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Date:   Thu, 27 Jan 2022 17:35:12 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Peter Oskolkov <posk@google.com>
Cc:     mingo@redhat.com, tglx@linutronix.de, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-api@vger.kernel.org, x86@kernel.org,
        pjt@google.com, avagin@google.com, jannh@google.com,
        tdelisle@uwaterloo.ca, posk@posk.io
Subject: Re: [RFC PATCH v2 5/5] sched: UMCG: allow to sys_umcg_kick UMCG
 servers
Message-ID: <20220127163512.GR20638@worktop.programming.kicks-ass.net>
References: <20220113233940.3608440-1-posk@google.com>
 <20220113233940.3608440-6-posk@google.com>
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On Thu, Jan 13, 2022 at 03:39:40PM -0800, Peter Oskolkov wrote:
> ---
>  include/uapi/linux/umcg.h | 10 ++++++++++
>  kernel/sched/umcg.c       |  7 +++++--
>  2 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/umcg.h b/include/uapi/linux/umcg.h
> index 93fccb44283b..a29e5e91a251 100644
> --- a/include/uapi/linux/umcg.h
> +++ b/include/uapi/linux/umcg.h
> @@ -148,4 +148,14 @@ enum umcg_ctl_flag {
>  	UMCG_CTL_WORKER		= 0x10000,
>  };
>  
> +/**
> + * enum umcg_kick_flag - flags to pass to sys_umcg_kick
> + * @UMCG_KICK_RESCHED: reschedule the task; used for worker preemption
> + * @UMCG_KICK_TTWU: wake the task; used to wake servers
> + */
> +enum umcg_kick_flag {
> +	UMCG_KICK_RESCHED	= 0x001,
> +	UMCG_KICK_TTWU		= 0x002,
> +};
> +
>  #endif /* _UAPI_LINUX_UMCG_H */
> diff --git a/kernel/sched/umcg.c b/kernel/sched/umcg.c
> index b85dec6b82e4..e33ec9eddc3e 100644
> --- a/kernel/sched/umcg.c
> +++ b/kernel/sched/umcg.c
> @@ -669,12 +669,15 @@ SYSCALL_DEFINE2(umcg_kick, u32, flags, pid_t, tid)
>  	if (!task)
>  		return -ESRCH;
>  
> -	if (flags)
> +	if (flags != UMCG_KICK_RESCHED && flags != UMCG_KICK_TTWU)
>  		return -EINVAL;
>  
>  #ifdef CONFIG_SMP
> -	smp_send_reschedule(task_cpu(task));
> +	if (flags == UMCG_KICK_RESCHED)
> +		smp_send_reschedule(task_cpu(task));
>  #endif
> +	if (flags == UMCG_KICK_TTWU)
> +		try_to_wake_up(task, TASK_NORMAL, 0);
>  
>  	return 0;
>  }

I'm thinking the simpler thing is this..
(also note the task ref leak fixed)

--- a/kernel/sched/umcg.c
+++ b/kernel/sched/umcg.c
@@ -719,16 +719,22 @@ void umcg_notify_resume(struct pt_regs *
  */
 SYSCALL_DEFINE2(umcg_kick, u32, flags, pid_t, tid)
 {
-	struct task_struct *task = umcg_get_task(tid);
-	if (!task)
-		return -ESRCH;
+	struct task_struct *task;
 
 	if (flags)
 		return -EINVAL;
+       
+	task = umcg_get_task(tid);
+	if (!task)
+		return -ESRCH;
 
+	if (!try_to_wake_up(task, TASK_NORMAL, WF_CURRENT_CPU)) {
 #ifdef CONFIG_SMP
-	smp_send_reschedule(task_cpu(task));
+		smp_send_reschedule(task_cpu(task));
 #endif
+	}
+
+	put_task_struct(task);
 
 	return 0;
 }