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Mon, 22 Sep 2025 02:41:56 -0700 (PDT) From: Julian Sun To: cgroups@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Cc: viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz, mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, akpm@linux-foundation.org, lance.yang@linux.dev, mhiramat@kernel.org, agruenba@redhat.com, hannes@cmpxchg.org, mhocko@kernel.org, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev Subject: [PATCH 1/3] sched: Introduce a new flag PF_DONT_HUNG. Date: Mon, 22 Sep 2025 17:41:44 +0800 Message-Id: <20250922094146.708272-2-sunjunchao@bytedance.com> X-Mailer: git-send-email 2.39.5 In-Reply-To: <20250922094146.708272-1-sunjunchao@bytedance.com> References: <20250922094146.708272-1-sunjunchao@bytedance.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" In the kernel, long waits can trigger hung task warnings. However, some warnings are undesirable and unnecessary - for example, a hung task warning triggered when a background kworker waits for writeback completion during resource cleanup(like the context of mem_cgroup_css_free()). This kworker does not affect any user behavior and there is no erroneous behavior at the kernel code level, yet it triggers an annoying hung task warning. This patch adds a new flag to task_struct to instruct the hung task detector to ignore such cases, as suggested by Andrew Morton in [1]. Also introduces current_set/clear_flags() to prepare for the next patch. [1]: https://lore.kernel.org/cgroups/20250917152155.5a8ddb3e4ff813289ea0b4c= 9@linux-foundation.org/ Signed-off-by: Julian Sun --- include/linux/sched.h | 12 +++++++++++- kernel/hung_task.c | 6 ++++++ 2 files changed, 17 insertions(+), 1 deletion(-) diff --git a/include/linux/sched.h b/include/linux/sched.h index f8188b833350..cd70ff051b2a 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1718,6 +1718,16 @@ static inline char task_state_to_char(struct task_st= ruct *tsk) return task_index_to_char(task_state_index(tsk)); } =20 +static inline void current_set_flags(unsigned int flags) +{ + current->flags |=3D flags; +} + +static inline void current_clear_flags(unsigned int flags) +{ + current->flags &=3D ~flags; +} + extern struct pid *cad_pid; =20 /* @@ -1747,7 +1757,7 @@ extern struct pid *cad_pid; * I am cleaning dirty pages from some other bdi. */ #define PF_KTHREAD 0x00200000 /* I am a kernel thread */ #define PF_RANDOMIZE 0x00400000 /* Randomize virtual address space */ -#define PF__HOLE__00800000 0x00800000 +#define PF_DONT_HUNG 0x00800000 /* Don't trigger hung task warning when w= aiting for something */ #define PF__HOLE__01000000 0x01000000 #define PF__HOLE__02000000 0x02000000 #define PF_NO_SETAFFINITY 0x04000000 /* Userland is not allowed to meddle = with cpus_mask */ diff --git a/kernel/hung_task.c b/kernel/hung_task.c index 8708a1205f82..b16d72276d01 100644 --- a/kernel/hung_task.c +++ b/kernel/hung_task.c @@ -208,6 +208,12 @@ static void check_hung_task(struct task_struct *t, uns= igned long timeout) t->last_switch_time =3D jiffies; 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Mon, 22 Sep 2025 02:42:01 -0700 (PDT) From: Julian Sun To: cgroups@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Cc: viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz, mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, akpm@linux-foundation.org, lance.yang@linux.dev, mhiramat@kernel.org, agruenba@redhat.com, hannes@cmpxchg.org, mhocko@kernel.org, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev Subject: [PATCH 2/3] writeback: Introduce wb_wait_for_completion_no_hung(). Date: Mon, 22 Sep 2025 17:41:45 +0800 Message-Id: <20250922094146.708272-3-sunjunchao@bytedance.com> X-Mailer: git-send-email 2.39.5 In-Reply-To: <20250922094146.708272-1-sunjunchao@bytedance.com> References: <20250922094146.708272-1-sunjunchao@bytedance.