On 2023/8/31 Benjamin Segall wrote:
Hi,
Bagas Sanjaya <bagasdotme@xxxxxxxxx> writes:
Hi,
I notice a regression report on Bugzilla [1]. Quoting from it:
Hello, we recently got a few kernel crashes with following backtrace. Happened on 6.4.12 (and 6.4.11 I think) but did not happen (I think) on 6.4.4.
[293790.928007] ------------[ cut here ]------------
[293790.929905] rq->clock_update_flags & RQCF_ACT_SKIP
[293790.929919] WARNING: CPU: 13 PID: 3837105 at kernel/sched/sched.h:1561 __cfsb_csd_unthrottle+0x149/0x160
[293790.933694] Modules linked in: [...]
[293790.946262] Unloaded tainted modules: edac_mce_amd(E):1
[293790.956625] CPU: 13 PID: 3837105 Comm: QueryWorker-30f Tainted: G W E 6.4.12-1.gdc.el9.x86_64 #1
[293790.957963] Hardware name: RDO OpenStack Compute/RHEL, BIOS edk2-20230301gitf80f052277c8-2.el9 03/01/2023
[293790.959681] RIP: 0010:__cfsb_csd_unthrottle+0x149/0x160
See Bugzilla for the full thread.
Anyway, I'm adding this regression to regzbot:
#regzbot introduced: ebb83d84e49b54 https://bugzilla.kernel.org/show_bug.cgi?id=217843
Thanks.
[1]: https://bugzilla.kernel.org/show_bug.cgi?id=217843
The code in question is literally "rq_lock; update_rq_clock;
rq_clock_start_loop_update (the warning)", which suggests to me that
RQCF_ACT_SKIP is somehow leaking from somewhere else?
If I understand correctly, rq->clock_update_flags may be set to
RQCF_ACT_SKIP after __schedule() holds the rq lock, and sometimes the rq
lock may be released briefly in __schedule(), such as newidle_balance().
At this time Other CPUs hold this rq lock, and then calling
rq_clock_start_loop_update() may trigger this warning.
This warning check might be wrong. We need to add assert_clock_updated()
to check that the rq clock has been updated before calling
rq_clock_start_loop_update().
Maybe some things can be like this?
From: Hao Jia <jiahao.os@xxxxxxxxxxxxx>
Date: Thu, 31 Aug 2023 11:38:54 +0800
Subject: [PATCH] sched/core: Fix wrong warning check in
rq_clock_start_loop_update()
Commit ebb83d84e49b54 ("sched/core: Avoid multiple
calling update_rq_clock() in __cfsb_csd_unthrottle()")
add "rq->clock_update_flags & RQCF_ACT_SKIP" warning in
rq_clock_start_loop_update().
But this warning is inaccurate and may be triggered
incorrectly in the following situations:
CPU0 CPU1
__schedule()
*rq->clock_update_flags <<= 1;* unregister_fair_sched_group()
pick_next_task_fair+0x4a/0x410 destroy_cfs_bandwidth()
newidle_balance+0x115/0x3e0 for_each_possible_cpu(i) *i=0*
rq_unpin_lock(this_rq, rf) __cfsb_csd_unthrottle()
raw_spin_rq_unlock(this_rq)
rq_lock(*CPU0_rq*, &rf)
rq_clock_start_loop_update()
rq->clock_update_flags &
RQCF_ACT_SKIP <--
raw_spin_rq_lock(this_rq)
So we remove this wrong check. Add assert_clock_updated() to
check that rq clock has been updated before calling
rq_clock_start_loop_update(). And use the variable rq_clock_flags
in rq_clock_start_loop_update() to record the previous state of
rq->clock_update_flags. Correspondingly, restore rq->clock_update_flags
through rq_clock_flags in rq_clock_stop_loop_update() to prevent
losing its previous information.
