Re: [PATCH] memcg: remove unneeded preempt_disable

Christoph Lameter
2011-08-25T16:31:40Z

On Thu, 25 Aug 2011, James Bottomley wrote:

> On Thu, 2011-08-25 at 10:11 -0500, Christoph Lameter wrote:
> > On Thu, 25 Aug 2011, Peter Zijlstra wrote:
> >
> > > On Thu, 2011-08-18 at 14:40 -0700, Andrew Morton wrote:
> > > >
> > > > I think I'll apply it, as the call frequency is low (correct?) and the
> > > > problem will correct itself as other architectures implement their
> > > > atomic this_cpu_foo() operations.
> > >
> > > Which leads me to wonder, can anything but x86 implement that this_cpu_*
> > > muck? I doubt any of the risk chips can actually do all this.
> > > Maybe Itanic, but then that seems to be dying fast.
> >
> > The cpu needs to have an RMW instruction that does something to a
> > variable relative to a register that points to the per cpu base.
> >
> > Thats generally possible. The problem is how expensive the RMW is going to
> > be.
>
> Risc systems generally don't have a single instruction for this, that's
> correct. Obviously we can do it as a non atomic sequence: read
> variable, compute relative, read, modify, write ... but there's
> absolutely no point hand crafting that in asm since the compiler can
> usually work it out nicely. And, of course, to have this atomic, we
> have to use locks, which ends up being very expensive.

ARM seems to have these LDREX/STREX instructions for that purpose which
seem to be used for generating atomic instructions without lockes. I guess
other RISC architectures have similar means of doing it?