Re: [rfc 03/45] Generic CPU operations: Core piece

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

Very interesting patch! I did not expect we could mix local atomic ops
with per CPU offsets in an atomic manner.. brilliant :)

Some nitpicking follows...

* clameter@sgi.com (clameter@sgi.com) wrote:
> Currently the per cpu subsystem is not able to use the atomic capabilities
> of the processors we have.
> 
> This adds new functionality that allows the optimizing of per cpu variable
> handliong. It in particular provides a simple way to exploit atomic operations

handling

> to avoid having to disable itnerrupts or add an per cpu offset.
interrupts

> 
> F.e. current implementations may do
> 
> unsigned long flags;
> struct stat_struct *p;
> 
> local_irq_save(flags);
> /* Calculate address of per processor area */
> p = CPU_PTR(stat, smp_processor_id());
> p->counter++;
> local_irq_restore(flags);
> 
> This whole segment can be replaced by a single CPU operation
> 
> CPU_INC(stat->counter);
> 
> And on most processors it is possible to perform the increment with
> a single processor instruction. Processors have segment registers,
> global registers and per cpu mappings of per cpu areas for that purpose.
> 
> The problem is that the current schemes cannot utilize those features.
> local_t is not really addressing the issue since the offset calculation
> is not solved. local_t is x86 processor specific. This solution here
> can utilize other methods than just the x86 instruction set.
> 
> On x86 the above CPU_INC translated into a single instruction:
> 
> inc %%gs:(&stat->counter)
> 
> This instruction is interrupt safe since it can either be completed
> or not.
> 
> The determination of the correct per cpu area for the current processor
> does not require access to smp_processor_id() (expensive...). The gs
> register is used to provide a processor specific offset to the respective
> per cpu area where the per cpu variabvle resides.

variable

> 
> Note tha the counter offset into the struct was added *before* the segment
that

> selector was added. This is necessary to avoid calculation, In the past
> we first determine the address of the stats structure on the respective
> processor and then added the field offset. However, the offset may as
> well be added earlier.
> 
> If stat was declared via DECLARE_PER_CPU then this patchset is capoable of
capable

> convincing the linker to provide the proper base address. In that case
> no calculations are necessary.
> 
> Should the stats structure be reachable via a register then the address
> calculation capabilities can be leverages to avoid calculations.
> 
> On IA64 the same will result in another single instruction using the
> factor that we have a virtual address that always maps to the local per cpu
> area.
> 
> fetchadd &stat->counter + (VCPU_BASE - __per_cpu_base)
> 
> The access is forced into the per cpu address reachable via the virtualized
> address. Again the counter field offset is eadded to the offset. The access

added

> is then similarly a singular instruction thing as on x86.
> 
> In order to be able to exploit the atomicity of this instructions we
> introduce a series of new functions that take a BASE pointer (a pointer
> into the area of cpu 0 which is the canonical base).
> 
> CPU_READ()
> CPU_WRITE()
> CPU_INC
> CPU_DEC
> CPU_ADD
> CPU_SUB
> CPU_XCHG
> CPU_CMPXCHG
> 
> 
> 
> 
> 
> 
> Signed-off-by: Christoph Lameter <clameter@sgi.com>
> 
> ---
>  include/linux/percpu.h |  156 +++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 156 insertions(+)
> 
> Index: linux-2.6/include/linux/percpu.h
> ===================================================================
> --- linux-2.6.orig/include/linux/percpu.h	2007-11-18 22:13:51.773274119 -0800
> +++ linux-2.6/include/linux/percpu.h	2007-11-18 22:15:10.396773779 -0800
> @@ -190,4 +190,160 @@ void cpu_free(void *cpu_pointer, unsigne
>   */
>  void *boot_cpu_alloc(unsigned long size);
>  
> +/*
> + * Fast Atomic per cpu operations.
> + *
> + * The following operations can be overridden by arches to implement fast
> + * and efficient operations. The operations are atomic meaning that the
> + * determination of the processor, the calculation of the address and the
> + * operation on the data is an atomic operation.
> + */
> +
> +#ifndef CONFIG_FAST_CPU_OPS
> +
> +/*
> + * The fallbacks are rather slow but they are safe
> + *
> + * The first group of macros is used when we it is
> + * safe to update the per cpu variable because
> + * preemption is off (per cpu variables that are not
> + * updated from interrupt cointext) or because

