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From: Kevin Hilman <khilman@linaro.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Russell King <rmk+kernel@arm.linux.org.uk>,
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        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linaro-kernel@lists.linaro.org
Subject: Re: [PATCH 0/2] cpustat: use atomic operations to read/update stats
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Date: Fri, 22 Feb 2013 11:01:39 -0800
In-Reply-To: <20130222142302.GC18149@somewhere.redhat.com> (Frederic
	Weisbecker's message of "Fri, 22 Feb 2013 15:23:05 +0100")
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Frederic Weisbecker <fweisbec@gmail.com> writes:

> On Fri, Feb 22, 2013 at 06:21:31AM -0800, Eric Dumazet wrote:
>> On Fri, 2013-02-22 at 15:09 +0100, Peter Zijlstra wrote:
>> > On Fri, 2013-02-22 at 13:50 +0100, Frederic Weisbecker wrote:
>> > > > Argh!! at what cost? 64bit atomics are like expensive. Wouldn't
>> > > adding
>> > > > a seqlock be saner?
>> > > 
>> > > Not sure. This requires a spinlock in the write side which is called
>> > > from
>> > > fast path like the timer interrupt.
>> > 
>> > A single spinlock is _way_ cheaper than a ton of cmpxchg8b()s to update
>> > a few variables.
>> 
>> We also have include/linux/u64_stats_sync.h since 2.6.36
>
> Interesting, we should probably use that instead.

OK, I'll spin a version using the u64_stats interface.

Unfortunately, for 32-bit platforms that have atomic 64-bit loads
stores[1], u64_stats leads to some unnecessary overhead, but I'll look
at possibly optimizing u64_stats for those platforms as a follow-up
patch.

Kevin

[1] ARM >= v6k devices have ldrexd/strexd instructions for 64-bit loads
    stores which are used by the atomic64 accessors on those devices.
    (c.f. arch/arm/include/asm/atomic.h:atomic64_read()