm, int cid) +{ + if (cid < 0) + return; + cpumask_clear_cpu(cid, mm_cidmask(mm)); +} + +/* + * The per-mm/cpu cid can have the MM_CID_LAZY_PUT flag set or transition to + * the MM_CID_UNSET state without holding the rq lock, but the rq lock needs to + * be held to transition to other states. + * + * State transitions synchronized with cmpxchg or try_cmpxchg need to be + * consistent across cpus, which prevents use of this_cpu_cmpxchg. + */ +static inline void mm_cid_put_lazy(struct task_struct *t) +{ + struct mm_struct *mm = t->mm; + struct mm_cid __percpu *pcpu_cid = mm->pcpu_cid; + int cid; + + lockdep_assert_irqs_disabled(); + cid = __this_cpu_read(pcpu_cid->cid); + if (!mm_cid_is_lazy_put(cid) || + !try_cmpxchg(&this_cpu_ptr(pcpu_cid)->cid, &cid, MM_CID_UNSET)) + return; + __mm_cid_put(mm, mm_cid_clear_lazy_put(cid)); +} + +static inline int mm_cid_pcpu_unset(struct mm_struct *mm) +{ + struct mm_cid __percpu *pcpu_cid = mm->pcpu_cid; + int cid, res; + + lockdep_assert_irqs_disabled(); + cid = __this_cpu_read(pcpu_cid->cid); + for (;;) { + if (mm_cid_is_unset(cid)) + return MM_CID_UNSET; + /* + * Attempt transition from valid or lazy-put to unset. + */ + res = cmpxchg(&this_cpu_ptr(pcpu_cid)->cid, cid, MM_CID_UNSET); + if (res == cid) + break; + cid = res; + } + return cid; +} + +static inline void mm_cid_put(struct mm_struct *mm) +{ + int cid; + + lockdep_assert_irqs_disabled(); + cid = mm_cid_pcpu_unset(mm); + if (cid == MM_CID_UNSET) + return; + __mm_cid_put(mm, mm_cid_clear_lazy_put(cid)); +} + +static inline int __mm_cid_try_get(struct mm_struct *mm) { struct cpumask *cpumask; int cid; cpumask = mm_cidmask(mm); - cid = cpumask_first_zero(cpumask); - if (cid >= nr_cpu_ids) + /* + * Retry finding first zero bit if the mask is temporarily + * filled. This only happens during concurrent remote-clear + * which owns a cid without holding a rq lock. + */ + for (;;) { + cid = cpumask_first_zero(cpumask); + if (cid < nr_cpu_ids) + break; + cpu_relax(); + } + if (cpumask_test_and_set_cpu(cid, cpumask)) return -1; - __cpumask_set_cpu(cid, cpumask); return cid; } -static inline void mm_cid_put(struct mm_struct *mm, int cid) +/* + * Save a snapshot of the current runqueue time of this cpu + * with the per-cpu cid value, allowing to estimate how recently it was used. + */ +static inline void mm_cid_snapshot_time(struct rq *rq, struct mm_struct *mm) { - lockdep_assert_irqs_disabled(); - if (cid < 0) - return; - raw_spin_lock(&mm->cid_lock); - __cpumask_clear_cpu(cid, mm_cidmask(mm)); - raw_spin_unlock(&mm->cid_lock); + struct mm_cid *pcpu_cid = per_cpu_ptr(mm->pcpu_cid, cpu_of(rq)); + + lockdep_assert_rq_held(rq); + WRITE_ONCE(pcpu_cid->time, rq->clock); } -static inline int mm_cid_get(struct mm_struct *mm) +static inline int __mm_cid_get(struct rq *rq, struct mm_struct *mm) { - int ret; + int cid; - lockdep_assert_irqs_disabled(); - raw_spin_lock(&mm->cid_lock); - ret = __mm_cid_get(mm); - raw_spin_unlock(&mm->cid_lock); - return ret; + /* + * All allocations (even those using the cid_lock) are lock-free. If + * use_cid_lock is set, hold the cid_lock to perform cid allocation to + * guarantee forward progress. + */ + if (!READ_ONCE(use_cid_lock)) { + cid = __mm_cid_try_get(mm); + if (cid >= 0) + goto end; + raw_spin_lock(&cid_lock); + } else { + raw_spin_lock(&cid_lock); + cid = __mm_cid_try_get(mm); + if (cid >= 0) + goto unlock; + } + + /* + * cid concurrently allocated. Retry while forcing following + * allocations