Re: [PATCH v3 2/2] mm/oom_kill: Only delay OOM reaper for processes using robust futex

[Michal Hocko <mhocko@suse.com>]

I have only noticed this now. Did you have any reason to repost v3
without any prior feedback on v2 and without any changelog from v2?

On Mon 04-08-25 11:03:41, zhongjinji@honor.com wrote:
> From: zhongjinji <zhongjinji@honor.com>
> 
> After merging the patch
> https://lore.kernel.org/all/20220414144042.677008-1-npache@redhat.com/T/#u,
> the OOM reaper runs less frequently because many processes exit within 2 seconds.
> 
> However, when a process is killed, timely handling by the OOM reaper allows
> its memory to be freed faster.
> 
> Since relatively few processes use robust futex, delaying the OOM reaper for
> all processes is undesirable, as many killed processes cannot release memory
> more quickly.
> 
> This patch modifies the behavior so that only processes using robust futex
> are delayed by the OOM reaper, allowing the OOM reaper to handle more
> processes in a timely manner.
> 
> Signed-off-by: zhongjinji <zhongjinji@honor.com>
> ---
>  mm/oom_kill.c | 41 +++++++++++++++++++++++++++++++----------
>  1 file changed, 31 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 25923cfec9c6..c558ac93ae7d 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -30,6 +30,7 @@
>  #include <linux/syscalls.h>
>  #include <linux/timex.h>
>  #include <linux/jiffies.h>
> +#include <linux/futex.h>
>  #include <linux/cpuset.h>
>  #include <linux/export.h>
>  #include <linux/notifier.h>
> @@ -692,7 +693,7 @@ static void wake_oom_reaper(struct timer_list *timer)
>   * before the exit path is able to wake the futex waiters.
>   */
>  #define OOM_REAPER_DELAY (2*HZ)
> -static void queue_oom_reaper(struct task_struct *tsk)
> +static void queue_oom_reaper(struct task_struct *tsk, bool delay)
>  {
>  	/* mm is already queued? */
>  	if (test_and_set_bit(MMF_OOM_REAP_QUEUED, &tsk->signal->oom_mm->flags))
> @@ -700,7 +701,7 @@ static void queue_oom_reaper(struct task_struct *tsk)
>  
>  	get_task_struct(tsk);
>  	timer_setup(&tsk->oom_reaper_timer, wake_oom_reaper, 0);
> -	tsk->oom_reaper_timer.expires = jiffies + OOM_REAPER_DELAY;
> +	tsk->oom_reaper_timer.expires = jiffies + (delay ? OOM_REAPER_DELAY : 0);
>  	add_timer(&tsk->oom_reaper_timer);
>  }
>  
> @@ -742,7 +743,7 @@ static int __init oom_init(void)
>  }
>  subsys_initcall(oom_init)
>  #else
> -static inline void queue_oom_reaper(struct task_struct *tsk)
> +static inline void queue_oom_reaper(struct task_struct *tsk, bool delay)
>  {
>  }
>  #endif /* CONFIG_MMU */
> @@ -871,11 +872,12 @@ static inline bool __task_will_free_mem(struct task_struct *task)
>   * Caller has to make sure that task->mm is stable (hold task_lock or
>   * it operates on the current).
>   */
> -static bool task_will_free_mem(struct task_struct *task)
> +static bool task_will_free_mem(struct task_struct *task, bool *delay_reap)
>  {
>  	struct mm_struct *mm = task->mm;
>  	struct task_struct *p;
>  	bool ret = true;
> +	bool has_robust = !delay_reap;
>  
>  	/*
>  	 * Skip tasks without mm because it might have passed its exit_mm and
> @@ -888,6 +890,15 @@ static bool task_will_free_mem(struct task_struct *task)
>  	if (!__task_will_free_mem(task))
>  		return false;
>  
> +	/*
> +	 * Check if a process is using robust futexes. If so, delay its handling by the
> +	 * OOM reaper. The reason is that if the owner of a robust futex lock is killed
> +	 * while waiters are still alive, the OOM reaper might free the robust futex
