From: Sebastian Andrzej Siewior bigeasy@linutronix.de
commit 95da84659226d75698a1ab958be0af21d9cc2a9c upstream.
During a context switch the scheduler invokes wq_worker_sleeping() with disabled preemption. Disabling preemption is needed because it protects access to `worker->sleeping'. As an optimisation it avoids invoking schedule() within the schedule path as part of possible wake up (thus preempt_enable_no_resched() afterwards).
The io-wq has been added to the mix in the same section with disabled preemption. This breaks on PREEMPT_RT because io_wq_worker_sleeping() acquires a spinlock_t. Also within the schedule() the spinlock_t must be acquired after tsk_is_pi_blocked() otherwise it will block on the sleeping lock again while scheduling out.
While playing with `io_uring-bench' I didn't notice a significant latency spike after converting io_wqe::lock to a raw_spinlock_t. The latency was more or less the same.
In order to keep the spinlock_t it would have to be moved after the tsk_is_pi_blocked() check which would introduce a branch instruction into the hot path.
The lock is used to maintain the `work_list' and wakes one task up at most. Should io_wqe_cancel_pending_work() cause latency spikes, while searching for a specific item, then it would need to drop the lock during iterations. revert_creds() is also invoked under the lock. According to debug cred::non_rcu is 0. Otherwise it should be moved outside of the locked section because put_cred_rcu()->free_uid() acquires a sleeping lock.
Convert io_wqe::lock to a raw_spinlock_t.c
Signed-off-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/io-wq.c | 52 ++++++++++++++++++++++++++-------------------------- 1 file changed, 26 insertions(+), 26 deletions(-)
--- a/fs/io-wq.c +++ b/fs/io-wq.c @@ -88,7 +88,7 @@ enum { */ struct io_wqe { struct { - spinlock_t lock; + raw_spinlock_t lock; struct io_wq_work_list work_list; unsigned long hash_map; unsigned flags; @@ -149,7 +149,7 @@ static bool __io_worker_unuse(struct io_
if (current->files != worker->restore_files) { __acquire(&wqe->lock); - spin_unlock_irq(&wqe->lock); + raw_spin_unlock_irq(&wqe->lock); dropped_lock = true;
task_lock(current); @@ -168,7 +168,7 @@ static bool __io_worker_unuse(struct io_ if (worker->mm) { if (!dropped_lock) { __acquire(&wqe->lock); - spin_unlock_irq(&wqe->lock); + raw_spin_unlock_irq(&wqe->lock); dropped_lock = true; } __set_current_state(TASK_RUNNING); @@ -222,17 +222,17 @@ static void io_worker_exit(struct io_wor worker->flags = 0; preempt_enable();
- spin_lock_irq(&wqe->lock); + raw_spin_lock_irq(&wqe->lock); hlist_nulls_del_rcu(&worker->nulls_node); list_del_rcu(&worker->all_list); if (__io_worker_unuse(wqe, worker)) { __release(&wqe->lock); - spin_lock_irq(&wqe->lock); + raw_spin_lock_irq(&wqe->lock); } acct->nr_workers--; nr_workers = wqe->acct[IO_WQ_ACCT_BOUND].nr_workers + wqe->acct[IO_WQ_ACCT_UNBOUND].nr_workers; - spin_unlock_irq(&wqe->lock); + raw_spin_unlock_irq(&wqe->lock);
/* all workers gone, wq exit can proceed */ if (!nr_workers && refcount_dec_and_test(&wqe->wq->refs)) @@ -508,7 +508,7 @@ get_next: else if (!wq_list_empty(&wqe->work_list)) wqe->flags |= IO_WQE_FLAG_STALLED;
- spin_unlock_irq(&wqe->lock); + raw_spin_unlock_irq(&wqe->lock); if (!work) break; io_assign_current_work(worker, work); @@ -542,17 +542,17 @@ get_next: io_wqe_enqueue(wqe, linked);
if (hash != -1U && !next_hashed) { - spin_lock_irq(&wqe->lock); + raw_spin_lock_irq(&wqe->lock); wqe->hash_map &= ~BIT_ULL(hash); wqe->flags &= ~IO_WQE_FLAG_STALLED; /* skip unnecessary unlock-lock wqe->lock */ if (!work) goto get_next; - spin_unlock_irq(&wqe->lock); + raw_spin_unlock_irq(&wqe->lock); } } while (work);
- spin_lock_irq(&wqe->lock); + raw_spin_lock_irq(&wqe->lock); } while (1); }
