io-wq: remove spin-for-work optimization (3030fd4c) · Commits · 戴 / test

Andres reports that buffered IO seems to suck up more cycles than we would like, and he narrowed it down to the fact that the io-wq workers will briefly spin for more work on completion of a work item. This was a win on the networking side, but apparently some other cases take a hit because of it. Remove the optimization to avoid burning more CPU than we have to for disk IO. Reported-by:

Andres Freund <andres@anarazel.de> Signed-off-by:

Jens Axboe <axboe@kernel.dk>

fs/io-wq.c

+0 −19

Original line number	Diff line number	Diff line
		@@ -535,42 +535,23 @@ next:
		} while (1);
		}

		static inline void io_worker_spin_for_work(struct io_wqe *wqe)
		{
		int i = 0;

		while (++i < 1000) {
		if (io_wqe_run_queue(wqe))
		break;
		if (need_resched())
		break;
		cpu_relax();
		}
		}

		static int io_wqe_worker(void *data)
		{
		struct io_worker *worker = data;
		struct io_wqe *wqe = worker->wqe;
		struct io_wq *wq = wqe->wq;
		bool did_work;

		io_worker_start(wqe, worker);

		did_work = false;
		while (!test_bit(IO_WQ_BIT_EXIT, &wq->state)) {
		set_current_state(TASK_INTERRUPTIBLE);
		loop:
		if (did_work)
		io_worker_spin_for_work(wqe);
		spin_lock_irq(&wqe->lock);
		if (io_wqe_run_queue(wqe)) {
		__set_current_state(TASK_RUNNING);
		io_worker_handle_work(worker);
		did_work = true;
		goto loop;
		}
		did_work = false;
		/* drops the lock on success, retry */
		if (__io_worker_idle(wqe, worker)) {
		__release(&wqe->lock);

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