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Date: Wed, 26 Jan 2022 02:34:31 +0000
From: "Zhang, Qiang1" <qiang1.zhang@...el.com>
To: "paulmck@...nel.org" <paulmck@...nel.org>,
Ammar Faizi <ammarfaizi2@...weeb.org>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v3] rcu: Add per-CPU rcuc task info to RCU CPU stall
warnings
On Tue, Jan 25, 2022 at 02:04:04PM +0700, Ammar Faizi wrote:
> On 1/25/22 9:47 AM, Zqiang wrote:
> > When the 'use_softirq' be set zero, all RCU_SOFTIRQ processing be
> > moved to per-CPU rcuc kthreads, if the rcuc kthreads is being
> > starved, quiescent state can not report in time. the RCU stall may
> > be triggered. this commit adds a stack trace of this CPU and dump
> > rcuc kthreads stack to help analyze what prevents rcuc kthreads from
> > running.
> >
> > Suggested-by: Ammar Faizi <ammarfaizi2@...weeb.org>
> > Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> > ---
>
> For
> https://lore.kernel.org/lkml/20220125024744.4186726-1-qiang1.zhang@int
> el.com/T/
>
> ```
>
> Reviewed-by: Ammar Faizi <ammarfaizi2@...weeb.org>
>
> ```
>I queued this wordsmithed as shown below. Please check to see if I messed anything up. And thank you both!
>
> Thanx, Paul
>
Thanks Paul, this description is clearer.
>------------------------------------------------------------------------
>
>commit 72bdc0a6f8cfb7cc56238dc850b7fe236e7371cb
>Author: Zqiang <qiang1.zhang@...el.com>
>Date: Tue Jan 25 10:47:44 2022 +0800
>
> rcu: Add per-CPU rcuc task dumps to RCU CPU stall warnings
>
> When the rcutree.use_softirq kernel boot parameter is set to zero, all
> RCU_SOFTIRQ processing is carried out by the per-CPU rcuc kthreads.
> If these kthreads are being starved, quiescent states will not be
> reported, which in turn means that the grace period will not end, which
> can in turn trigger RCU CPU stall warnings. This commit therefore dumps
> stack traces of stalled CPUs' rcuc kthreads, which can help identify
> what is preventing those kthreads from running.
>
> Suggested-by: Ammar Faizi <ammarfaizi2@...weeb.org>
> Reviewed-by: Ammar Faizi <ammarfaizi2@...weeb.org>
> Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
>diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index f071c49b37118..e6ad532cffe78 100644
>--- a/kernel/rcu/tree.c
>+++ b/kernel/rcu/tree.c
>@@ -2859,10 +2859,12 @@ static void rcu_cpu_kthread(unsigned int cpu) {
> unsigned int *statusp = this_cpu_ptr(&rcu_data.rcu_cpu_kthread_status);
> char work, *workp = this_cpu_ptr(&rcu_data.rcu_cpu_has_work);
>+ unsigned long *j = this_cpu_ptr(&rcu_data.rcuc_activity);
> int spincnt;
>
> trace_rcu_utilization(TPS("Start CPU kthread@..._run"));
> for (spincnt = 0; spincnt < 10; spincnt++) {
>+ WRITE_ONCE(*j, jiffies);
> local_bh_disable();
> *statusp = RCU_KTHREAD_RUNNING;
> local_irq_disable();
>@@ -2883,6 +2885,7 @@ static void rcu_cpu_kthread(unsigned int cpu)
> schedule_timeout_idle(2);
> trace_rcu_utilization(TPS("End CPU kthread@..._yield"));
> *statusp = RCU_KTHREAD_WAITING;
>+ WRITE_ONCE(*j, jiffies);
> }
>
> static struct smp_hotplug_thread rcu_cpu_thread_spec = { diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index a87f31b1701db..926673ebe355f 100644
>--- a/kernel/rcu/tree.h
>+++ b/kernel/rcu/tree.h
>@@ -241,6 +241,7 @@ struct rcu_data {
> /* rcuc per-CPU kthread or NULL. */
> unsigned int rcu_cpu_kthread_status;
> char rcu_cpu_has_work;
>+ unsigned long rcuc_activity;
>
> /* 7) Diagnostic data, including RCU CPU stall warnings. */
> unsigned int softirq_snap; /* Snapshot of softirq activity. */
>diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 4781aa338c7c1..6082dd23408f8 100644
>--- a/kernel/rcu/tree_plugin.h
>+++ b/kernel/rcu/tree_plugin.h
>@@ -994,12 +994,15 @@ dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
> */
> static void rcu_cpu_kthread_setup(unsigned int cpu) {
>+ struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> #ifdef CONFIG_RCU_BOOST
> struct sched_param sp;
>
> sp.sched_priority = kthread_prio;
> sched_setscheduler_nocheck(current, SCHED_FIFO, &sp); #endif /* #ifdef CONFIG_RCU_BOOST */
>+
>+ WRITE_ONCE(rdp->rcuc_activity, jiffies);
> }
>
> #ifdef CONFIG_RCU_BOOST
>diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h index 21bebf7c9030b..0c5d8516516af 100644
>--- a/kernel/rcu/tree_stall.h
>+++ b/kernel/rcu/tree_stall.h
>@@ -379,6 +379,15 @@ static bool rcu_is_gp_kthread_starving(unsigned long *jp)
> return j > 2 * HZ;
> }
>
>+static bool rcu_is_rcuc_kthread_starving(struct rcu_data *rdp, unsigned
>+long *jp) {
>+ unsigned long j = jiffies - READ_ONCE(rdp->rcuc_activity);
>+
>+ if (jp)
>+ *jp = j;
>+ return j > 2 * HZ;
>+}
>+
> /*
> * Print out diagnostic information for the specified stalled CPU.
> *
>@@ -430,6 +439,29 @@ static void print_cpu_stall_info(int cpu)
> falsepositive ? " (false positive?)" : ""); }
>
>+static void rcuc_kthread_dump(struct rcu_data *rdp) {
>+ int cpu;
>+ unsigned long j;
>+ struct task_struct *rcuc;
>+
>+ rcuc = rdp->rcu_cpu_kthread_task;
>+ if (!rcuc)
>+ return;
>+
>+ cpu = task_cpu(rcuc);
>+ if (cpu_is_offline(cpu) || idle_cpu(cpu))
>+ return;
>+
>+ if (!rcu_is_rcuc_kthread_starving(rdp, &j))
>+ return;
>+
>+ pr_err("%s kthread starved for %ld jiffies\n", rcuc->comm, j);
>+ sched_show_task(rcuc);
>+ if (!trigger_single_cpu_backtrace(cpu))
>+ dump_cpu_task(cpu);
>+}
>+
> /* Complain about starvation of grace-period kthread. */ static void rcu_check_gp_kthread_starvation(void)
> {
>@@ -601,6 +633,9 @@ static void print_cpu_stall(unsigned long gps)
> rcu_check_gp_kthread_expired_fqs_timer();
> rcu_check_gp_kthread_starvation();
>
>+ if (!use_softirq)
>+ rcuc_kthread_dump(rdp);
>+
> rcu_dump_cpu_stacks();
>
> raw_spin_lock_irqsave_rcu_node(rnp, flags);
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