linux-kernel - Re: [PATCH 1/2] Add support for deferrable timers

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Message-ID: <20070322121816.GA15473@linux-os.sc.intel.com>
Date:	Thu, 22 Mar 2007 05:18:17 -0700
From:	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>, tglx@...utronix.de
Subject: Re: [PATCH 1/2] Add support for deferrable timers

On Thu, Mar 22, 2007 at 03:18:26AM -0800, Andrew Morton wrote:
> On Wed, 21 Mar 2007 13:22:17 -0700 Venkatesh Pallipadi <venkatesh.pallipadi@...el.com> wrote:
> 
> > 
> > Introduce a new flag for timers - 'deferrable timer'
> > Timers that work normally when system is busy. But, will not cause CPU to
> > come out of idle (just to service this timer), when CPU is idle. Instead,
> > this timer will be serviced when CPU eventually wakes up with a subsequent
> > non-deferrable timer or any other event.
> > 
> > The main advantage of this is to avoid unnecessary timer interrupts when
> > CPU is idle. If the routine currently called by a timer can wait until next
> > event without any issues, this new timer can be used while setting up timer
> > for that routine. This, with dynticks, allows CPUs to be lazy, allowing them
> > to stay in idle for extended period of time by reducing unnecesary wakeup and
> > thereby reducing the power consumption.
> > 
> > This patch:
> > Builds this new timer on top of existing timer infrastructure. It uses
> > last bit in 'base' pointer of timer_list structure to store this
> > deferrable timer flag. __next_timer_interrupt() function
> > skips over these deferrable timers when CPU looks for
> > next timer event for which it has to wake up.
> > 
> > This is exported by a new interface init_timer_deferrable() that can
> > be called in place of regular init_timer().
> > 
> 
> These patches cause a lockup during execuion of `halt -p'.  See phuzzy foto
> at http://userweb.kernel.org/~akpm/s5000486.jpg
> 
> It's fairly obvious why, when one looks at lock_timer_base().

Hmm.. Wonder how it worked for me and why I did not see this hang.
I had thought I covered all accesses to ->base. Obviously, I have missed
this "base == timer->base" check here. Will fix it and respin a patch later
today with below suggestions as well.

Thanks,
Venki
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