[lttng-dev] urcu workqueue thread uses 99% of cpu while workqueue is empty
Mathieu Desnoyers
mathieu.desnoyers at efficios.com
Fri Jun 17 09:37:23 EDT 2022
----- On Jun 16, 2022, at 4:09 AM, Minlan Wang wangminlan at szsandstone.com wrote:
> Hi, Mathieu,
> I tried to write a simple program to reproduce this issue, but failed.
> The environment we used to produce this issue is a storage cluster.
> It has 3 nodes, each with an NVME and several SATA disks, urcu is part of the
> storage cluster software.
> The storage software is pretty big, 3+ GB in size. I guess it's hard for you to
> build a environment like ours.
>
> Besides, I made some change into workqueue.c to debug this issue today, here's
> the patch i used for debuging:
>
> From 18f3fea1436e0c999b346b998b7153150b92e62f Mon Sep 17 00:00:00 2001
> From: wangminlan <wangminlan at szsandstone.com>
> Date: Wed, 15 Jun 2022 14:28:53 +0800
> Subject: [PATCH] <urcu> debug workqueue->futex
>
> ---
> .../userspace-rcu-0.12.1/src/workqueue.c | 49 ++++++++++++++++++----
> 1 file changed, 42 insertions(+), 7 deletions(-)
>
> diff --git
> a/ceph/thirdpart/ascache/thirdparty/userspace-rcu-0.12.1/src/workqueue.c
> b/ceph/thirdpart/ascache/thirdparty/userspace-rcu-0.12.1/src/workqueue.c
> index 59eb21d..eed1e21 100644
> --- a/ceph/thirdpart/ascache/thirdparty/userspace-rcu-0.12.1/src/workqueue.c
> +++ b/ceph/thirdpart/ascache/thirdparty/userspace-rcu-0.12.1/src/workqueue.c
> @@ -132,12 +132,21 @@ static int set_thread_cpu_affinity(struct urcu_workqueue
> *workqueue)
>
> static void futex_wait(int32_t *futex)
> {
> + int ret;
> + int err;
> /* Read condition before read futex */
> cmm_smp_mb();
> - if (uatomic_read(futex) != -1)
> + if (uatomic_read(futex) != -1) {
> + fprintf(stderr, "%lu: wq %p: %s futex != -1, don't wait\n",
> + pthread_self(), caa_container_of(futex, struct urcu_workqueue, futex),
> __func__);
> +
> return;
> - while (futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
> - switch (errno) {
> + }
> + while ((ret = futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0))) {
> + err = errno;
> + fprintf(stderr, "%lu: wq %p: %s failed, errno %d\n",
> + pthread_self(), caa_container_of(futex, struct urcu_workqueue, futex),
> __func__, err);
> + switch (err) {
> case EWOULDBLOCK:
> /* Value already changed. */
> return;
> @@ -146,17 +155,35 @@ static void futex_wait(int32_t *futex)
> break; /* Get out of switch. */
> default:
> /* Unexpected error. */
> - urcu_die(errno);
> + urcu_die(err);
> }
> }
> +
> + fprintf(stderr, "%lu: wq %p: %s wait return %d\n",
> + pthread_self(), caa_container_of(futex, struct urcu_workqueue, futex),
> __func__, ret);
> +
> }
>
> static void futex_wake_up(int32_t *futex)
> {
> /* Write to condition before reading/writing futex */
> + int32_t old;
> +
> cmm_smp_mb();
> - if (caa_unlikely(uatomic_read(futex) == -1)) {
> - uatomic_set(futex, 0);
> + old = uatomic_read(futex);
> + if (caa_unlikely(old == -1)) {
> + old = uatomic_xchg(futex, 0);
> + if (old == -1) {
> + fprintf(stderr, "%lu: wq %p, wakeup succeed: old %d\n",
> + pthread_self(),
> + caa_container_of(futex, struct urcu_workqueue, futex),
> + old);
> + } else {
> + fprintf(stderr, "%lu: wq %p, wakeup failed: old %d\n",
> + pthread_self(),
> + caa_container_of(futex, struct urcu_workqueue, futex),
> + old);
> + }
> if (futex_async(futex, FUTEX_WAKE, 1,
> NULL, NULL, 0) < 0)
> urcu_die(errno);
> @@ -237,8 +264,16 @@ static void *workqueue_thread(void *arg)
> if (!rt) {
> if (cds_wfcq_empty(&workqueue->cbs_head,
> &workqueue->cbs_tail)) {
> + int32_t new;
> futex_wait(&workqueue->futex);
> - uatomic_dec(&workqueue->futex);
> + new = uatomic_add_return(&workqueue->futex, -1);
> + if (new == -1) {
> + fprintf(stderr, "%lu: wq %p dec succeed: old %d, new %d\n",
> + pthread_self(), workqueue, new + 1, new);
> + } else {
> + fprintf(stderr, "%lu: wq %p dec failed: old %d\n",
> + pthread_self(), workqueue, new + 1);
> + }
> /*
> * Decrement futex before reading
> * urcu_work list.
