linux-kernel - Re: [RFC] time: xtime

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Date:	Fri, 06 May 2011 13:24:03 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	lkml <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Anton Blanchard <anton@...ba.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC] time: xtime_lock is held too long

On Fri, 2011-05-06 at 22:04 +0200, Eric Dumazet wrote:
> Le vendredi 06 mai 2011 à 21:26 +0200, Eric Dumazet a écrit :
> > Le vendredi 06 mai 2011 à 19:50 +0200, Andi Kleen a écrit :
> > > On Fri, May 06, 2011 at 07:42:47PM +0200, Eric Dumazet wrote:
> > > > Le vendredi 06 mai 2011 à 18:59 +0200, Andi Kleen a écrit :
> > > > 
> > > > > If you have a better way to make it faster please share it.
> > > > 
> > > > Ideally we could use RCU :)
> > > 
> > > Hmm, I didn't think my case had a lot of loops in the seqlock -- just
> > > expensive cache misses -- but I should double check.
> > > 
> > > For the lots of loop case we probably need to understand first why you
> > > iterate that often.
> > 
> > Yep, I'll try to investigate on this
> > 
> 
> So apparently some calls to tick_do_update_jiffies64() are pretty
> expensive :

So would the easier solution be to just break out timekeeper locking
from the xtime_lock?

So basically we would just add a timekeeper.lock seqlock and use it to
protect only the timekeeping code? We can still keep xtime_lock around
for the tick/jiffies protection (well, until tglx kills jiffies :), but
gettimeofday and friends wouldn't be blocked for so long.

That should be pretty straight forward now that the timekeeper data is
completely static to timkeeeping.c.

thanks
-john


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