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Subject: Re: scheduler problems in -next (was: Re: [PATCH 6.4 000/227]
 6.4.7-rc1 review)
From:   Roy Hopkins <rhopkins@suse.de>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Guenter Roeck <linux@roeck-us.net>,
        Joel Fernandes <joel@joelfernandes.org>, paulmck@kernel.org,
        Pavel Machek <pavel@denx.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, patches@lists.linux.dev,
        linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
        akpm@linux-foundation.org, shuah@kernel.org, patches@kernelci.org,
        lkft-triage@lists.linaro.org, jonathanh@nvidia.com,
        f.fainelli@gmail.com, sudipm.mukherjee@gmail.com,
        srw@sladewatkins.net, rwarsow@gmx.de, conor@kernel.org,
        rcu@vger.kernel.org, Ingo Molnar <mingo@kernel.org>
Date:   Mon, 31 Jul 2023 17:30:17 +0100
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On Mon, 2023-07-31 at 18:14 +0200, Peter Zijlstra wrote:
> Ha!, I was poking around the same thing. My hack below seems to (so far,
> <20 boots) help things.
>=20
>=20
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index 56c470a489c8..b083b5a30025 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -652,7 +658,11 @@ static void __init rcu_spawn_tasks_kthread_generic(s=
truct rcu_tasks *rtp)
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0t =3D kthread_run(rcu_tas=
ks_kthread, rtp, "%s_kthread", rtp->kname);
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if (WARN_ONCE(IS_ERR(t), =
"%s: Could not start %s grace-period kthread, OOM is now expected behavior\=
n", __func__, rtp->name))
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0return;
> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0smp_mb(); /* Ensure others see=
 full kthread. */
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0for (;;) {
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0cond_resched();
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0if (smp_load_acquire(&rtp->kthread_ptr))
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0break;
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0}
> =C2=A0}
> =C2=A0
> =C2=A0#ifndef CONFIG_TINY_RCU

FWIW, here's my hack which seems to fix it.

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 9b9ce09f8f35..2e76fbfff9c6 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -52,6 +52,7 @@ struct rcu_tasks_percpu {
  * @cbs_gbl_lock: Lock protecting callback list.
  * @tasks_gp_mutex: Mutex protecting grace period, needed during mid-boot =
dead zone.
  * @kthread_ptr: This flavor's grace-period/callback-invocation kthread.
+ * @kthread_started: Flag that indicates whether kthread has been launched=
.
  * @gp_func: This flavor's grace-period-wait function.
  * @gp_state: Grace period's most recent state transition (debugging).
  * @gp_sleep: Per-grace-period sleep to prevent CPU-bound looping.
@@ -92,6 +93,7 @@ struct rcu_tasks {
        unsigned long n_ipis;
        unsigned long n_ipis_fails;
        struct task_struct *kthread_ptr;
+       int kthread_started;
        rcu_tasks_gp_func_t gp_func;
        pregp_func_t pregp_func;
        pertask_func_t pertask_func;
@@ -582,7 +584,7 @@ static void synchronize_rcu_tasks_generic(struct rcu_ta=
sks *rtp)
                return;
=20
        // If the grace-period kthread is running, use it.
-       if (READ_ONCE(rtp->kthread_ptr)) {
+       if (READ_ONCE(rtp->kthread_started)) {
                wait_rcu_gp(rtp->call_func);
                return;
        }
@@ -595,6 +597,7 @@ static void __init rcu_spawn_tasks_kthread_generic(stru=
ct rcu_tasks *rtp)
        struct task_struct *t;
=20
        t =3D kthread_run(rcu_tasks_kthread, rtp, "%s_kthread", rtp->kname)=
;
+       rtp->kthread_started =3D 1;
        if (WARN_ONCE(IS_ERR(t), "%s: Could not start %s grace-period kthre=
ad, OOM is now expected behavior\n", __func__, rtp->name))
                return;
        smp_mb(); /* Ensure others see full kthread. */