linux-kernel - Re: [01/22] sched: Provide scheduler_ipi() callback in response to smp_send

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Message-Id: <sched-ipi-callback-comment@mdm.bga.com>
Date:	Fri, 11 Mar 2011 09:07:13 -0600
From:	Milton Miller <miltonm@....com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Frank Rowand <frank.rowand@...sony.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>,
	Oleg Nesterov <oleg@...hat.com>, Paul Turner <pjt@...gle.com>,
	Jens Axboe <axboe@...nel.dk>,
	Yong Zhang <yong.zhang0@...il.com>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Jesper Nilsson <jesper.nilsson@...s.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [01/22] sched: Provide scheduler_ipi() callback in response to
 smp_send_reschedule()

On Wed, 02 Mar 2011 17:38:32 -0000, To: Peter Zijlstra wrote:

> For future rework of try_to_wake_up() we'd like to push part of that
> onto the CPU the task is actually going to run on, in order to do so we
> need a generic callback from the existing scheduler IPI.
> 
> This patch introduces such a generic callback: scheduler_ipi() and
> implements it as a NOP.
> 
> BenH notes: PowerPC might use this IPI on offline CPUs under rare
> conditions!!
..
> Index: linux-2.6/arch/powerpc/kernel/smp.c
> ===================================================================
> --- linux-2.6.orig/arch/powerpc/kernel/smp.c
> +++ linux-2.6/arch/powerpc/kernel/smp.c
> @@ -98,6 +98,7 @@ void smp_message_recv(int msg)
>  		break;
>  	case PPC_MSG_RESCHEDULE:
>  		/* we notice need_resched on exit */

This comment should also be removed, as it was documenting the empty action.

> +		scheduler_ipi();
>  		break;
>  	case PPC_MSG_CALL_FUNC_SINGLE:
>  		generic_smp_call_function_single_interrupt();
> @@ -127,7 +128,7 @@ static irqreturn_t call_function_action(
>  
>  static irqreturn_t reschedule_action(int irq, void *data)
>  {
> -	/* we just need the return path side effect of checking need_resched */
> +	scheduler_ipi();
>  	return IRQ_HANDLED;
>  }
>  

Thanks,
milton
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