@@ -567,7 +567,7 @@ static int io_wqe_worker(void *data) while (!test_bit(IO_WQ_BIT_EXIT, &wq->state)) { set_current_state(TASK_INTERRUPTIBLE); loop: - spin_lock_irq(&wqe->lock); + raw_spin_lock_irq(&wqe->lock); if (io_wqe_run_queue(wqe)) { __set_current_state(TASK_RUNNING); io_worker_handle_work(worker); @@ -578,7 +578,7 @@ loop: __release(&wqe->lock); goto loop; } - spin_unlock_irq(&wqe->lock); + raw_spin_unlock_irq(&wqe->lock); if (signal_pending(current)) flush_signals(current); if (schedule_timeout(WORKER_IDLE_TIMEOUT)) @@ -590,11 +590,11 @@ loop: }
if (test_bit(IO_WQ_BIT_EXIT, &wq->state)) { - spin_lock_irq(&wqe->lock); + raw_spin_lock_irq(&wqe->lock); if (!wq_list_empty(&wqe->work_list)) io_worker_handle_work(worker); else - spin_unlock_irq(&wqe->lock); + raw_spin_unlock_irq(&wqe->lock); }
io_worker_exit(worker); @@ -634,9 +634,9 @@ void io_wq_worker_sleeping(struct task_s
worker->flags &= ~IO_WORKER_F_RUNNING;
- spin_lock_irq(&wqe->lock); + raw_spin_lock_irq(&wqe->lock); io_wqe_dec_running(wqe, worker); - spin_unlock_irq(&wqe->lock); + raw_spin_unlock_irq(&wqe->lock); }
static bool create_io_worker(struct io_wq *wq, struct io_wqe *wqe, int index) @@ -660,7 +660,7 @@ static bool create_io_worker(struct io_w return false; }
- spin_lock_irq(&wqe->lock); + raw_spin_lock_irq(&wqe->lock); hlist_nulls_add_head_rcu(&worker->nulls_node, &wqe->free_list); list_add_tail_rcu(&worker->all_list, &wqe->all_list); worker->flags |= IO_WORKER_F_FREE; @@ -669,7 +669,7 @@ static bool create_io_worker(struct io_w if (!acct->nr_workers && (worker->flags & IO_WORKER_F_BOUND)) worker->flags |= IO_WORKER_F_FIXED; acct->nr_workers++; - spin_unlock_irq(&wqe->lock); + raw_spin_unlock_irq(&wqe->lock);
if (index == IO_WQ_ACCT_UNBOUND) atomic_inc(&wq->user->processes); @@ -724,12 +724,12 @@ static int io_wq_manager(void *data) if (!node_online(node)) continue;
- spin_lock_irq(&wqe->lock); + raw_spin_lock_irq(&wqe->lock); if (io_wqe_need_worker(wqe, IO_WQ_ACCT_BOUND)) fork_worker[IO_WQ_ACCT_BOUND] = true; if (io_wqe_need_worker(wqe, IO_WQ_ACCT_UNBOUND)) fork_worker[IO_WQ_ACCT_UNBOUND] = true; - spin_unlock_irq(&wqe->lock); + raw_spin_unlock_irq(&wqe->lock); if (fork_worker[IO_WQ_ACCT_BOUND]) create_io_worker(wq, wqe, IO_WQ_ACCT_BOUND); if (fork_worker[IO_WQ_ACCT_UNBOUND]) @@ -825,10 +825,10 @@ static void io_wqe_enqueue(struct io_wqe }
work_flags = work->flags; - spin_lock_irqsave(&wqe->lock, flags); + raw_spin_lock_irqsave(&wqe->lock, flags); io_wqe_insert_work(wqe, work); wqe->flags &= ~IO_WQE_FLAG_STALLED; - spin_unlock_irqrestore(&wqe->lock, flags); + raw_spin_unlock_irqrestore(&wqe->lock, flags);
if ((work_flags & IO_WQ_WORK_CONCURRENT) || !atomic_read(&acct->nr_running)) @@ -955,13 +955,13 @@ static void io_wqe_cancel_pending_work(s unsigned long flags;
retry: - spin_lock_irqsave(&wqe->lock, flags); + raw_spin_lock_irqsave(&wqe->lock, flags); wq_list_for_each(node, prev, &wqe->work_list) { work = container_of(node, struct io_wq_work, list); if (!match->fn(work, match->data)) continue; io_wqe_remove_pending(wqe, work, prev); - spin_unlock_irqrestore(&wqe->lock, flags); + raw_spin_unlock_irqrestore(&wqe->lock, flags); io_run_cancel(work, wqe); match->nr_pending++; if (!match->cancel_all) @@ -970,7 +970,7 @@ retry: /* not safe to continue after unlock */ goto retry; } - spin_unlock_irqrestore(&wqe->lock, flags); + raw_spin_unlock_irqrestore(&wqe->lock, flags); }
static void io_wqe_cancel_running_work(struct io_wqe *wqe, @@ -1078,7 +1078,7 @@ struct io_wq *io_wq_create(unsigned boun } atomic_set(&wqe->acct[IO_WQ_ACCT_UNBOUND].nr_running, 0); wqe->wq = wq; - spin_lock_init(&wqe->lock); + raw_spin_lock_init(&wqe->lock); INIT_WQ_LIST(&wqe->work_list); INIT_HLIST_NULLS_HEAD(&wqe->free_list, 0); INIT_LIST_HEAD(&wqe->all_list);