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

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Message-ID: <87y5egyxyk.fsf@linaro.org>
Date:	Fri, 22 Feb 2013 11:01:39 -0800
From:	Kevin Hilman <khilman@...aro.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH 0/2] cpustat: use atomic operations to read/update stats

Frederic Weisbecker <fweisbec@...il.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()
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