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Message-ID: <20251031125951.GA430420@pauld.westford.csb>
Date: Fri, 31 Oct 2025 08:59:51 -0400
From: Phil Auld <pauld@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
Michal Koutný <mkoutny@...e.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Bjorn Helgaas <bhelgaas@...gle.com>,
Catalin Marinas <catalin.marinas@....com>,
Danilo Krummrich <dakr@...nel.org>,
"David S . Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Gabriele Monaco <gmonaco@...hat.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Ingo Molnar <mingo@...hat.com>, Jakub Kicinski <kuba@...nel.org>,
Jens Axboe <axboe@...nel.dk>, Johannes Weiner <hannes@...xchg.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
Marco Crivellari <marco.crivellari@...e.com>,
Michal Hocko <mhocko@...e.com>, Muchun Song <muchun.song@...ux.dev>,
Paolo Abeni <pabeni@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
"Rafael J . Wysocki" <rafael@...nel.org>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Shakeel Butt <shakeel.butt@...ux.dev>,
Simon Horman <horms@...nel.org>, Tejun Heo <tj@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Vlastimil Babka <vbabka@...e.cz>, Waiman Long <longman@...hat.com>,
Will Deacon <will@...nel.org>, cgroups@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, linux-block@...r.kernel.org,
linux-mm@...ck.org, linux-pci@...r.kernel.org,
netdev@...r.kernel.org
Subject: Re: [PATCH 13/33] cpuset: Update HK_TYPE_DOMAIN cpumask from cpuset
Hi Frederic,
On Mon, Oct 13, 2025 at 10:31:26PM +0200 Frederic Weisbecker wrote:
> Until now, HK_TYPE_DOMAIN used to only include boot defined isolated
> CPUs passed through isolcpus= boot option. Users interested in also
> knowing the runtime defined isolated CPUs through cpuset must use
> different APIs: cpuset_cpu_is_isolated(), cpu_is_isolated(), etc...
>
> There are many drawbacks to that approach:
>
> 1) Most interested subsystems want to know about all isolated CPUs, not
> just those defined on boot time.
>
> 2) cpuset_cpu_is_isolated() / cpu_is_isolated() are not synchronized with
> concurrent cpuset changes.
>
> 3) Further cpuset modifications are not propagated to subsystems
>
> Solve 1) and 2) and centralize all isolated CPUs within the
> HK_TYPE_DOMAIN housekeeping cpumask.
>
> Subsystems can rely on RCU to synchronize against concurrent changes.
>
> The propagation mentioned in 3) will be handled in further patches.
>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> ---
> include/linux/sched/isolation.h | 2 +
> kernel/cgroup/cpuset.c | 2 +
> kernel/sched/isolation.c | 75 ++++++++++++++++++++++++++++++---
> kernel/sched/sched.h | 1 +
> 4 files changed, 74 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/sched/isolation.h b/include/linux/sched/isolation.h
> index da22b038942a..94d5c835121b 100644
> --- a/include/linux/sched/isolation.h
> +++ b/include/linux/sched/isolation.h
> @@ -32,6 +32,7 @@ extern const struct cpumask *housekeeping_cpumask(enum hk_type type);
> extern bool housekeeping_enabled(enum hk_type type);
> extern void housekeeping_affine(struct task_struct *t, enum hk_type type);
> extern bool housekeeping_test_cpu(int cpu, enum hk_type type);
> +extern int housekeeping_update(struct cpumask *mask, enum hk_type type);
> extern void __init housekeeping_init(void);
>
> #else
> @@ -59,6 +60,7 @@ static inline bool housekeeping_test_cpu(int cpu, enum hk_type type)
> return true;
> }
>
> +static inline int housekeeping_update(struct cpumask *mask, enum hk_type type) { return 0; }
> static inline void housekeeping_init(void) { }
> #endif /* CONFIG_CPU_ISOLATION */
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index aa1ac7bcf2ea..b04a4242f2fa 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1403,6 +1403,8 @@ static void update_unbound_workqueue_cpumask(bool isolcpus_updated)
