Re: [RFC PATCH] PM: Optionally block user fork during freeze to improve performance

[David Hildenbrand <david@redhat.com>]

Hi,

On 06.06.25 08:25, Zihuan Zhang wrote:
> Currently, the freezer traverses all tasks to freeze them during
> system suspend or hibernation. If a user process forks during this
> window, the new child may escape freezing and require a second
> traversal or retry, adding non-trivial overhead.
> 
> This patch introduces a CONFIG_PM_DISABLE_USER_FORK_DURING_FREEZE

Not sure if a Kconfig is really the right choice here ...

> option. When enabled, it prevents user processes from creating new
> processes (via fork/clone) during the freezing period. This guarantees
> a stable task list and avoids re-traversing the process list due to
> late-created user tasks, thereby improving performance.

Any performance numbers to back your claims?

> 
> The restriction is only active during the window when the system is
> freezing user tasks. Once all tasks are frozen, or if the system aborts
> the suspend/hibernate process, the restriction is lifted.
> No kernel threads are affected, and kernel_create_* functions remain
> unrestricted.
> 
> Signed-off-by: Zihuan Zhang <zhangzihuan@kylinos.cn>
> ---
>   include/linux/suspend.h |  8 ++++++++
>   kernel/fork.c           |  6 ++++++
>   kernel/power/Kconfig    | 10 ++++++++++
>   kernel/power/main.c     | 44 +++++++++++++++++++++++++++++++++++++++++
>   kernel/power/power.h    |  4 ++++
>   kernel/power/process.c  |  7 +++++++
>   6 files changed, 79 insertions(+)
> 
> diff --git a/include/linux/suspend.h b/include/linux/suspend.h
> index b1c76c8f2c82..2dd8b3eb50f0 100644
> --- a/include/linux/suspend.h
> +++ b/include/linux/suspend.h
> @@ -591,4 +591,12 @@ enum suspend_stat_step {
>   void dpm_save_failed_dev(const char *name);
>   void dpm_save_failed_step(enum suspend_stat_step step);
>   
> +#ifdef CONFIG_PM_DISABLE_USER_FORK_DURING_FREEZE
> +extern bool pm_block_user_fork;
> +bool pm_should_block_fork(void);
> +bool pm_freeze_process_in_progress(void);
> +#else
> +static inline bool pm_should_block_fork(void) { return false; };
> +static inline bool pm_freeze_process_in_progress(void) { return false; };
> +#endif /* CONFIG_PM_DISABLE_USER_FORK_DURING_FREEZE */
>   #endif /* _LINUX_SUSPEND_H */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 1ee8eb11f38b..b0bd0206b644 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -105,6 +105,7 @@
>   #include <uapi/linux/pidfd.h>
>   #include <linux/pidfs.h>
>   #include <linux/tick.h>
> +#include <linux/suspend.h>
>   
>   #include <asm/pgalloc.h>
>   #include <linux/uaccess.h>
> @@ -2596,6 +2597,11 @@ pid_t kernel_clone(struct kernel_clone_args *args)
>   			trace = 0;
>   	}
>   
> +#ifdef CONFIG_PM_DISABLE_USER_FORK_DURING_FREEZE
> +	if (pm_should_block_fork() && !(current->flags & PF_KTHREAD))
> +		return -EBUSY;
> +#endif

fork() is not documented to return EBUSY and for clone3() it's 
documented to only happen in specific cases.

So user space is not prepared for that.

-- 
Cheers,

David / dhildenb