Re: [PATCH] io_uring: add a schedule condition in io_submit_sqes
From: Guo Xuenan
Date: Tue May 24 2022 - 02:59:01 EST
Hi Jens,
this piece of code is used to reproduce the problem.
it always successfuly reproduce this bug on my
arm64 qemu virtual machine.
```c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "liburing.h"
int main(void)
{
int ret, i = 0;
struct io_uring uring;
struct io_uring_params params;
struct io_uring_sqe *sqe;
struct io_uring_cqe *cqe;
struct __kernel_timespec ts = { .tv_sec = 30, .tv_nsec = 0, };
memset(¶ms, 0, sizeof(params));
ret = io_uring_queue_init_params(32768, &uring, ¶ms);
if (ret < 0) {
printf("init err %d\n", errno);
return 0;
}
io_uring_register_personality(&uring);
while (i++ < 32768) {
sqe = io_uring_get_sqe(&uring);
io_uring_prep_timeout(sqe, &ts, rand(), 0);
}
io_uring_submit(&uring);
printf("to_submit %d\n", i - 1);
io_uring_wait_cqe_nr(&uring, &cqe, i - 1);
return 0;
}
```
------------------------------------------------------------------------------------------
considering the performance issue when we add cond_resched() in "the
very fast path"
you have mentationed, may we add some restriction to avoid extreme
situtaions while
also reduceing the impact on performance.
I tested the following patch, it did work, there is no soft lockup here.
diff --git a/fs/io_uring.c b/fs/io_uring.c
index e0823f58f795..3c2e019fd124 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -7873,6 +7873,9 @@ static int io_submit_sqes(struct io_ring_ctx *ctx,
unsigned int nr)
if (!(ctx->flags & IORING_SETUP_SUBMIT_ALL))
break;
}
+ /* to avoid doing too much in one submit round */
+ if (submitted > IORING_MAX_ENTRIES / 2)
+ cond_resched();
} while (submitted < nr);
if (unlikely(submitted != nr)) {
Best regards
Xuenan
On 2022/5/24 0:27, Jens Axboe wrote:
On 5/23/22 8:45 AM, Guo Xuenan wrote:
Hi Jens
On 2022/5/22 10:42, Jens Axboe wrote:
On 5/21/22 8:33 AM, Guo Xuenan wrote:
when set up sq ring size with IORING_MAX_ENTRIES, io_submit_sqes may
looping ~32768 times which may trigger soft lockups. add need_resched
condition to avoid this bad situation.
set sq ring size 32768 and using io_sq_thread to perform stress test
as follows:
watchdog: BUG: soft lockup - CPU#2 stuck for 26s! [iou-sqp-600:601]
Kernel panic - not syncing: softlockup: hung tasks
CPU: 2 PID: 601 Comm: iou-sqp-600 Tainted: G L 5.18.0-rc7+ #3
Hardware name: linux,dummy-virt (DT)
Call trace:
dump_backtrace+0x218/0x228
show_stack+0x20/0x68
dump_stack_lvl+0x68/0x84
dump_stack+0x1c/0x38
panic+0x1ec/0x3ec
watchdog_timer_fn+0x28c/0x300
__hrtimer_run_queues+0x1d8/0x498
hrtimer_interrupt+0x238/0x558
arch_timer_handler_virt+0x48/0x60
handle_percpu_devid_irq+0xdc/0x270
generic_handle_domain_irq+0x50/0x70
gic_handle_irq+0x8c/0x4bc
call_on_irq_stack+0x2c/0x38
do_interrupt_handler+0xc4/0xc8
el1_interrupt+0x48/0xb0
el1h_64_irq_handler+0x18/0x28
el1h_64_irq+0x74/0x78
console_unlock+0x5d0/0x908
vprintk_emit+0x21c/0x470
vprintk_default+0x40/0x50
vprintk+0xd0/0x128
_printk+0xb4/0xe8
io_issue_sqe+0x1784/0x2908
io_submit_sqes+0x538/0x2880
io_sq_thread+0x328/0x7b0
ret_from_fork+0x10/0x20
SMP: stopping secondary CPUs
Kernel Offset: 0x40f1e8600000 from 0xffff800008000000
PHYS_OFFSET: 0xfffffa8c80000000
CPU features: 0x110,0000cf09,00001006
Memory Limit: none
---[ end Kernel panic - not syncing: softlockup: hung tasks ]---
Signed-off-by: Guo Xuenan <guoxuenan@xxxxxxxxxx>
---
fs/io_uring.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 92ac50f139cd..d897c6798f00 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -7864,7 +7864,7 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr)
if (!(ctx->flags & IORING_SETUP_SUBMIT_ALL))
break;
}
- } while (submitted < nr);
+ } while (submitted < nr && !need_resched());
if (unlikely(submitted != nr)) {
int ref_used = (submitted == -EAGAIN) ? 0 : submitted;
This is wrong, you'll potentially end up doing random short submits for
non-sqpoll as well.
Sorry, Indeed, this is not a good solution. Since, the function
io_submit_sqes not only called by io_sq_thread, it also called by
syscall io_uring_enter sending large amounts of requests, will also
trigger soft lockup.
Exactly.
sqpoll already supports capping how many it submits in one go, it just
doesn't do it if it's only running one ring. As simple as the below,
with 1024 pulled out of thin air. Would be great if you could experiment
and submit a v2 based on this principle instead. Might still need a
yes, Jens, your patch sloved sq-poll-thread problem, but the problem
may not completely solved; when using syscall io_uring_enter to
subimit large amounts of requests.So in my opinion How about 1) add
cond_resched() in the while cycle part of io_submit_sqes ?. OR 2) set
macro IORING_MAX_ENTRIES smaller? (i'm curious about the value,why we
set it with 32768)
I did suspect this isn't specific to SQPOLL at all.
Might make sense to cap batches of non-sqpoll as well, and for each
batch, have a cond_resched() just in case. If you change
IORING_MAX_ENTRIES to something smaller, you risk breaking applications
that currently (for whatever reason) may have set up an SQ ring of that
side. So that is not a viable solution, and honestly wouldn't be a good
option even if that weren't the case.
So the simple solution is just to do it in io_submit_sqes() itself, but
would need to be carefully benchmarked to make sure that it doesn't
regress anything. It's the very fast path, and for real use cases you'd
never run into this problem. Even for a synthetic use case, it sounds
highly suspicious that nothing in the call path ends up doing a
conditional reschedule. What kind of requests are being submitted when
you hit this?