Re: [PATCH] sched/rt: Fix rt_runtime leaks with cpu hotplug and RT_RUNTIME_SHARE

[zhaowenhui (A)]

Friendly Ping.

Regards,
Zhao Wenhui

On 2024/5/24 11:42, Zhao Wenhui Wrote:
> When using cgroup rt_bandwidth with RT_RUNTIME_SHARE, if there are cpu
> hotplug and cpu.rt_runtime_us changing concurrently, the warning in
> __disable_runtime may occur:
> [  991.697692] WARNING: CPU: 0 PID: 49573 at kernel/sched/rt.c:802
> rq_offline_rt+0x24d/0x260
> [  991.697795] CPU: 0 PID: 49573 Comm: kworker/1:0 Kdump: loaded Not
> tainted 6.9.0-rc1+ #4
> [  991.697800] Workqueue: events cpuset_hotplug_workfn
> [  991.697803] RIP: 0010:rq_offline_rt+0x24d/0x260
> [  991.697825] Call Trace:
> [  991.697827]  <TASK>
> [  991.697858]  set_rq_offline.part.125+0x2d/0x70
> [  991.697864]  rq_attach_root+0xda/0x110
> [  991.697867]  cpu_attach_domain+0x433/0x860
> [  991.697880]  partition_sched_domains_locked+0x2a8/0x3a0
> [  991.697885]  rebuild_sched_domains_locked+0x608/0x800
> [  991.697895]  rebuild_sched_domains+0x1b/0x30
> [  991.697897]  cpuset_hotplug_workfn+0x4b6/0x1160
> [  991.697909]  process_scheduled_works+0xad/0x430
> [  991.697917]  worker_thread+0x105/0x270
> [  991.697922]  kthread+0xde/0x110
> [  991.697928]  ret_from_fork+0x2d/0x50
> [  991.697935]  ret_from_fork_asm+0x11/0x20
> [  991.697940]  </TASK>
> [  991.697941] ---[ end trace 0000000000000000 ]---
>
> That's how it happens:
> CPU0                                   CPU1
> -----                                  -----
>
> set_rq_offline(rq)
>      __disable_runtime(rq) (1)
>                                        tg_set_rt_bandwidth (2)
>                                        do_balance_runtime  (3)
> set_rq_online(rq)
>      __enable_runtime(rq)  (4)
>
> In step(1) rt_rq->rt_runtime is set to RUNTIME_INF, and this rtrq's
> runtime is not supposed to change until its rq gets online. However,
> in step(2) tg_set_rt_bandwidth can set rt_rq->rt_runtime to
> rt_bandwidth.rt_runtime. Then, in step(3) rtrq's runtime is not
> RUNTIME_INF, so others can borrow rt_runtime from it. Finally, in
> step(4) the rq gets online, so its rtrq's runtime is set to
> rt_bandwidth.rt_runtime again, and Since then the total rt_runtime in
> the domain is increased by this way. After these steps, when offline cpu,
> rebuilding the sched_domain will offline all rq, and the last rq will
> find the rt_runtime is increased but nowhere to return.
>
> To fix this, we can add a state RUNTIME_DISABLED, which means the runtime
> is disabled and should not be used. When rq get offline, we can set its
> rtrq's rt_runtime to RUNTIME_DISABLED, and when rq get online, reset it.
> And in tg_set_rt_bandwidth and do_balance_runtime, never change a
> disabled rt_runtime.
>
> Fixes: 7def2be1dc67 ("sched: fix hotplug cpus on ia64")
> Closes: https://lore.kernel.org/all/47b4a790-9a27-2fc5-f2aa-f9981c6da015@xxxxxxxxxx/
> Co-developed-by: Hui Tang <tanghui20@xxxxxxxxxx>
> Signed-off-by: Hui Tang <tanghui20@xxxxxxxxxx>
> Signed-off-by: Zhao Wenhui <zhaowenhui8@xxxxxxxxxx>
> ---
>   kernel/sched/rt.c    | 15 +++++++++------
>   kernel/sched/sched.h |  5 +++++
>   2 files changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index aa4c1c874fa4..44b8cc5a2f5f 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -704,7 +704,8 @@ static void do_balance_runtime(struct rt_rq *rt_rq)
>   		 * or __disable_runtime() below sets a specific rq to inf to
>   		 * indicate its been disabled and disallow stealing.
>   		 */
> -		if (iter->rt_runtime == RUNTIME_INF)
> +		if (iter->rt_runtime == RUNTIME_INF ||
> +				iter->rt_runtime == RUNTIME_DISABLED)
>   			goto next;
>   
>   		/*
> @@ -775,7 +776,9 @@ static void __disable_runtime(struct rq *rq)
>   			/*
>   			 * Can't reclaim from ourselves or disabled runqueues.
>   			 */
> -			if (iter == rt_rq || iter->rt_runtime == RUNTIME_INF)
> +			if (iter == rt_rq ||
> +					iter->rt_runtime == RUNTIME_INF ||
> +					iter->rt_runtime == RUNTIME_DISABLED)
>   				continue;
>   
>   			raw_spin_lock(&iter->rt_runtime_lock);
> @@ -801,10 +804,9 @@ static void __disable_runtime(struct rq *rq)
>   		WARN_ON_ONCE(want);
>   balanced:
>   		/*
> -		 * Disable all the borrow logic by pretending we have inf
> -		 * runtime - in which case borrowing doesn't make sense.
> +		 * Disable all the borrow logic by marking runtime disabled.
>   		 */
> -		rt_rq->rt_runtime = RUNTIME_INF;
> +		rt_rq->rt_runtime = RUNTIME_DISABLED;
>   		rt_rq->rt_throttled = 0;
>   		raw_spin_unlock(&rt_rq->rt_runtime_lock);
>   		raw_spin_unlock(&rt_b->rt_runtime_lock);
> @@ -2827,7 +2829,8 @@ static int tg_set_rt_bandwidth(struct task_group *tg,
>   		struct rt_rq *rt_rq = tg->rt_rq[i];
>   
>   		raw_spin_lock(&rt_rq->rt_runtime_lock);
> -		rt_rq->rt_runtime = rt_runtime;
> +		if (rt_rq->rt_runtime != RUNTIME_DISABLED)
> +			rt_rq->rt_runtime = rt_runtime;
>   		raw_spin_unlock(&rt_rq->rt_runtime_lock);
>   	}
>   	raw_spin_unlock_irq(&tg->rt_bandwidth.rt_runtime_lock);
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index a831af102070..c2ad9102b8fa 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -183,6 +183,11 @@ extern struct list_head asym_cap_list;
>    */
>   #define RUNTIME_INF		((u64)~0ULL)
>   
> +/*
> + * Single value that denotes runtime is disabled, and it should not be used.
> + */
> +#define RUNTIME_DISABLED	(-2ULL)
> +
>   static inline int idle_policy(int policy)
>   {
>   	return policy == SCHED_IDLE;

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