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Message-ID: <9a085dcd-be41-4244-a35e-014e0a1d0fd6@redhat.com>
Date: Wed, 3 Sep 2025 09:14:21 -0400
From: Waiman Long <llong@...hat.com>
To: Yi Tao <escape@...ux.alibaba.com>, tj@...nel.org, hannes@...xchg.org,
mkoutny@...e.com
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cgroup: replace global percpu_rwsem with
signal_struct->group_rwsem when writing cgroup.procs/threads
On 9/3/25 7:11 AM, Yi Tao wrote:
> As computer hardware advances, modern systems are typically equipped
> with many CPU cores and large amounts of memory, enabling the deployment
> of numerous applications. On such systems, container creation and
> deletion become frequent operations, making cgroup process migration no
> longer a cold path. This leads to noticeable contention with common
> process operations such as fork, exec, and exit.
>
> To alleviate the contention between cgroup process migration and
> operations like process fork, this patch modifies lock to take the write
> lock on signal_struct->group_rwsem when writing pid to
> cgroup.procs/threads instead of holding a global write lock.
>
> Cgroup process migration has historically relied on
> signal_struct->group_rwsem to protect thread group integrity. In commit
> <1ed1328792ff> ("sched, cgroup: replace signal_struct->group_rwsem with
> a global percpu_rwsem"), this was changed to a global
> cgroup_threadgroup_rwsem. The advantage of using a global lock was
> simplified handling of process group migrations. This patch retains the
> use of the global lock for protecting process group migration, while
> reducing contention by using per thread group lock during
> cgroup.procs/threads writes.
>
> The locking behavior is as follows:
>
> write cgroup.procs/threads | process fork,exec,exit | process group migration
> ------------------------------------------------------------------------------
> cgroup_lock() | down_read(&g_rwsem) | cgroup_lock()
> down_write(&p_rwsem) | down_read(&p_rwsem) | down_write(&g_rwsem)
> critical section | critical section | critical section
> up_write(&p_rwsem) | up_read(&p_rwsem) | up_write(&g_rwsem)
> cgroup_unlock() | up_read(&g_rwsem) | cgroup_unlock()
>
> g_rwsem denotes cgroup_threadgroup_rwsem, p_rwsem denotes
> signal_struct->group_rwsem.
>
> This patch eliminates contention between cgroup migration and fork
> operations for threads that belong to different thread groups, thereby
> reducing the long-tail latency of cgroup migrations and lowering system
> load.
Do you have any performance numbers to showcase how much performance
benefit does this change provide?
> Signed-off-by: Yi Tao <escape@...ux.alibaba.com>
> ---
> include/linux/cgroup-defs.h | 2 ++
> include/linux/sched/signal.h | 4 +++
> init/init_task.c | 3 ++
> kernel/cgroup/cgroup-internal.h | 6 ++--
> kernel/cgroup/cgroup-v1.c | 8 ++---
> kernel/cgroup/cgroup.c | 56 ++++++++++++++++++++-------------
> kernel/fork.c | 4 +++
> 7 files changed, 55 insertions(+), 28 deletions(-)
>
> diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
> index 6b93a64115fe..8e0fdad8a440 100644
> --- a/include/linux/cgroup-defs.h
> +++ b/include/linux/cgroup-defs.h
> @@ -838,6 +838,7 @@ struct cgroup_of_peak {
> static inline void cgroup_threadgroup_change_begin(struct task_struct *tsk)
> {
> percpu_down_read(&cgroup_threadgroup_rwsem);
> + down_read(&tsk->signal->group_rwsem);
> }
>
> /**
> @@ -848,6 +849,7 @@ static inline void cgroup_threadgroup_change_begin(struct task_struct *tsk)
> */
> static inline void cgroup_threadgroup_change_end(struct task_struct *tsk)
> {
> + up_read(&tsk->signal->group_rwsem);
> percpu_up_read(&cgroup_threadgroup_rwsem);
> }
>
> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
> index 1ef1edbaaf79..86fbc99a9174 100644
