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Message-ID: <6df6a8d3-6c5e-4ea7-8f55-08c2a56928f6@linux.dev>
Date: Wed, 24 Jul 2024 10:29:27 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Chuyi Zhou <zhouchuyi@...edance.com>, mingo@...hat.com,
peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org,
dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
mgorman@...e.de, vschneid@...hat.com
Cc: linux-kernel@...r.kernel.org, joshdon@...gle.com
Subject: Re: [PATCH v2 1/2] sched/fair: Decrease cfs bandwidth usage in
task_group destruction
On 2024/7/23 20:20, Chuyi Zhou wrote:
> The static key __cfs_bandwidth_used is used to indicate whether bandwidth
> control is enabled in the system. Currently, it is only decreased when a
> task group disables bandwidth control. This is incorrect because if there
> was a task group in the past that enabled bandwidth control, the
> __cfs_bandwidth_used will never go to zero, even if there are no task_group
> using bandwidth control now.
>
> This patch tries to fix this issue by decrsasing bandwidth usage in
> destroy_cfs_bandwidth(). cfs_bandwidth_usage_dec() calls
> static_key_slow_dec_cpuslocked which needs to hold hotplug lock, but cfs
> bandwidth destroy maybe run in a rcu callback. Move the call to
> destroy_cfs_bandwidth() from unregister_fair_sched_group() to
> cpu_cgroup_css_free() which runs in process context.
>
> Signed-off-by: Chuyi Zhou <zhouchuyi@...edance.com>
Yeah, autogroup can't have bandwidth set, so it's ok to just destroy
bandwidth in .css_free().
Reviewed-by: Chengming Zhou <chengming.zhou@...ux.dev>
Just some nits below:
> ---
> kernel/sched/core.c | 2 ++
> kernel/sched/fair.c | 13 +++++++------
> kernel/sched/sched.h | 2 ++
> 3 files changed, 11 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 6d35c48239be..7720d34bd71b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8816,6 +8816,8 @@ static void cpu_cgroup_css_free(struct cgroup_subsys_state *css)
> {
> struct task_group *tg = css_tg(css);
>
> + destroy_cfs_bandwidth(tg_cfs_bandwidth(tg));
Instead of exporting this tg_cfs_bandwidth(), how about just changing
the parameter of init_cfs_bandwidth()/destroy_cfs_bandwidth() to tg?
Which maybe clearer? but this is your call.
Thanks.
> +
> /*
> * Relies on the RCU grace period between css_released() and this.
> */
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index da3cdd86ab2e..c56b6d5b8ed7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5615,7 +5615,7 @@ void __refill_cfs_bandwidth_runtime(struct cfs_bandwidth *cfs_b)
> cfs_b->runtime_snap = cfs_b->runtime;
> }
>
> -static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
> +struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
> {
> return &tg->cfs_bandwidth;
> }
> @@ -6438,7 +6438,7 @@ void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
> hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED);
> }
>
> -static void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
> +void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
> {
> int __maybe_unused i;
>
> @@ -6472,6 +6472,9 @@ static void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
> local_irq_restore(flags);
> }
> #endif
> + guard(cpus_read_lock)();
> + if (cfs_b->quota != RUNTIME_INF)
> + cfs_bandwidth_usage_dec();
> }
>
> /*
> @@ -6614,11 +6617,11 @@ void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b, struct cfs_bandwidth *paren
> static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq) {}
> #endif
>
> -static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
> +struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
> {
> return NULL;
> }
> -static inline void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b) {}
> +void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b) {}
> static inline void update_runtime_enabled(struct rq *rq) {}
> static inline void unthrottle_offline_cfs_rqs(struct rq *rq) {}
> #ifdef CONFIG_CGROUP_SCHED
> @@ -12992,8 +12995,6 @@ void unregister_fair_sched_group(struct task_group *tg)
> struct rq *rq;
> int cpu;
>
> - destroy_cfs_bandwidth(tg_cfs_bandwidth(tg));
> -
> for_each_possible_cpu(cpu) {
> if (tg->se[cpu])
> remove_entity_load_avg(tg->se[cpu]);
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 8a071022bdec..d251842867ce 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2938,6 +2938,8 @@ extern void init_dl_rq(struct dl_rq *dl_rq);
> extern void cfs_bandwidth_usage_inc(void);
> extern void cfs_bandwidth_usage_dec(void);
>
> +extern struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg);
> +extern void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b);
> #ifdef CONFIG_NO_HZ_COMMON
>
> #define NOHZ_BALANCE_KICK_BIT 0
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