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

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Message-ID: <877gm2rfca.fsf@linaro.org>
Date:	Wed, 20 Feb 2013 10:48:53 -0800
From:	Kevin Hilman <khilman@...aro.org>
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

Frederic Weisbecker <fweisbec@...il.com> writes:

> 2013/2/15 Kevin Hilman <khilman@...aro.org>:
>> Frederic Weisbecker <fweisbec@...il.com> writes:
>>
>>> Ingo,
>>>
>>> Please pull the new full dynticks cputime accounting code that
>>> can be found at:
>>>
>>> git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
>>>       tags/full-dynticks-cputime-for-mingo
>>>
>>> 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.
>>
>> Actually, moderately recent 32-bit ARMs can do atomic 64-bit load/stores
>> too.
>
> Does gcc automatically handle 64 bit store/loads in one way or does
> that require specific CPU instructions?

For atomic load/stores, it requires specific instructions that gcc will
not generate, so the atomic64 accessors are needed on ARM.

Kevin
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