Re: [patch 5/8] Immediate Values - x86 Optimization (simplified)

Mathieu Desnoyers
2007-11-19T14:28:18Z

* Rusty Russell (rusty@rustcorp.com.au) wrote:
> On Saturday 17 November 2007 01:03:35 Mathieu Desnoyers wrote:
> > x86 optimization of the immediate values which uses a movl with code
> > patching to set/unset the value used to populate the register used as
> > variable source.
>
> Since immediate values are by definition an optimization, I think it makes
> sense to insist they be 1, 2, 4 or 8 bytes. A BUILD_BUG_ON() in the right
> place should ensure this (probably in generic code rather than x86).
>
sure,

> > ===================================================================
> > --- linux-2.6-lttng.orig/arch/x86/kernel/traps_32.c
>
> I don't think you need any modification to this file now.
>
yep,

> > + * Create the instruction in a discarded section to calculate its size. This is
> > + * how we can align the beginning of the instruction on an address that will
> > + * permit atomic modificatino of the immediate value without knowing the size of
> > + * the opcode used by the compiler. The operand size is known in advance.
> > + */
>
> This alignment is also now unnecessary.
>

right,

> > +++ linux-2.6-lttng/arch/x86/kernel/immediate.c 2007-11-16
> > 08:56:22.000000000 -0500 @@ -0,0 +1,143 @@
> > +/*
> > + * Immediate Value - x86 architecture specific code.
>
> This is now almost entirely generic code, but I suppose we can let the next
> architecture hoist it out.
>

I'll move it to kernel/immediate.c. We can therefore remove the powerpc
immediate.c as well.

> > +/**
> > + * arch_immediate_update_early - update one immediate value at boot time
> > + * @immediate: pointer of type const struct __immediate to update
> > + *
> > + * Update one immediate value at boot time.
> > + */
> > +void arch_immediate_update_early(const struct __immediate *immediate)
>
> I think it would be easier to just fast-path the num_online_cpus == 1 case,
> even if you want to keep this "update_early" interface.
>

Nope, that could lead to problems. I call core_immediate_update()
_very_ early, before boot_cpu_init() is called. Therefore,
cpu_online_map is not set yet. I am not sure the benefit of using
num_online_cpus outweights the added fragility wrt other boot process
initializations.

> But I like your IPI algorithm: very tight.
>

Thanks. Many thanks for the review, I'll repost the patchset for
comments.

Mathieu