Re: [PATCH 0/2] cpustat: use atomic operations to read/update stats
From: Kevin Hilman
Date: Fri Feb 22 2013 - 14:01:48 EST
Frederic Weisbecker <fweisbec@xxxxxxxxx> 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, u64_stats leads to some unnecessary overhead, but I'll look
at possibly optimizing u64_stats for those platforms as a follow-up
 ARM >= v6k devices have ldrexd/strexd instructions for 64-bit loads
stores which are used by the atomic64 accessors on those devices.
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