Fixes: ebb83d84e49b ("sched/core: Avoid multiple calling
update_rq_clock() in __cfsb_csd_unthrottle()")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Igor Raits <igor.raits@xxxxxxxxx>
Reported-by: Bagas Sanjaya <bagasdotme@xxxxxxxxx>
Signed-off-by: Hao Jia <jiahao.os@xxxxxxxxxxxxx>
---
kernel/sched/fair.c | 10 ++++++----
kernel/sched/sched.h | 12 +++++++-----
2 files changed, 13 insertions(+), 9 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 911d0063763c..0f6557c82a4c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5679,6 +5679,7 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
#ifdef CONFIG_SMP
static void __cfsb_csd_unthrottle(void *arg)
{
+ unsigned int rq_clock_flags;
struct cfs_rq *cursor, *tmp;
struct rq *rq = arg;
struct rq_flags rf;
@@ -5691,7 +5692,7 @@ static void __cfsb_csd_unthrottle(void *arg)
* Do it once and skip the potential next ones.
*/
update_rq_clock(rq);
- rq_clock_start_loop_update(rq);
+ rq_clock_start_loop_update(rq, &rq_clock_flags);
/*
* Since we hold rq lock we're safe from concurrent manipulation of
@@ -5712,7 +5713,7 @@ static void __cfsb_csd_unthrottle(void *arg)
rcu_read_unlock();
- rq_clock_stop_loop_update(rq);
+ rq_clock_stop_loop_update(rq, &rq_clock_flags);
rq_unlock(rq, &rf);
}
@@ -6230,6 +6231,7 @@ static void __maybe_unused
update_runtime_enabled(struct rq *rq)
/* cpu offline callback */
static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
{
+ unsigned int rq_clock_flags;
struct task_group *tg;
lockdep_assert_rq_held(rq);
@@ -6239,7 +6241,7 @@ static void __maybe_unused
unthrottle_offline_cfs_rqs(struct rq *rq)
* set_rq_offline(), so we should skip updating
* the rq clock again in unthrottle_cfs_rq().
*/
- rq_clock_start_loop_update(rq);
+ rq_clock_start_loop_update(rq, &rq_clock_flags);
rcu_read_lock();
list_for_each_entry_rcu(tg, &task_groups, list) {
@@ -6264,7 +6266,7 @@ static void __maybe_unused
unthrottle_offline_cfs_rqs(struct rq *rq)
}
rcu_read_unlock();
- rq_clock_stop_loop_update(rq);
+ rq_clock_stop_loop_update(rq, &rq_clock_flags);
}
bool cfs_task_bw_constrained(struct task_struct *p)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 04846272409c..ff2864f202f5 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1558,20 +1558,22 @@ static inline void
rq_clock_cancel_skipupdate(struct rq *rq)
* when using list_for_each_entry_*)
* rq_clock_start_loop_update() can be called after updating the clock
* once and before iterating over the list to prevent multiple update.
+ * And use @rq_clock_flags to record the previous state of
rq->clock_update_flags.
* After the iterative traversal, we need to call
rq_clock_stop_loop_update()
- * to clear RQCF_ACT_SKIP of rq->clock_update_flags.
+ * to restore rq->clock_update_flags through @rq_clock_flags.
*/
-static inline void rq_clock_start_loop_update(struct rq *rq)
+static inline void rq_clock_start_loop_update(struct rq *rq, unsigned
int *rq_clock_flags)
{
lockdep_assert_rq_held(rq);
- SCHED_WARN_ON(rq->clock_update_flags & RQCF_ACT_SKIP);
+ assert_clock_updated(rq);
+ *rq_clock_flags = rq->clock_update_flags;
rq->clock_update_flags |= RQCF_ACT_SKIP;
}
-static inline void rq_clock_stop_loop_update(struct rq *rq)
+static inline void rq_clock_stop_loop_update(struct rq *rq, unsigned
int *rq_clock_flags)
{
lockdep_assert_rq_held(rq);
- rq->clock_update_flags &= ~RQCF_ACT_SKIP;
+ rq->clock_update_flags = *rq_clock_flags;
}
struct rq_flags {
--
2.20.1
