linux-kernel - Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

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Date:	Tue, 03 Nov 2009 07:12:02 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Glauber Costa <glommer@...hat.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	kurt.hackel@...cle.com, the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Keir Fraser <keir.fraser@...citrix.com>, zach.brown@...cle.com,
	chris.mason@...cle.com, Ingo Molnar <mingo@...hat.com>
Subject: Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall	implementation

On 11/02/2009 05:46 PM, Dan Magenheimer wrote:
>>> I don't have any public data available for this DB usage,
>>>        
>> Sorry, that doesn't explain anything.
>>      
> Well for now just consider the DB usage as another use
> of profiling.  But one can easily draw scenarios where
> a monotonic timestamp is also used to guarantee transaction
> ordering.
>    

In this case we should provide a facility for this.  Providing a global 
monotonic counter may be easier than providing a monotonic clock.  Hence 
my question.

>>> Search for "flight recorder".  This feature is intended to
>>> be enabled all the time, but with non-vsyscall gettimeofday
>>> the performance impact is unacceptably high, so they are using
>>>        
>> For profiling work fast timestamping is of course great, but surely
>> there is no monotonicity requirement?
>>      
> Yes and no.  Monotonicity is a poor substitute for a more
> generic mechanism that might provide an indication that a
> discontinuity has occurred (forward or backward); if an app
> could get both the timestamp AND some kind of "continuity
> generation counter" (basically a much more sophisticated
> form of TSC_AUX that changes whenever the timestamp is
> coming from a different source), perhaps all problems could be solved.
>    

I doubt it.  A discontinuity has occured, but what do we know about it?  
nothing.

>> I don't think we'll be able to provide monotonicity with vsyscall on
>> tsc-broken hosts, so we'll be limited to correcting the tsc frequency
>> after migration for good-tsc hosts.
>>      
> True, though clock_gettime(CLOCK_MONOTONIC) can provide
> the monotonicity where it is required.
>    

We have that already.  The question is how to implement it in a vsyscall.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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