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From:   "Zhang, Qiang1" <qiang1.zhang@intel.com>
To:     "paulmck@kernel.org" <paulmck@kernel.org>
CC:     "frederic@kernel.org" <frederic@kernel.org>,
        "rcu@vger.kernel.org" <rcu@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        lkp <lkp@intel.com>
Subject: RE: [PATCH v2] rcu-tasks: Stop RCU Tasks scanning tasks which record
 on dyntick-idle entry
Thread-Topic: [PATCH v2] rcu-tasks: Stop RCU Tasks scanning tasks which record
 on dyntick-idle entry
Thread-Index: AQHYbCV/lb12vPYjukKy63FEgFIY2K0tG/kAgAAGzJA=
Date:   Mon, 23 May 2022 23:45:13 +0000
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References: <20220520084155.1745726-1-qiang1.zhang@intel.com>
 <20220523230920.GG1790663@paulmck-ThinkPad-P17-Gen-1>
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On Fri, May 20, 2022 at 04:41:55PM +0800, Zqiang wrote:
> When the RCU Tasks scanning on-rq tasks, there is no need to scanning=20
> tasks which record on dyntick-idle entry, at this time, these tasks=20
> are not remain within an RCU Tasks read-side critical section.
>=20
> This commit skip scanning tasks which record on dyntick-idle entry in=20
> rcu_tasks_pertask().
>=20
> Reported-by: kernel test robot <lkp@intel.com>
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>
>
>You lost me with this one.  Doesn't the rcu_tasks_pertask() function alrea=
dy exclude the idle tasks that might possibly be running in dyntick-idle st=
ate in non-CONFIG_NO_HZ_FULL systems?
>
>What am I missing here?

Thanks for reply.
Yes, the is_idle_task() can remove from task_is_on_dyntick_idle().
And the is_idle_task() can also remove from warnings in check_holdout_task(=
), because=20
the rcu _tasks_pertask() already exclude the idle tasks. Is my understandin=
g correct?

Thanks
Zqiang


>							Thanx, Paul
>
> ---
>  v1->v2:
>  fix build test ERROR due to CONFIG_TASKS_RCU=3Dn
>=20
>  kernel/rcu/tasks.h | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
>=20
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index=20
> f6459343e4b6..7cb0f922478e 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -809,10 +809,17 @@ static void rcu_tasks_pregp_step(struct list_head *=
hop)
>  	synchronize_rcu();
>  }
> =20
> +static bool task_is_on_dyntick_idle(struct task_struct *t) {
> +	return IS_ENABLED(CONFIG_NO_HZ_FULL) && !is_idle_task(t) &&
> +				t->rcu_tasks_idle_cpu >=3D 0;
> +}
> +
>  /* Per-task initial processing. */
>  static void rcu_tasks_pertask(struct task_struct *t, struct list_head=20
> *hop)  {
> -	if (t !=3D current && READ_ONCE(t->on_rq) && !is_idle_task(t)) {
> +	if (t !=3D current && READ_ONCE(t->on_rq) && !is_idle_task(t) &&
> +				!task_is_on_dyntick_idle(t)) {
>  		get_task_struct(t);
>  		t->rcu_tasks_nvcsw =3D READ_ONCE(t->nvcsw);
>  		WRITE_ONCE(t->rcu_tasks_holdout, true); @@ -842,8 +849,7 @@ static=20
> void check_holdout_task(struct task_struct *t,
>  	if (!READ_ONCE(t->rcu_tasks_holdout) ||
>  	    t->rcu_tasks_nvcsw !=3D READ_ONCE(t->nvcsw) ||
>  	    !READ_ONCE(t->on_rq) ||
> -	    (IS_ENABLED(CONFIG_NO_HZ_FULL) &&
> -	     !is_idle_task(t) && t->rcu_tasks_idle_cpu >=3D 0)) {
> +	    task_is_on_dyntick_idle(t)) {
>  		WRITE_ONCE(t->rcu_tasks_holdout, false);
>  		list_del_init(&t->rcu_tasks_holdout_list);
>  		put_task_struct(t);
> --
> 2.25.1
>=20