linux-kernel - Re: [PATCH 06/12] irq: implement irq_schedule

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Message-ID: <20100615114058.32e58afd@bike.lwn.net>
Date:	Tue, 15 Jun 2010 11:40:58 -0600
From:	Jonathan Corbet <corbet@....net>
To:	Tejun Heo <tj@...nel.org>
Cc:	mingo@...e.hu, tglx@...utronix.de, bphilips@...e.de,
	yinghai@...nel.org, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	jeff@...zik.org
Subject: Re: [PATCH 06/12] irq: implement irq_schedule_poll()

On Sun, 13 Jun 2010 17:31:32 +0200
Tejun Heo <tj@...nel.org> wrote:

> Implement and use irq_schedule_poll() to schedule desc->poll_timer
> instead of calling mod_timer directly.  irq_schedule_poll() is called
> with desc->lock held and schedules the timer iff necessary - ie. if
> the timer is offline or scheduled to expire later than requested.
> This will be used to share desc->poll_timer.

One quick question...

> diff --git a/kernel/irq/spurious.c b/kernel/irq/spurious.c
> index 0bce0e3..0b8fd0a 100644
> --- a/kernel/irq/spurious.c
> +++ b/kernel/irq/spurious.c
> @@ -23,6 +23,8 @@ enum {
>  
>  	/* IRQ polling common parameters */
>  	IRQ_POLL_INTV			= HZ / 100,	/* from the good ol' 100HZ tick */
> +
> +	IRQ_POLL_SLACK			= HZ / 1000,	/* 10% slack */
>  };

Why would IRQ_POLL_SLACK be anything other than zero for HZ < 1000?
It seems like you'd get no slack at all on most systems.  Am I missing
something?

Thanks,

jon
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