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Message-ID: <aJBNbnNWa5KLciOq@tiehlicka>
Date: Mon, 4 Aug 2025 08:04:30 +0200
From: Michal Hocko <mhocko@...e.com>
To: zhongjinji@...or.com
Cc: linux-mm@...ck.org, akpm@...ux-foundation.org, rientjes@...gle.com,
shakeel.butt@...ux.dev, npache@...hat.com,
linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
peterz@...radead.org, dvhart@...radead.org, dave@...olabs.net,
andrealmeid@...lia.com, liulu.liu@...or.com, feng.han@...or.com
Subject: Re: [PATCH v3 2/2] mm/oom_kill: Only delay OOM reaper for processes
using robust futex
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@...or.com wrote:
> From: zhongjinji <zhongjinji@...or.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@...or.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
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