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" This patch introduces wait_event_no_hung() and wb_wait_for_completion_no_hung() to make hung task detector ignore the current task if it waits for a long time. Signed-off-by: Julian Sun --- fs/fs-writeback.c | 15 +++++++++++++++ include/linux/backing-dev.h | 1 + include/linux/wait.h | 15 +++++++++++++++ 3 files changed, 31 insertions(+) diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index a07b8cf73ae2..eebb7f145734 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -216,6 +216,21 @@ void wb_wait_for_completion(struct wb_completion *done) wait_event(*done->waitq, !atomic_read(&done->cnt)); } =20 +/* + * Same as wb_wait_for_completion() but hung task warning will not be + * triggered if it wait for a long time. Use this function with caution + * unless you are sure that the hung task is undesirable. + * When is this undesirable? From the kernel code perspective, there is + * no misbehavior and it has no impact on user behavior. For example, a + * *background* kthread/kworker used to clean resources while waiting a + * long time for writeback to finish. + */ + void wb_wait_for_completion_no_hung(struct wb_completion *done) + { + atomic_dec(&done->cnt); /* put down the initial count */ + wait_event_no_hung(*done->waitq, !atomic_read(&done->cnt)); + } + #ifdef CONFIG_CGROUP_WRITEBACK =20 /* diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h index e721148c95d0..9d3335866f6f 100644 --- a/include/linux/backing-dev.h +++ b/include/linux/backing-dev.h @@ -40,6 +40,7 @@ void wb_start_background_writeback(struct bdi_writeback *= wb); void wb_workfn(struct work_struct *work); =20 void wb_wait_for_completion(struct wb_completion *done); +void wb_wait_for_completion_no_hung(struct wb_completion *done); =20 extern spinlock_t bdi_lock; extern struct list_head bdi_list; diff --git a/include/linux/wait.h b/include/linux/wait.h index 09855d819418..8f05d0bb8db7 100644 --- a/include/linux/wait.h +++ b/include/linux/wait.h @@ -330,6 +330,21 @@ __out: __ret; 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Mon, 22 Sep 2025 02:42:06 -0700 (PDT) Received: from localhost ([106.38.226.98]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-b55268e73f6sm7544132a12.21.2025.09.22.02.42.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 22 Sep 2025 02:42:06 -0700 (PDT) From: Julian Sun To: cgroups@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Cc: viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz, mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, akpm@linux-foundation.org, lance.yang@linux.dev, mhiramat@kernel.org, agruenba@redhat.com, hannes@cmpxchg.org, mhocko@kernel.org, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev Subject: [PATCH 3/3] memcg: Don't trigger hung task warnings when memcg is releasing resources. Date: Mon, 22 Sep 2025 17:41:46 +0800 Message-Id: <20250922094146.708272-4-sunjunchao@bytedance.com> X-Mailer: git-send-email 2.39.5 In-Reply-To: <20250922094146.708272-1-sunjunchao@bytedance.com> References: <20250922094146.708272-1-sunjunchao@bytedance.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Hung task warning in mem_cgroup_css_free() is undesirable and unnecessary since it does not affect any user behavior and there is no misbehavior at the kernel code level. Use wb_wait_for_completion_no_hung() to eliminate the possible hung task warning. Signed-off-by: Julian Sun --- mm/memcontrol.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 8dd7fbed5a94..b7d9e795dd64 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -3913,7 +3913,7 @@ static void mem_cgroup_css_free(struct cgroup_subsys_= state *css) =20 #ifdef CONFIG_CGROUP_WRITEBACK for (i =3D 0; i < MEMCG_CGWB_FRN_CNT; i++) - wb_wait_for_completion(&memcg->cgwb_frn[i].done); + wb_wait_for_completion_no_hung(&memcg->cgwb_frn[i].done); #endif if (cgroup_subsys_on_dfl(memory_cgrp_subsys) && !cgroup_memory_nosocket) static_branch_dec(&memcg_sockets_enabled_key); --=20 2.39.5