>
> ret = workqueue_unbound_exclude_cpumask(isolated_cpus);
> WARN_ON_ONCE(ret < 0);
> + ret = housekeeping_update(isolated_cpus, HK_TYPE_DOMAIN);
> + WARN_ON_ONCE(ret < 0);
> }
>
> /**
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index b46c20b5437f..95d69c2102f6 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -29,18 +29,48 @@ static struct housekeeping housekeeping;
>
> bool housekeeping_enabled(enum hk_type type)
> {
> - return !!(housekeeping.flags & BIT(type));
> + return !!(READ_ONCE(housekeeping.flags) & BIT(type));
> }
> EXPORT_SYMBOL_GPL(housekeeping_enabled);
>
> +static bool housekeeping_dereference_check(enum hk_type type)
> +{
> + if (IS_ENABLED(CONFIG_LOCKDEP) && type == HK_TYPE_DOMAIN) {
> + /* Cpuset isn't even writable yet? */
> + if (system_state <= SYSTEM_SCHEDULING)
> + return true;
> +
> + /* CPU hotplug write locked, so cpuset partition can't be overwritten */
> + if (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_write_held())
> + return true;
> +
> + /* Cpuset lock held, partitions not writable */
> + if (IS_ENABLED(CONFIG_CPUSETS) && lockdep_is_cpuset_held())
> + return true;
> +
> + return false;
> + }
> +
> + return true;
> +}
> +
> +static inline struct cpumask *housekeeping_cpumask_dereference(enum hk_type type)
> +{
> + return rcu_dereference_check(housekeeping.cpumasks[type],
> + housekeeping_dereference_check(type));
> +}
> +
> const struct cpumask *housekeeping_cpumask(enum hk_type type)
> {
> + const struct cpumask *mask = NULL;
> +
> if (static_branch_unlikely(&housekeeping_overridden)) {
> - if (housekeeping.flags & BIT(type)) {
> - return rcu_dereference_check(housekeeping.cpumasks[type], 1);
> - }
> + if (READ_ONCE(housekeeping.flags) & BIT(type))
> + mask = housekeeping_cpumask_dereference(type);
> }
> - return cpu_possible_mask;
> + if (!mask)
> + mask = cpu_possible_mask;
> + return mask;
> }
> EXPORT_SYMBOL_GPL(housekeeping_cpumask);
>
> @@ -80,12 +110,45 @@ EXPORT_SYMBOL_GPL(housekeeping_affine);
>
> bool housekeeping_test_cpu(int cpu, enum hk_type type)
> {
> - if (housekeeping.flags & BIT(type))
> + if (READ_ONCE(housekeeping.flags) & BIT(type))
> return cpumask_test_cpu(cpu, housekeeping_cpumask(type));
> return true;
> }
> EXPORT_SYMBOL_GPL(housekeeping_test_cpu);
>
> +int housekeeping_update(struct cpumask *mask, enum hk_type type)
> +{
> + struct cpumask *trial, *old = NULL;
> +
> + if (type != HK_TYPE_DOMAIN)
> + return -ENOTSUPP;
> +
> + trial = kmalloc(sizeof(*trial), GFP_KERNEL);
> + if (!trial)
> + return -ENOMEM;
> +
> + cpumask_andnot(trial, housekeeping_cpumask(HK_TYPE_DOMAIN_BOOT), mask);
> + if (!cpumask_intersects(trial, cpu_online_mask)) {
> + kfree(trial);
> + return -EINVAL;
> + }
> +
> + if (!housekeeping.flags)
> + static_branch_enable(&housekeeping_overridden);
> +
> + if (!(housekeeping.flags & BIT(type)))
> + old = housekeeping_cpumask_dereference(type);
> + else
> + WRITE_ONCE(housekeeping.flags, housekeeping.flags | BIT(type));
Isn't this backwards? If the bit is not set you save old to free it
and if the bit is set you set it again.
Cheers,
Phil
> + rcu_assign_pointer(housekeeping.cpumasks[type], trial);
> +
> + synchronize_rcu();
> +
> + kfree(old);
> +
> + return 0;
> +}
> +
> void __init housekeeping_init(void)
> {
> enum hk_type type;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 0c0ef8999fd6..8fac8aa451c6 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -30,6 +30,7 @@
> #include <linux/context_tracking.h>
> #include <linux/cpufreq.h>
> #include <linux/cpumask_api.h>
> +#include <linux/cpuset.h>
> #include <linux/ctype.h>
> #include <linux/file.h>
> #include <linux/fs_api.h>
> --
> 2.51.0
>
--
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