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

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Date:	Thu, 5 May 2011 17:59:39 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	john stultz <johnstul@...ibm.com>,
	Andi Kleen <andi@...stfloor.org>,
	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 Thu, 5 May 2011, Eric Dumazet wrote:

> Le jeudi 05 mai 2011 à 16:39 +0200, Thomas Gleixner a écrit :
> > On Thu, 5 May 2011, Eric Dumazet wrote:
> > 
> > > I feel xtime_lock seqlock is abused these days.
> > > 
> > > seqlock abstraction is somewhat lazy/dangerous because write_sequnlock()
> > > does both the seqcount increment and spinlock release.
> > > 
> > > I am concerned by fact that readers might wait for long times, because
> > > writers hold the whole seqlock, while sometime they only want to guard
> > > other writers to come in.
> > > 
> > > Maybe it's time to separate the things (the seqcount and the spinlock)
> > > so that writer can manipulate data in different sections : 
> > > - Sections while holding spinlock, allowing "readers" to run
> > > - Very small sections enclosed in a pair of seqcount increments, to
> > > synchronize with readers.
> > 
> > Well, in the case of timekeeping that might be problematic. I'm not
> > sure whether we can calculate the new values under the spinlock and
> > then update the timekeeper under the seqlock because we might adjust
> > the mult/shift pair which then can result in observabcle time going
> > backwards problems. It might be worth a try, though. John ???
> >  
> > The only thing which really can move right away outside the xtime
> > seqlock region is calc_global_load().
> > 
> 
> That would be a start, but we also could have finer granularity in
> locks :
> 
> update_vsyscall() has its own protection and could be done outside of
> the seqcount inc pair used for ktime_get().

Yeah, we could move that out, but it might be interesting to add a few
tracepoints into update_wall_time() first to see which part takes the
most time.

> [ but my patch numbers were for a 32bit kernel, so vsyscall is not
> accounted for. ]

:)
 
> Another idea would be to prime cache lines to be dirtied in cpu cache
> before taking locks, and better pack variables to reduce number of cache
> lines.

Most variables are packed already in struct timekeeper, which should
be pretty cache hot anyway, so I don't know whether we gain much.

Thanks,

	tglx