linux-kernel - Re: [GIT PULL] cputime: Full dynticks task/cputime accounting v7

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Date:	Mon, 28 Jan 2013 19:18:00 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung.kim@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sedat Dilek <sedat.dilek@...il.com>,
	Gleb Natapov <gleb@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [GIT PULL] cputime: Full dynticks task/cputime accounting v7

On Mon, 28 Jan 2013, Frederic Weisbecker wrote:

> My last concern is the dependency on CONFIG_64BIT. We rely on cputime_t
> being u64 for reasonable nanosec granularity implementation. And therefore
> we need a single instruction fetch to read kernel cpustat for atomicity
> requirement against concurrent incrementation, which only 64 bit archs
> can provide.

Most x86 cpus support cmpxchg8b on 32bit which can be abused for
64 bit reads (see cmpxchg64 in cmpxchg_32.h). Simply do a cmpxchg with
zero and use whatever it returns.

A percpu version that uses the instruction is called
this_cpu_cmpxchg_double().

> There is just no emergency though as this new option depends on the context
> tracking subsystem that only x86-64 (and soon ppc64) implements yet. And
> this set is complex enough already. I think we can deal with that later.

Ok then this may not be that useful.

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