Re: [PATCH 0/2] cpustat: use atomic operations to read/update stats

From: Peter Zijlstra
Date: Fri Feb 22 2013 - 03:46:39 EST

On Thu, 2013-02-21 at 21:56 -0800, Kevin Hilman wrote:
> On 64-bit platforms, reads/writes of the various cpustat fields are
> atomic due to native 64-bit loads/stores. However, on non 64-bit
> platforms, reads/writes of the cpustat fields are not atomic and could
> lead to inconsistent statistics.

Which is a problem how?

> This problem was originally reported by Frederic Weisbecker as a
> 64-bit limitation with the nsec granularity cputime accounting for
> full dynticks, but then we realized that it's a problem that's been
> around for awhile and not specific to the new cputime accounting.
> This series fixes this by first converting all access to the cputime
> fields to use accessor functions, and then converting the accessor
> functions to use the atomic64 functions.

Argh!! at what cost? 64bit atomics are like expensive. Wouldn't adding
a seqlock be saner?

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