6.14-stable review patch. If anyone has any objections, please let me know.
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From: Gabriel Krisman Bertazi krisman@suse.de
[ Upstream commit 92835cebab120f8a5f023a26a792a2ac3f816c4f ]
Our QA team reported a 10%-23%, throughput reduction on an io_uring sqpoll testcase doing IO to a null_blk, that I traced back to a reduction of the device submission queue depth utilization. It turns out that, after commit af5d68f8892f ("io_uring/sqpoll: manage task_work privately"), we capped the number of task_work entries that can be completed from a single spin of sqpoll to only 8 entries, before the sqpoll goes around to (potentially) sleep. While this cap doesn't drive the submission side directly, it impacts the completion behavior, which affects the number of IO queued by fio per sqpoll cycle on the submission side, and io_uring ends up seeing less ios per sqpoll cycle. As a result, block layer plugging is less effective, and we see more time spent inside the block layer in profilings charts, and increased submission latency measured by fio.
There are other places that have increased overhead once sqpoll sleeps more often, such as the sqpoll utilization calculation. But, in this microbenchmark, those were not representative enough in perf charts, and their removal didn't yield measurable changes in throughput. The major overhead comes from the fact we plug less, and less often, when submitting to the block layer.
My benchmark is:
fio --ioengine=io_uring --direct=1 --iodepth=128 --runtime=300 --bs=4k \ --invalidate=1 --time_based --ramp_time=10 --group_reporting=1 \ --filename=/dev/nullb0 --name=RandomReads-direct-nullb-sqpoll-4k-1 \ --rw=randread --numjobs=1 --sqthread_poll
In one machine, tested on top of Linux 6.15-rc1, we have the following baseline: READ: bw=4994MiB/s (5236MB/s), 4994MiB/s-4994MiB/s (5236MB/s-5236MB/s), io=439GiB (471GB), run=90001-90001msec
With this patch: READ: bw=5762MiB/s (6042MB/s), 5762MiB/s-5762MiB/s (6042MB/s-6042MB/s), io=506GiB (544GB), run=90001-90001msec
which is a 15% improvement in measured bandwidth. The average submission latency is noticeably lowered too. As measured by fio:
Baseline: lat (usec): min=20, max=241, avg=99.81, stdev=3.38 Patched: lat (usec): min=26, max=226, avg=86.48, stdev=4.82
If we look at blktrace, we can also see the plugging behavior is improved. In the baseline, we end up limited to plugging 8 requests in the block layer regardless of the device queue depth size, while after patching we can drive more io, and we manage to utilize the full device queue.
In the baseline, after a stabilization phase, an ordinary submission looks like: 254,0 1 49942 0.016028795 5977 U N [iou-sqp-5976] 7
After patching, I see consistently more requests per unplug. 254,0 1 4996 0.001432872 3145 U N [iou-sqp-3144] 32
Ideally, the cap size would at least be the deep enough to fill the device queue, but we can't predict that behavior, or assume all IO goes to a single device, and thus can't guess the ideal batch size. We also don't want to let the tw run unbounded, though I'm not sure it would really be a problem. Instead, let's just give it a more sensible value that will allow for more efficient batching. I've tested with different cap values, and initially proposed to increase the cap to 1024. Jens argued it is too big of a bump and I observed that, with 32, I'm no longer able to observe this bottleneck in any of my machines.
Fixes: af5d68f8892f ("io_uring/sqpoll: manage task_work privately") Signed-off-by: Gabriel Krisman Bertazi krisman@suse.de Link: https://lore.kernel.org/r/20250508181203.3785544-1-krisman@suse.de Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Sasha Levin sashal@kernel.org --- io_uring/sqpoll.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/io_uring/sqpoll.c b/io_uring/sqpoll.c index d037cc68e9d3e..03c699493b5ab 100644 --- a/io_uring/sqpoll.c +++ b/io_uring/sqpoll.c @@ -20,7 +20,7 @@ #include "sqpoll.h"
#define IORING_SQPOLL_CAP_ENTRIES_VALUE 8 -#define IORING_TW_CAP_ENTRIES_VALUE 8 +#define IORING_TW_CAP_ENTRIES_VALUE 32
enum { IO_SQ_THREAD_SHOULD_STOP = 0,