> --- a/include/linux/sched/signal.h
> +++ b/include/linux/sched/signal.h
> @@ -226,6 +226,10 @@ struct signal_struct {
> struct tty_audit_buf *tty_audit_buf;
> #endif
>
> +#ifdef CONFIG_CGROUPS
> + struct rw_semaphore group_rwsem;
> +#endif
> +
> /*
> * Thread is the potential origin of an oom condition; kill first on
> * oom
> diff --git a/init/init_task.c b/init/init_task.c
> index e557f622bd90..0450093924a7 100644
> --- a/init/init_task.c
> +++ b/init/init_task.c
> @@ -27,6 +27,9 @@ static struct signal_struct init_signals = {
> },
> .multiprocess = HLIST_HEAD_INIT,
> .rlim = INIT_RLIMITS,
> +#ifdef CONFIG_CGROUPS
> + .group_rwsem = __RWSEM_INITIALIZER(init_signals.group_rwsem),
> +#endif
> .cred_guard_mutex = __MUTEX_INITIALIZER(init_signals.cred_guard_mutex),
> .exec_update_lock = __RWSEM_INITIALIZER(init_signals.exec_update_lock),
> #ifdef CONFIG_POSIX_TIMERS
> diff --git a/kernel/cgroup/cgroup-internal.h b/kernel/cgroup/cgroup-internal.h
> index b14e61c64a34..572c24b7e947 100644
> --- a/kernel/cgroup/cgroup-internal.h
> +++ b/kernel/cgroup/cgroup-internal.h
> @@ -249,8 +249,10 @@ int cgroup_migrate(struct task_struct *leader, bool threadgroup,
>
> int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
> bool threadgroup);
> -void cgroup_attach_lock(bool lock_threadgroup);
> -void cgroup_attach_unlock(bool lock_threadgroup);
> +void cgroup_attach_lock(struct task_struct *tsk, bool lock_threadgroup,
> + bool global);
> +void cgroup_attach_unlock(struct task_struct *tsk, bool lock_threadgroup,
> + bool global);
> struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup,
> bool *locked)
> __acquires(&cgroup_threadgroup_rwsem);
> diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
> index 2a4a387f867a..3e5ead8c5bc5 100644
> --- a/kernel/cgroup/cgroup-v1.c
> +++ b/kernel/cgroup/cgroup-v1.c
> @@ -68,7 +68,7 @@ int cgroup_attach_task_all(struct task_struct *from, struct task_struct *tsk)
> int retval = 0;
>
> cgroup_lock();
> - cgroup_attach_lock(true);
> + cgroup_attach_lock(NULL, true, true);
> for_each_root(root) {
> struct cgroup *from_cgrp;
>
> @@ -80,7 +80,7 @@ int cgroup_attach_task_all(struct task_struct *from, struct task_struct *tsk)
> if (retval)
> break;
> }
> - cgroup_attach_unlock(true);
> + cgroup_attach_unlock(NULL, true, true);
> cgroup_unlock();
>
> return retval;
> @@ -117,7 +117,7 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
>
> cgroup_lock();
>
> - cgroup_attach_lock(true);
> + cgroup_attach_lock(NULL, true, true);
>
> /* all tasks in @from are being moved, all csets are source */
> spin_lock_irq(&css_set_lock);
> @@ -153,7 +153,7 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
> } while (task && !ret);
> out_err:
> cgroup_migrate_finish(&mgctx);
> - cgroup_attach_unlock(true);
> + cgroup_attach_unlock(NULL, true, true);
> cgroup_unlock();
> return ret;
> }
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 312c6a8b55bb..4e1d80a2741f 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -2480,21 +2480,31 @@ EXPORT_SYMBOL_GPL(cgroup_path_ns);
> * write-locking cgroup_threadgroup_rwsem. This allows ->attach() to assume that
> * CPU hotplug is disabled on entry.
> */
> -void cgroup_attach_lock(bool lock_threadgroup)
> +void cgroup_attach_lock(struct task_struct *tsk,
> + bool lock_threadgroup, bool global)
> {
> cpus_read_lock();
> - if (lock_threadgroup)
> - percpu_down_write(&cgroup_threadgroup_rwsem);
> + if (lock_threadgroup) {
> + if (global)
> + percpu_down_write(&cgroup_threadgroup_rwsem);
> + else
> + down_write(&tsk->signal->group_rwsem);
> + }
> }
tsk is only used when global is false. So you can eliminate the
redundant global argument and use the presence of tsk to indicate which
lock to take.
Cheers,
Longman
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