linux-kernel - Re: [PATCH 2/2] Add a thread cpu time implementation to vDSO

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Date:	Mon, 12 Dec 2011 15:52:53 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Arun Sharma <asharma@...com>, linux-kernel@...r.kernel.org,
	Kumar Sundararajan <kumar@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/2] Add a thread cpu time implementation to vDSO

On Mon, 2011-12-12 at 15:41 -0800, Andrew Lutomirski wrote:
> On Mon, Dec 12, 2011 at 3:32 PM, john stultz <johnstul@...ibm.com> wrote:
> > Is it just that you're concerned about the clockid switch costs being
> > too high?
> 
> From my (old, from memory) measurements, the switch cost is very very
> low if it's predicted correctly, and the main case where it's likely
> to be mispredicted is when it hasn't been called in awhile, in which
> case either probably no one cares or the cache misses will dominate.
> 
> How worried are you about introducing a year 2554 bug?  Python says:
> 
> >>> dateutil.parser.parse('1/1/1970') + datetime.timedelta(seconds = 2**64 // 1000000000)
> datetime.datetime(2554, 7, 21, 23, 34, 33)

Not very. :) 

Worse case the syscall can expose a nsec_t or something that can be
bumped to u128 when that becomes common.

thanks
-john

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