> --
> 1.8.3.1
>
> And the instresting log began from here:
> ---
> ...
> 2097 140196498532096: wq 0x55f69a99ea00, wakeup succeed: old -1 /* enqueue op
> set futex: -1 -> 0 */
> 2098 140196272453376: wq 0x55f69a99ea00: futex_wait wait return 0 /*
> workqueue_thread wait success */
> 2099 140196272453376: wq 0x55f69a99ea00 dec failed: old -1 /* workqueue_thread
> got futex == -1 ? */
> 2100 140196272453376: wq 0x55f69a99ea00: futex_wait futex != -1, don't wait
> 2101 140196272453376: wq 0x55f69a99ea00 dec failed: old -2
> ...
> ---
> In line 2099, how did workqueue_thread got futex as -1? This is wierd.
>
> I put the patch i used to debug, and the complete log of the reproduce into the
> attachment, may this helps.
Here is my current line of investigation:
grep -e wakeup -e "futex_wait wait return 0" 8.osd.mail.log |tail
shows:
140196498532096: wq 0x55f69a99ea00, wakeup succeed: old -1
140196272453376: wq 0x55f69a99ea00: futex_wait wait return 0
140196498532096: wq 0x55f69a99ea00, wakeup succeed: old -1
140196272453376: wq 0x55f69a99ea00: futex_wait wait return 0
140196498532096: wq 0x55f69a99ea00, wakeup succeed: old -1
140196272453376: wq 0x55f69a99ea00: futex_wait wait return 0
140196498532096: wq 0x55f69a99ea00, wakeup succeed: old -1
140196498532096: wq 0x55f69a99ea00, wakeup succeed: old -1
140196498532096: wq 0x55f69a99ea00, wakeup succeed: old -1
140196272453376: wq 0x55f69a99ea00: futex_wait wait return 0
Which basically shows that shortly before things go awry, many wakeups succeed
(and are thus potentially queued within the kernel) before a single futex_wait
succeeds.
I therefore suspect that this particular last futex_wait success has been caused
by a prior queued wakeup.
This could indeed be the culprit here, because it would cause the futex_wait to
return and proceed with uatomic_add_return(&workqueue->futex, -1); when the state
was not set to 0 by the wakeup side.
We should not rely too much on the ordering of fprintf stderr output between threads,
because those can be interleaved in any order (each fprintf to stderr is unbuffered).
If we want more precision in the order of events I would recommend using LTTng-UST for
fine-grained user-space tracing. This might explain why the futex has an unexpected value
at line 2099.
Can you try the following patch on top of v0.12.1 + your debug patch ?
diff --git a/src/workqueue.c b/src/workqueue.c
index eed1e21e..19fc89c8 100644
--- a/src/workqueue.c
+++ b/src/workqueue.c
@@ -142,6 +142,7 @@ static void futex_wait(int32_t *futex)
return;
}
+wait:
while ((ret = futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0))) {
err = errno;
fprintf(stderr, "%lu: wq %p: %s failed, errno %d\n",
@@ -161,7 +162,16 @@ static void futex_wait(int32_t *futex)
fprintf(stderr, "%lu: wq %p: %s wait return %d\n",
pthread_self(), caa_container_of(futex, struct urcu_workqueue, futex), __func__, ret);
-
+ /*
+ * Prior queued wakeups can cause futex wait to return even
+ * though the futex value is still -1. If this happens, wait
+ * again.
+ */
+ if (uatomic_read(futex) == -1) {
+ fprintf(stderr, "%lu: wq %p: %s futex == -1, wait again\n",
+ pthread_self(), caa_container_of(futex, struct urcu_workqueue, futex), __func__);
+ goto wait;
+ }
}
static void futex_wake_up(int32_t *futex)
Thanks!
Mathieu
>
> Thanks,
> Minlan
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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