linux-kernel - Re: [PATCH RFC nohz_full 0/8] Provide infrastructure for full-system idle

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Message-ID: <20130625220152.GH3828@linux.vnet.ibm.com>
Date:	Tue, 25 Jun 2013 15:01:52 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	niv@...ibm.com, peterz@...radead.org, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu
Subject: Re: [PATCH RFC nohz_full 0/8] Provide infrastructure for full-system
 idle

On Tue, Jun 25, 2013 at 11:49:47PM +0200, Thomas Gleixner wrote:
> On Tue, 25 Jun 2013, Paul E. McKenney wrote:
> > Note that this version pays attention to CPUs that have taken an NMI
> > from idle.  It is not clear to me that NMI handlers can safely access
> > the time on a system that is long-term idle.  Unless someone tells me
> > that it is somehow safe to access time from an NMI from idle, I will
> > remove NMI support in the next version.
> 
> NMI cannot access any time related functions independent of NOHZ, long
> term idle or whatever you come up with:
> 
>        write_seqcount_begin(&timekeeper_seq);
> 
> ---> NMI
> 	...
> 	do {
> 	   seq = read_seqcount_begin(&timekeeper_seq);
> 	} while (read_seqcount_retry(&timekeeper_seq, seq));
> 
> Guess how well that works ....

Woo-hoo!!!  I will yank the NMI support on the next version.  Thank
you for the info!

							Thanx, Paul

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