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Date: Sat, 30 Sep 2006 01:37:00 -0700
From: Andrew Morton <akpm@osdl.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
       Jim Gettys <jg@laptop.org>, John Stultz <johnstul@us.ibm.com>,
       David Woodhouse <dwmw2@infradead.org>,
       Arjan van de Ven <arjan@infradead.org>, Dave Jones <davej@redhat.com>
Subject: Re: [patch 09/23] dynticks: extend next_timer_interrupt() to use a
 reference jiffie
Message-Id: <20060930013700.784b57f8.akpm@osdl.org>
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	<20060929234439.836620000@cruncher.tec.linutronix.de>
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On Fri, 29 Sep 2006 23:58:28 -0000
Thomas Gleixner <tglx@linutronix.de> wrote:

> From: Thomas Gleixner <tglx@linutronix.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?