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Message-ID: <aGfgUq9Tfn8xlcar@tiehlicka>
Date: Fri, 4 Jul 2025 16:08:18 +0200
From: Michal Hocko <mhocko@...e.com>
To: Chen Yu <yu.c.chen@...el.com>
Cc: Ingo Molnar <mingo@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Peter Zijlstra <peterz@...radead.org>,
Jirka Hladky <jhladky@...hat.com>,
Srikanth Aithal <Srikanth.Aithal@....com>,
Suneeth D <Suneeth.D@....com>, Libo Chen <libo.chen@...cle.com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Revert "sched/numa: add statistics of numa balance task"
On Fri 04-07-25 21:56:20, Chen Yu wrote:
> This reverts commit ad6b26b6a0a79166b53209df2ca1cf8636296382.
>
> This commit introduces per-memcg/task NUMA balance statistics,
> but unfortunately it introduced a NULL pointer exception due
> to the following race condition: After a swap task candidate
> was chosen, its mm_struct pointer was set to NULL due to task
> exit. Later, when performing the actual task swapping, the
> p->mm caused the problem.
>
> CPU0 CPU1
> :
> ...
> task_numa_migrate
> task_numa_find_cpu
> task_numa_compare
> # a normal task p is chosen
> env->best_task = p
>
> # p exit:
> exit_signals(p);
> p->flags |= PF_EXITING
> exit_mm
> p->mm = NULL;
>
> migrate_swap_stop
> __migrate_swap_task((arg->src_task, arg->dst_cpu)
> count_memcg_event_mm(p->mm, NUMA_TASK_SWAP)# p->mm is NULL
>
> task_lock() should be held and the PF_EXITING flag needs to
> be checked to prevent this from happening. After discussion,
> the conclusion was that adding a lock is not worthwhile for
> some statistics calculations. Revert the change and rely on
> the tracepoint for this purpose.
>
> Fixes: ad6b26b6a0a7 ("sched/numa: add statistics of numa balance task")
> Reported-by: Jirka Hladky <jhladky@...hat.com>
> Closes: https://lore.kernel.org/all/CAE4VaGBLJxpd=NeRJXpSCuw=REhC5LWJpC29kDy-Zh2ZDyzQZA@mail.gmail.com/
> Reported-by: Srikanth Aithal <Srikanth.Aithal@....com>
> Reported-by: Suneeth D <Suneeth.D@....com>
> Signed-off-by: Chen Yu <yu.c.chen@...el.com>
Acked-by: Michal Hocko <mhocko@...e.com>
Thanks!
> ---
> Documentation/admin-guide/cgroup-v2.rst | 6 ------
> include/linux/sched.h | 4 ----
> include/linux/vm_event_item.h | 2 --
> kernel/sched/core.c | 9 ++-------
> kernel/sched/debug.c | 4 ----
> mm/memcontrol.c | 2 --
> mm/vmstat.c | 2 --
> 7 files changed, 2 insertions(+), 27 deletions(-)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 0cc35a14afbe..bd98ea3175ec 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1732,12 +1732,6 @@ The following nested keys are defined.
> numa_hint_faults (npn)
> Number of NUMA hinting faults.
>
> - numa_task_migrated (npn)
> - Number of task migration by NUMA balancing.
> -
> - numa_task_swapped (npn)
> - Number of task swap by NUMA balancing.
> -
> pgdemote_kswapd
> Number of pages demoted by kswapd.
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 4f78a64beb52..aa9c5be7a632 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -548,10 +548,6 @@ struct sched_statistics {
> u64 nr_failed_migrations_running;
> u64 nr_failed_migrations_hot;
> u64 nr_forced_migrations;
> -#ifdef CONFIG_NUMA_BALANCING
> - u64 numa_task_migrated;
> - u64 numa_task_swapped;
> -#endif
>
> u64 nr_wakeups;
> u64 nr_wakeups_sync;
> diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
> index 91a3ce9a2687..9e15a088ba38 100644
> --- a/include/linux/vm_event_item.h
> +++ b/include/linux/vm_event_item.h
> @@ -66,8 +66,6 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
> NUMA_HINT_FAULTS,
> NUMA_HINT_FAULTS_LOCAL,
> NUMA_PAGE_MIGRATE,
> - NUMA_TASK_MIGRATE,
> - NUMA_TASK_SWAP,
> #endif
> #ifdef CONFIG_MIGRATION
> PGMIGRATE_SUCCESS, PGMIGRATE_FAIL,
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 8988d38d46a3..ca0be74e865b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3362,10 +3362,6 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
> #ifdef CONFIG_NUMA_BALANCING
> static void __migrate_swap_task(struct task_struct *p, int cpu)
> {
> - __schedstat_inc(p->stats.numa_task_swapped);
> - count_vm_numa_event(NUMA_TASK_SWAP);
> - count_memcg_event_mm(p->mm, NUMA_TASK_SWAP);
> -
> if (task_on_rq_queued(p)) {
> struct rq *src_rq, *dst_rq;
> struct rq_flags srf, drf;
> @@ -7934,9 +7930,8 @@ int migrate_task_to(struct task_struct *p, int target_cpu)
> if (!cpumask_test_cpu(target_cpu, p->cpus_ptr))
> return -EINVAL;
>
> - __schedstat_inc(p->stats.numa_task_migrated);
> - count_vm_numa_event(NUMA_TASK_MIGRATE);
> - count_memcg_event_mm(p->mm, NUMA_TASK_MIGRATE);
> + /* TODO: This is not properly updating schedstats */
> +
> trace_sched_move_numa(p, curr_cpu, target_cpu);
> return stop_one_cpu(curr_cpu, migration_cpu_stop, &arg);
> }
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 9d71baf08075..557246880a7e 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -1210,10 +1210,6 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
> P_SCHEDSTAT(nr_failed_migrations_running);
> P_SCHEDSTAT(nr_failed_migrations_hot);
> P_SCHEDSTAT(nr_forced_migrations);
> -#ifdef CONFIG_NUMA_BALANCING
> - P_SCHEDSTAT(numa_task_migrated);
> - P_SCHEDSTAT(numa_task_swapped);
> -#endif
> P_SCHEDSTAT(nr_wakeups);
> P_SCHEDSTAT(nr_wakeups_sync);
> P_SCHEDSTAT(nr_wakeups_migrate);
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 902da8a9c643..70fdeda1120b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -474,8 +474,6 @@ static const unsigned int memcg_vm_event_stat[] = {
> NUMA_PAGE_MIGRATE,
> NUMA_PTE_UPDATES,
> NUMA_HINT_FAULTS,
> - NUMA_TASK_MIGRATE,
> - NUMA_TASK_SWAP,
> #endif
> };
>
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 429ae5339bfe..a78d70ddeacd 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1346,8 +1346,6 @@ const char * const vmstat_text[] = {
> "numa_hint_faults",
> "numa_hint_faults_local",
> "numa_pages_migrated",
> - "numa_task_migrated",
> - "numa_task_swapped",
> #endif
> #ifdef CONFIG_MIGRATION
> "pgmigrate_success",
> --
> 2.25.1
>
--
Michal Hocko
SUSE Labs
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