linux-kernel - Re: [PATCH 01/11] x86: Fix vtime/file timestamp inconsistencies

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Date:	Thu, 15 Jul 2010 10:51:13 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	john stultz <johnstul@...ibm.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	LKML <linux-kernel@...r.kernel.org>,
	Jiri Olsa <jolsa@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 01/11] x86: Fix vtime/file timestamp inconsistencies

> On Wed, 2010-07-14 at 11:40 +0900, KOSAKI Motohiro wrote:
> > Hi
> > 
> > > Due to vtime calling vgettimeofday(), its possible that an application
> > > could call  time();create("stuff",O_RDRW);  only to see the file's
> > > creation timestamp to be before the value returned by time.
> > 
> > Just dumb question.
> > 
> > Almost application are using gettimeofday() instead time(). It mean
> > your fix don't solve almost application.
> 
> Correct,  filesystem timestamps and gettimeofday can still seem
> inconsistently ordered. But that is expected.
> 
> Because of granularity differences (one interface is only tick
> resolution, the other is clocksource resolution), we can't interleave
> the two interfaces (time and gettimeofday, respectively) and expect to
> get ordered results.

hmmm...
Yes, times() vs gettimeofday() mekes no sense. nobody want this. but
I don't understand why we can ignore gettimeofday() vs file-tiemstamp.


> This is why the fix I'm proposing is important: Filesystem timestamps
> have always been tick granular, so when vtime() was made clocksource
> granular (by using vgettime internally) we broke the historic
> expectation that the time() interface could be interleaved with
> filesystem operations.
> 
> Side note: For full nanosecond resolution of the tick-granular
> timestamps, check out the clock_gettime(CLOCK_REALTIME_COARSE, ...)
> interface.
> 
> 
> > So, Why can't we fix vgettimeofday() vs create() inconsistency?
> > This is just question, I don't intend to disagree you.
> 
> The only way to make gettimeofday and create consistent is to use
> gettimeofday clocksource resolution timestamps for files. This however
> would potentially cause a large performance hit, since each every file
> timestamp would require a possibly expensive read of the clocksource.

Why clocksource() reading is so slow? the implementation of current
tsc clocksource ->read method is here.


	static cycle_t read_tsc(struct clocksource *cs)
	{
	        cycle_t ret = (cycle_t)get_cycles();
	
	        return ret >= clocksource_tsc.cycle_last ?
	                ret : clocksource_tsc.cycle_last;
	}

It mean, the difference is almost only one rdtsc.
And, now we have RELATIME. then crazy atime frequently updating issue
has been solved.

Can you please elaborate your worry? I think I haven't get which case
you worry.

Thanks.


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