From: Jens Axboe <[email protected]> When a vCPU thread handles MMIO (holding BQL), aio_co_enter() runs the block I/O coroutine inline on the vCPU thread because qemu_get_current_aio_context() returns the main AioContext when BQL is held. The coroutine calls luring_co_submit() which queues an SQE via fdmon_io_uring_add_sqe(), but the actual io_uring_submit() only happens in gsource_prepare() on the main loop thread.
Since the coroutine ran inline (not via aio_co_schedule()), no BH is scheduled and aio_notify() is never called. The main loop remains asleep in ppoll() with up to a 499ms timeout, leaving the SQE unsubmitted until the next timer fires. Fix this by calling aio_notify() after queuing the SQE. This wakes the main loop via the eventfd so it can run gsource_prepare() and submit the pending SQE promptly. This is a generic fix that benefits all devices using aio=io_uring. Without it, AHCI/SATA devices see MUCH worse I/O latency since they use MMIO (not ioeventfd like virtio) and have no other mechanism to wake the main loop after queuing block I/O. This is usually a bit hard to detect, as it also relies on the ppoll loop not waking up for other activity, and micro benchmarks tend not to see it because they don't have any real processing time. With a synthetic test case that has a few usleep() to simulate processing of read data, it's very noticeable. The below example reads 128MB with O_DIRECT in 128KB chunks in batches of 16, and has a 1ms delay before each batch submit, and a 1ms delay after processing each completion. Running it on /dev/sda yields: time sudo ./iotest /dev/sda ________________________________________________________ Executed in 25.76 secs fish external usr time 6.19 millis 783.00 micros 5.41 millis sys time 12.43 millis 642.00 micros 11.79 millis while on a virtio-blk or NVMe device we get: time sudo ./iotest /dev/vdb ________________________________________________________ Executed in 1.25 secs fish external usr time 1.40 millis 0.30 millis 1.10 millis sys time 17.61 millis 1.43 millis 16.18 millis time sudo ./iotest /dev/nvme0n1 ________________________________________________________ Executed in 1.26 secs fish external usr time 6.11 millis 0.52 millis 5.59 millis sys time 13.94 millis 1.50 millis 12.43 millis where the latter are consistent. If we run the same test but keep the socket for the ssh connection active by having activity there, then the sda test looks as follows: time sudo ./iotest /dev/sda ________________________________________________________ Executed in 1.23 secs fish external usr time 2.70 millis 39.00 micros 2.66 millis sys time 4.97 millis 977.00 micros 3.99 millis as now the ppoll loop is woken all the time anyway. After this fix, on an idle system: time sudo ./iotest /dev/sda ________________________________________________________ Executed in 1.30 secs fish external usr time 2.14 millis 0.14 millis 2.00 millis sys time 16.93 millis 1.16 millis 15.76 millis Signed-off-by: Jens Axboe <[email protected]> Message-Id: <[email protected]> Reviewed-by: Kevin Wolf <[email protected]> [Generalize the comment since this applies to all vCPU thread activity, not just coroutines, as suggested by Kevin Wolf <[email protected]>. --Stefan] Signed-off-by: Stefan Hajnoczi <[email protected]> --- util/aio-posix.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/util/aio-posix.c b/util/aio-posix.c index e24b955fd9..488d964611 100644 --- a/util/aio-posix.c +++ b/util/aio-posix.c @@ -23,6 +23,7 @@ #include "qemu/rcu_queue.h" #include "qemu/sockets.h" #include "qemu/cutils.h" +#include "system/iothread.h" #include "trace.h" #include "aio-posix.h" @@ -813,5 +814,13 @@ void aio_add_sqe(void (*prep_sqe)(struct io_uring_sqe *sqe, void *opaque), { AioContext *ctx = qemu_get_current_aio_context(); ctx->fdmon_ops->add_sqe(ctx, prep_sqe, opaque, cqe_handler); + + /* + * Wake the main loop if it is sleeping in ppoll(). When a vCPU thread + * queues SQEs, the actual io_uring_submit() only happens in + * gsource_prepare() in the main loop thread. Without this notify, the + * main loop thread's ppoll() can sleep up to 499ms before submitting. + */ + aio_notify(ctx); } #endif /* CONFIG_LINUX_IO_URING */ -- 2.53.0
