linux-kernel - Re: [patch 09/23] dynticks: extend next_timer

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Date:	Sat, 30 Sep 2006 01:37:00 -0700
From:	Andrew Morton <akpm@...l.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Jim Gettys <jg@...top.org>, John Stultz <johnstul@...ibm.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: [patch 09/23] dynticks: extend next_timer_interrupt() to use a
 reference jiffie

On Fri, 29 Sep 2006 23:58:28 -0000
Thomas Gleixner <tglx@...utronix.de> wrote:

> From: Thomas Gleixner <tglx@...utronix.de>
> 
> For CONFIG_NO_HZ we need to calculate the next timer wheel event based
> to a given jiffie value. Extend the existing code to allow the extra now
> argument. Provide a compability function for the existing implementations
> to call the function with now = jiffies.
> This also solves the racyness of the original code vs. jiffies changing
> during the iteration.
> 

I think this change has the potential to significantly increase the hold
time of tvec_base_t.lock.  Quite a lot of code has been moved inside that
lock, but most worrisome is that hrtimer_get_next_event() is also now
inside it.

What workloads is this change likely to impact, and how can we set about
verifying that we aren't introducing a problem?

Was that (unchangelogged) locking change even needed?  If so, why?

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