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Message-ID: <20230510143244.GM4253@hirez.programming.kicks-ass.net>
Date: Wed, 10 May 2023 16:32:44 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Hao Jia <jiahao.os@...edance.com>
Cc: mingo@...hat.com, mingo@...nel.org, juri.lelli@...hat.com,
vincent.guittot@...aro.org, dietmar.eggemann@....com,
rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
bristot@...hat.com, vschneid@...hat.com,
mgorman@...hsingularity.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] sched/core: Avoid double calling update_rq_clock()
On Wed, May 10, 2023 at 04:34:50PM +0800, Hao Jia wrote:
> There are some double rq clock update warnings are triggered.
> ------------[ cut here ]------------
> rq->clock_update_flags & RQCF_UPDATED
> WARNING: CPU: 17 PID: 138 at kernel/sched/core.c:741
> update_rq_clock+0xaf/0x180
> Call Trace:
> <TASK>
> __balance_push_cpu_stop+0x146/0x180
> ? migration_cpu_stop+0x2a0/0x2a0
> cpu_stopper_thread+0xa3/0x140
> smpboot_thread_fn+0x14f/0x210
> ? sort_range+0x20/0x20
> kthread+0xe6/0x110
> ? kthread_complete_and_exit+0x20/0x20
> ret_from_fork+0x1f/0x30
>
> ------------[ cut here ]------------
> rq->clock_update_flags & RQCF_UPDATED
> WARNING: CPU: 54 PID: 0 at kernel/sched/core.c:741
> update_rq_clock+0xaf/0x180
> Call Trace:
> <TASK>
> unthrottle_cfs_rq+0x4b/0x300
> __cfsb_csd_unthrottle+0xe0/0x100
> __flush_smp_call_function_queue+0xaf/0x1d0
> flush_smp_call_function_queue+0x49/0x90
> do_idle+0x17c/0x270
> cpu_startup_entry+0x19/0x20
> start_secondary+0xfa/0x120
> secondary_startup_64_no_verify+0xce/0xdb
>
> ------------[ cut here ]------------
> rq->clock_update_flags & RQCF_UPDATED
> WARNING: CPU: 0 PID: 3323 at kernel/sched/core.c:741
> update_rq_clock+0xaf/0x180
> Call Trace:
> <TASK>
> unthrottle_cfs_rq+0x4b/0x300
> rq_offline_fair+0x89/0x90
> set_rq_offline.part.118+0x28/0x60
> rq_attach_root+0xc4/0xd0
> cpu_attach_domain+0x3dc/0x7f0
> partition_sched_domains_locked+0x2a5/0x3c0
> rebuild_sched_domains_locked+0x477/0x830
> rebuild_sched_domains+0x1b/0x30
> cpuset_hotplug_workfn+0x2ca/0xc90
> ? balance_push+0x56/0xf0
> ? _raw_spin_unlock+0x15/0x30
> ? finish_task_switch+0x98/0x2f0
> ? __switch_to+0x291/0x410
> ? __schedule+0x65e/0x1310
> process_one_work+0x1bc/0x3d0
> worker_thread+0x4c/0x380
> ? preempt_count_add+0x92/0xa0
> ? rescuer_thread+0x310/0x310
> kthread+0xe6/0x110
> ? kthread_complete_and_exit+0x20/0x20
> ret_from_fork+0x1f/0x30
>
> For the __balance_push_cpu_stop() case, we remove update_rq_clock() from
> the __migrate_task() function to avoid double updating the rq clock.
> And in order to avoid missing rq clock update, add update_rq_clock()
> call before migration_cpu_stop() calls __migrate_task().
>
> For the unthrottle_cfs_rq() case, we use rq_clock_start_loop_update() to
> prevent multiple calls to update_rq_clock() in unthrottle_cfs_rq().
>
> Note that the rq clock has been updated before the set_rq_offline()
> function runs, so we don't need to call update_rq_clock() in
> unthrottle_offline_cfs_rqs().
This reads like 3 separate issues -- in which case this ought to be 3
separate patches.
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index ec7b3e0a2b20..9c712f29e5a4 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1536,6 +1536,27 @@ static inline void rq_clock_skip_update(struct rq *rq)
> rq->clock_update_flags |= RQCF_REQ_SKIP;
> }
>
> +/*
> + * During cpu offlining and rq wide unthrottling, we can trigger
> + * an update_rq_clock() for several cfs and rt runqueues (Typically
> + * 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.
> + * After the iterative traversal, we need to call rq_clock_stop_loop_update()
> + * to restore rq->clock_update_flags.
^^^^^^^
> + */
> +static inline void rq_clock_start_loop_update(struct rq *rq)
> +{
> + lockdep_assert_rq_held(rq);
> + rq->clock_update_flags |= RQCF_ACT_SKIP;
> +}
> +
> +static inline void rq_clock_stop_loop_update(struct rq *rq)
> +{
> + lockdep_assert_rq_held(rq);
> + rq->clock_update_flags &= ~RQCF_ACT_SKIP;
> +}
This does not restore the flag, it clears the flag.
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