On 2023/8/31 Benjamin Segall wrote: Hi,

Bagas Sanjaya bagasdotme@gmail.com writes:

...

Hi,

I notice a regression report on Bugzilla [1]. Quoting from it:

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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.

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@bytedance.com 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@vger.kernel.org Reported-by: Igor Raits igor.raits@gmail.com Reported-by: Bagas Sanjaya bagasdotme@gmail.com Signed-off-by: Hao Jia jiahao.os@bytedance.com --- 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