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Message-ID: <885de7ad-f168-4826-b96d-bb136ee30333@linux.ibm.com>
Date: Fri, 7 Mar 2025 11:06:56 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Juri Lelli <juri.lelli@...hat.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
Waiman Long <longman@...hat.com>, Tejun Heo <tj@...nel.org>,
Johannes Weiner <hannes@...xchg.org>,
Michal Koutný
<mkoutny@...e.com>,
Qais Yousef <qyousef@...alina.io>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Swapnil Sapkal <swapnil.sapkal@....com>, Phil Auld <pauld@...hat.com>,
luca.abeni@...tannapisa.it, tommaso.cucinotta@...tannapisa.it,
Jon Hunter <jonathanh@...dia.com>, cgroups@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/8] sched/deadline: Generalize unique visiting of root
domains
On 3/6/25 19:40, Juri Lelli wrote:
> Bandwidth checks and updates that work on root domains currently employ
> a cookie mechanism for efficiency. This mechanism is very much tied to
> when root domains are first created and initialized.
>
> Generalize the cookie mechanism so that it can be used also later at
> runtime while updating root domains. Also, additionally guard it with
> sched_domains_mutex, since domains need to be stable while updating them
> (and it will be required for further dynamic changes).
>
> Reported-by: Jon Hunter <jonathanh@...dia.com>
> Fixes: 53916d5fd3c0 ("sched/deadline: Check bandwidth overflow earlier for hotplug")
> Signed-off-by: Juri Lelli <juri.lelli@...hat.com>
> ---
> include/linux/sched/deadline.h | 3 +++
> kernel/sched/deadline.c | 23 +++++++++++++----------
> kernel/sched/rt.c | 2 ++
> kernel/sched/sched.h | 2 +-
> kernel/sched/topology.c | 2 +-
> 5 files changed, 20 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/sched/deadline.h b/include/linux/sched/deadline.h
> index 3a912ab42bb5..6ec578600b24 100644
> --- a/include/linux/sched/deadline.h
> +++ b/include/linux/sched/deadline.h
> @@ -37,4 +37,7 @@ extern void dl_clear_root_domain(struct root_domain *rd);
>
> #endif /* CONFIG_SMP */
>
> +extern u64 dl_cookie;
> +extern bool dl_bw_visited(int cpu, u64 cookie);
> +
> #endif /* _LINUX_SCHED_DEADLINE_H */
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index ab565a151355..339434271cba 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -166,14 +166,14 @@ static inline unsigned long dl_bw_capacity(int i)
> }
> }
>
> -static inline bool dl_bw_visited(int cpu, u64 gen)
> +static inline bool dl_bw_visited(int cpu, u64 cookie)
> {
> struct root_domain *rd = cpu_rq(cpu)->rd;
>
> - if (rd->visit_gen == gen)
> + if (rd->visit_cookie == cookie)
> return true;
>
> - rd->visit_gen = gen;
> + rd->visit_cookie = cookie;
> return false;
> }
>
> @@ -207,7 +207,7 @@ static inline unsigned long dl_bw_capacity(int i)
> return SCHED_CAPACITY_SCALE;
> }
>
> -static inline bool dl_bw_visited(int cpu, u64 gen)
> +static inline bool dl_bw_visited(int cpu, u64 cookie)
> {
> return false;
> }
> @@ -3171,15 +3171,18 @@ DEFINE_SCHED_CLASS(dl) = {
> #endif
> };
>
> -/* Used for dl_bw check and update, used under sched_rt_handler()::mutex */
> -static u64 dl_generation;
> +/*
> + * Used for dl_bw check and update, used under sched_rt_handler()::mutex and
> + * sched_domains_mutex.
> + */
> +u64 dl_cookie;
>
> int sched_dl_global_validate(void)
> {
> u64 runtime = global_rt_runtime();
> u64 period = global_rt_period();
> u64 new_bw = to_ratio(period, runtime);
> - u64 gen = ++dl_generation;
> + u64 cookie = ++dl_cookie;
> struct dl_bw *dl_b;
> int cpu, cpus, ret = 0;
> unsigned long flags;
> @@ -3192,7 +3195,7 @@ int sched_dl_global_validate(void)
> for_each_possible_cpu(cpu) {
This has been changed in 14672f059d83f591afb2ee1fff56858efe055e5a to
online CPUs. So patch didn't apply cleanly to me.
> rcu_read_lock_sched();
>
> - if (dl_bw_visited(cpu, gen))
> + if (dl_bw_visited(cpu, cookie))
> goto next;
>
> dl_b = dl_bw_of(cpu);
> @@ -3229,7 +3232,7 @@ static void init_dl_rq_bw_ratio(struct dl_rq *dl_rq)
> void sched_dl_do_global(void)
> {
> u64 new_bw = -1;
> - u64 gen = ++dl_generation;
> + u64 cookie = ++dl_cookie;
> struct dl_bw *dl_b;
> int cpu;
> unsigned long flags;
> @@ -3240,7 +3243,7 @@ void sched_dl_do_global(void)
> for_each_possible_cpu(cpu) {
> rcu_read_lock_sched();
>
> - if (dl_bw_visited(cpu, gen)) {
> + if (dl_bw_visited(cpu, cookie)) {
> rcu_read_unlock_sched();
> continue;
> }
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 4b8e33c615b1..8cebe71d2bb1 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -2910,6 +2910,7 @@ static int sched_rt_handler(const struct ctl_table *table, int write, void *buff
> int ret;
>
> mutex_lock(&mutex);
> + sched_domains_mutex_lock();
> old_period = sysctl_sched_rt_period;
> old_runtime = sysctl_sched_rt_runtime;
>
> @@ -2936,6 +2937,7 @@ static int sched_rt_handler(const struct ctl_table *table, int write, void *buff
> sysctl_sched_rt_period = old_period;
> sysctl_sched_rt_runtime = old_runtime;
> }
> + sched_domains_mutex_unlock();
> mutex_unlock(&mutex);
>
> return ret;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index c8512a9fb022..c978abe38c07 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -998,7 +998,7 @@ struct root_domain {
> * Also, some corner cases, like 'wrap around' is dangerous, but given
> * that u64 is 'big enough'. So that shouldn't be a concern.
> */
> - u64 visit_gen;
> + u64 visit_cookie;
>
> #ifdef HAVE_RT_PUSH_IPI
> /*
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 296ff2acfd32..44093339761c 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -568,7 +568,7 @@ static int init_rootdomain(struct root_domain *rd)
> rd->rto_push_work = IRQ_WORK_INIT_HARD(rto_push_irq_work_func);
> #endif
>
> - rd->visit_gen = 0;
> + rd->visit_cookie = 0;
> init_dl_bw(&rd->dl_bw);
> if (cpudl_init(&rd->cpudl) != 0)
> goto free_rto_mask;
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