Re: [PATCH 0/2] cpustat: use atomic operations to read/update stats
From: Frederic Weisbecker
Date: Fri Feb 22 2013 - 09:15:42 EST
On Fri, Feb 22, 2013 at 03:05:39PM +0100, Peter Zijlstra wrote:
> On Fri, 2013-02-22 at 13:50 +0100, Frederic Weisbecker wrote:
> > > 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.
> Sure I get that, but again, why is that a problem,.. who relies on
> these statistics that makes it a problem?
I guess we want to provide at least some minimal reliability in /proc/stat
I mean we don't mind if the read is slightly off, reading stats from userspace
is inherently racy anyway, but if it suddenly shows a wrong increase of 4 billions
which disappear soon after, it looks like a bug to me.
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