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

[Frederic Weisbecker]

On Fri, Feb 22, 2013 at 09:46:07AM +0100, Peter Zijlstra wrote:
> 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?

So here is a possible scenario, CPU 0 reads a kcpustat value, and CPU 1 writes
it at the same time:

    //Initial value of "cpustat" is 0xffffffff
         == CPU 0 ==           == CPU 1 ==

       //load low part
       mov %eax, [cpustat]
                             inc [cpustat]
                             //Update the high part if necessary
                             jnc 1f
                             inc [cpustat + 4]
                             1:
       //load high part
       mov %edx, [cpustat + 4]


Afterward, CPU 0 will think the value is 0x1ffffffff while it's actually
0x100000000.

atomic64_read() and atomic64_set() are supposed to take care of that, without
even the need for _inc() or _add() parts that use LOCK.


> 
> > 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?

Not sure. This requires a spinlock in the write side which is called from
fast path like the timer interrupt.
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