to use the cid_lock to ensure forward progress. + */ + WRITE_ONCE(use_cid_lock, 1); + /* + * Set use_cid_lock before allocation. Only care about program order + * because this is only required for forward progress. + */ + barrier(); + /* + * Retry until it succeeds. It is guaranteed to eventually succeed once + * all newcoming allocations observe the use_cid_lock flag set. + */ + do { + cid = __mm_cid_try_get(mm); + cpu_relax(); + } while (cid < 0); + /* + * Allocate before clearing use_cid_lock. Only care about + * program order because this is for forward progress. + */ + barrier(); + WRITE_ONCE(use_cid_lock, 0); +unlock: + raw_spin_unlock(&cid_lock); +end: + mm_cid_snapshot_time(rq, mm); + return cid; } -static inline void switch_mm_cid(struct task_struct *prev, struct task_struct *next) +static inline int mm_cid_get(struct rq *rq, struct mm_struct *mm) { + struct mm_cid __percpu *pcpu_cid = mm->pcpu_cid; + struct cpumask *cpumask; + int cid; + + lockdep_assert_rq_held(rq); + cpumask = mm_cidmask(mm); + cid = __this_cpu_read(pcpu_cid->cid); + if (mm_cid_is_valid(cid)) { + mm_cid_snapshot_time(rq, mm); + return cid; + } + if (mm_cid_is_lazy_put(cid)) { + if (try_cmpxchg(&this_cpu_ptr(pcpu_cid)->cid, &cid, MM_CID_UNSET)) + __mm_cid_put(mm, mm_cid_clear_lazy_put(cid)); + } + cid = __mm_cid_get(rq, mm); + __this_cpu_write(pcpu_cid->cid, cid); + return cid; +} + +static inline void switch_mm_cid(struct rq *rq, + struct task_struct *prev, + struct task_struct *next) +{ + /* + * Provide a memory barrier between rq->curr store and load of + * {prev,next}->mm->pcpu_cid[cpu] on rq->curr->mm transition. + * + * Should be adapted if context_switch() is modified. + */ + if (!next->mm) { // to kernel + /* + * user -> kernel transition does not guarantee a barrier, but + * we can use the fact that it performs an atomic operation in + * mmgrab(). + */ + if (prev->mm) // from user + smp_mb__after_mmgrab(); + /* + * kernel -> kernel transition does not change rq->curr->mm + * state. It stays NULL. + */ + } else { // to user + /* + * kernel -> user transition does not provide a barrier + * between rq->curr store and load of {prev,next}->mm->pcpu_cid[cpu]. + * Provide it here. + */ + if (!prev->mm) // from kernel + smp_mb(); + /* + * user -> user transition guarantees a memory barrier through + * switch_mm() when current->mm changes. If current->mm is + * unchanged, no barrier is needed. + */ + } if (prev->mm_cid_active) { - if (next->mm_cid_active && next->mm == prev->mm) { - /* - * Context switch between threads in same mm, hand over - * the mm_cid from prev to next. - */ - next->mm_cid = prev->mm_cid; - prev->mm_cid = -1; - return; - } - mm_cid_put(prev->mm, prev->mm_cid); + mm_cid_snapshot_time(rq, prev->mm); + mm_cid_put_lazy(prev); prev->mm_cid = -1; } if (next->mm_cid_active) - next->mm_cid = mm_cid_get(next->mm); + next->last_mm_cid = next->mm_cid = mm_cid_get(rq, next->mm); } #else -static inline void switch_mm_cid(struct task_struct *prev, struct task_struct *next) { } +static inline void switch_mm_cid(struct rq *rq, struct task_struct *prev, struct task_struct *next) { } +static inline void sched_mm_cid_migrate_from(struct task_struct *t) { } +static inline void sched_mm_cid_migrate_to(struct rq *dst_rq, struct task_struct *t) { } +static inline void task_tick_mm_cid(struct rq *rq, struct task_struct *curr) { } +static inline void init_sched_mm_cid(struct task_struct *t) { } #endif #endif /* _KERNEL_SCHED_SCHED_H */ |