> +	 * resources before futex_cleanup runs, causing the waiters to wait indefinitely.
> +	 */
> +	if (!has_robust)
> +		has_robust = check_robust_futex_rcu(task);
> +
>  	/*
>  	 * This task has already been drained by the oom reaper so there are
>  	 * only small chances it will free some more
> @@ -912,8 +923,12 @@ static bool task_will_free_mem(struct task_struct *task)
>  		ret = __task_will_free_mem(p);
>  		if (!ret)
>  			break;
> +		if (!has_robust)
> +			has_robust = check_robust_futex(p);
>  	}
>  	rcu_read_unlock();
> +	if (delay_reap)
> +		*delay_reap = has_robust;
>  
>  	return ret;
>  }
> @@ -923,6 +938,7 @@ static void __oom_kill_process(struct task_struct *victim, const char *message)
>  	struct task_struct *p;
>  	struct mm_struct *mm;
>  	bool can_oom_reap = true;
> +	bool delay_reap;
>  
>  	p = find_lock_task_mm(victim);
>  	if (!p) {
> @@ -950,6 +966,7 @@ static void __oom_kill_process(struct task_struct *victim, const char *message)
>  	 * reserves from the user space under its control.
>  	 */
>  	do_send_sig_info(SIGKILL, SEND_SIG_PRIV, victim, PIDTYPE_TGID);
> +	delay_reap = check_robust_futex_rcu(victim);
>  	mark_oom_victim(victim);
>  	pr_err("%s: Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB, UID:%u pgtables:%lukB oom_score_adj:%hd\n",
>  		message, task_pid_nr(victim), victim->comm, K(mm->total_vm),
> @@ -990,11 +1007,13 @@ static void __oom_kill_process(struct task_struct *victim, const char *message)
>  		if (unlikely(p->flags & PF_KTHREAD))
>  			continue;
>  		do_send_sig_info(SIGKILL, SEND_SIG_PRIV, p, PIDTYPE_TGID);
> +		if (!delay_reap)
> +			delay_reap = check_robust_futex(p);
>  	}
>  	rcu_read_unlock();
>  
>  	if (can_oom_reap)
> -		queue_oom_reaper(victim);
> +		queue_oom_reaper(victim, delay_reap);
>  
>  	mmdrop(mm);
>  	put_task_struct(victim);
> @@ -1020,6 +1039,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
>  	struct mem_cgroup *oom_group;
>  	static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
>  					      DEFAULT_RATELIMIT_BURST);
> +	bool delay_reap = false;
>  
>  	/*
>  	 * If the task is already exiting, don't alarm the sysadmin or kill
> @@ -1027,9 +1047,9 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
>  	 * so it can die quickly
>  	 */
>  	task_lock(victim);
> -	if (task_will_free_mem(victim)) {
> +	if (task_will_free_mem(victim, &delay_reap)) {
>  		mark_oom_victim(victim);
> -		queue_oom_reaper(victim);
> +		queue_oom_reaper(victim, delay_reap);
>  		task_unlock(victim);
>  		put_task_struct(victim);
>  		return;
> @@ -1112,6 +1132,7 @@ EXPORT_SYMBOL_GPL(unregister_oom_notifier);
>  bool out_of_memory(struct oom_control *oc)
>  {
>  	unsigned long freed = 0;
> +	bool delay_reap = false;
>  
>  	if (oom_killer_disabled)
>  		return false;
> @@ -1128,9 +1149,9 @@ bool out_of_memory(struct oom_control *oc)
>  	 * select it.  The goal is to allow it to allocate so that it may
>  	 * quickly exit and free its memory.
>  	 */
> -	if (task_will_free_mem(current)) {
> +	if (task_will_free_mem(current, &delay_reap)) {
>  		mark_oom_victim(current);
> -		queue_oom_reaper(current);
> +		queue_oom_reaper(current, delay_reap);
>  		return true;
>  	}
>  
> @@ -1231,7 +1252,7 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
>  	mm = p->mm;
>  	mmgrab(mm);
>  
> -	if (task_will_free_mem(p))
> +	if (task_will_free_mem(p, NULL))
>  		reap = true;
>  	else {
>  		/* Error only if the work has not been done already */
> -- 
> 2.17.1

-- 
Michal Hocko
SUSE Labs