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

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Message-ID: <alpine.DEB.2.02.1306252345210.4013@ionos.tec.linutronix.de>
Date:	Tue, 25 Jun 2013 23:49:47 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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, 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 ....

Thanks,

	tglx
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