On 7/24/23 9:35?AM, Phil Elwell wrote:
Hi Andres,
With this commit applied to the 6.1 and later kernels (others not tested) the iowait time ("wa" field in top) in an ARM64 build running on a 4 core CPU (a Raspberry Pi 4 B) increases to 25%, as if one core is permanently blocked on I/O. The change can be observed after installing mariadb-server (no configuration or use is required). After reverting just this commit, "wa" drops to zero again.
There are a few other threads on this...
I can believe that this change hasn't negatively affected performance, but the result is misleading. I also think it's pushing the boundaries of what a back-port to stable should do.
It's just a cosmetic thing, to be fair, and it makes quite a large difference on important cases. This is why it also went to stable, which btw was not Andres's decision at all. I've posted this patch in another thread as well, but here it is in this thread too - this will limit the cases that are marked as iowait.
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c index 89a611541bc4..f4591b912ea8 100644 --- a/io_uring/io_uring.c +++ b/io_uring/io_uring.c @@ -2493,11 +2493,20 @@ int io_run_task_work_sig(struct io_ring_ctx *ctx) return 0; }
+static bool current_pending_io(void) +{ + struct io_uring_task *tctx = current->io_uring; + + if (!tctx) + return false; + return percpu_counter_read_positive(&tctx->inflight); +} + /* when returns >0, the caller should retry */ static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx, struct io_wait_queue *iowq) { - int token, ret; + int io_wait, ret;
if (unlikely(READ_ONCE(ctx->check_cq))) return 1; @@ -2511,17 +2520,19 @@ static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx, return 0;
/* - * Use io_schedule_prepare/finish, so cpufreq can take into account - * that the task is waiting for IO - turns out to be important for low - * QD IO. + * Mark us as being in io_wait if we have pending requests, so cpufreq + * can take into account that the task is waiting for IO - turns out + * to be important for low QD IO. */ - token = io_schedule_prepare(); + io_wait = current->in_iowait; + if (current_pending_io()) + current->in_iowait = 1; ret = 0; if (iowq->timeout == KTIME_MAX) schedule(); else if (!schedule_hrtimeout(&iowq->timeout, HRTIMER_MODE_ABS)) ret = -ETIME; - io_schedule_finish(token); + current->in_iowait = io_wait; return ret; }