context

> + * interrupts are already off.
> + */
> +
> +#define __CPU_READ(obj)				\
> +({						\
> +	typeof(obj) x;				\
> +	x = *THIS_CPU(&(obj));			\
> +	(x);					\
> +})
> +
> +#define __CPU_WRITE(obj, value)			\
> +({						\
> +	*THIS_CPU((&(obj)) = value;		\
> +})
> +
> +#define __CPU_ADD(obj, value)			\
> +({						\
> +	*THIS_CPU(&(obj)) += value;		\
> +})
> +
> +
> +#define __CPU_INC(addr) __CPU_ADD(addr, 1)
> +#define __CPU_DEC(addr) __CPU_ADD(addr, -1)
> +#define __CPU_SUB(addr, value) __CPU_ADD(addr, -(value))
> +
> +#define __CPU_CMPXCHG(obj, old, new)		\
> +({						\
> +	typeof(obj) x;				\
> +	typeof(obj) *p = THIS_CPU(&(obj));	\
> +	x = *p;					\
> +	if (x == old)				\
> +		*p = new;			\

I think you could use extra () around old, new etc.. ?

> +	(x);					\
> +})
> +
> +#define __CPU_XCHG(obj, new)			\
> +({						\
> +	typeof(obj) x;				\
> +	typeof(obj) *p = THIS_CPU(&(obj));	\
> +	x = *p;					\
> +	*p = new;				\

Same here.

> +	(x);					\

() seems unneeded here, since x is local.

> +})
> +
> +/*
> + * Second group used for per cpu variables that
> + * are not updated from an interrupt context.
> + * In that case we can simply disable preemption which
> + * may be free if the kernel is compiled without preemption.
> + */
> +
> +#define _CPU_READ(addr)				\
> +({						\
> +	(__CPU_READ(addr));			\
> +})

({ }) seems to be unneeded here.

> +
> +#define _CPU_WRITE(addr, value)			\
> +({						\
> +	__CPU_WRITE(addr, value);		\
> +})

and here..

> +
> +#define _CPU_ADD(addr, value)			\
> +({						\
> +	preempt_disable();			\
> +	__CPU_ADD(addr, value);			\
> +	preempt_enable();			\
> +})
> +

Add ()

> +#define _CPU_INC(addr) _CPU_ADD(addr, 1)
> +#define _CPU_DEC(addr) _CPU_ADD(addr, -1)
> +#define _CPU_SUB(addr, value) _CPU_ADD(addr, -(value))
> +
> +#define _CPU_CMPXCHG(addr, old, new)		\
> +({						\
> +	typeof(addr) x;				\
> +	preempt_disable();			\
> +	x = __CPU_CMPXCHG(addr, old, new);	\

add ()

> +	preempt_enable();			\
> +	(x);					\
> +})
> +
> +#define _CPU_XCHG(addr, new)			\
> +({						\
> +	typeof(addr) x;				\
> +	preempt_disable();			\
> +	x = __CPU_XCHG(addr, new);		\

()

> +	preempt_enable();			\
> +	(x);					\

() seems unneeded here, since x is local.

> +})
> +
> +/*
> + * Interrupt safe CPU functions
> + */
> +
> +#define CPU_READ(addr)				\
> +({						\
> +	(__CPU_READ(addr));			\
> +})
> +

Unnecessary ({ })

> +#define CPU_WRITE(addr, value)			\
> +({						\
> +	__CPU_WRITE(addr, value);		\
> +})
> +
> +#define CPU_ADD(addr, value)			\
> +({						\
> +	unsigned long flags;			\
> +	local_irq_save(flags);			\
> +	__CPU_ADD(addr, value);			\
> +	local_irq_restore(flags);		\
> +})
> +
> +#define CPU_INC(addr) CPU_ADD(addr, 1)
> +#define CPU_DEC(addr) CPU_ADD(addr, -1)
> +#define CPU_SUB(addr, value) CPU_ADD(addr, -(value))
> +
> +#define CPU_CMPXCHG(addr, old, new)		\
> +({						\
> +	unsigned long flags;			\
> +	typeof(*addr) x;			\
> +	local_irq_save(flags);			\
> +	x = __CPU_CMPXCHG(addr, old, new);	\

()

> +	local_irq_restore(flags);		\
> +	(x);					\

() seems unneeded here, since x is local.

> +})
> +
> +#define CPU_XCHG(addr, new)			\
> +({						\
> +	unsigned long flags;			\
> +	typeof(*addr) x;			\
> +	local_irq_save(flags);			\
> +	x = __CPU_XCHG(addr, new);		\

()

> +	local_irq_restore(flags);		\
> +	(x);					\

() seems unneeded here, since x is local.

> +})
> +
> +#endif /* CONFIG_FAST_CPU_OPS */
> +
>  #endif /* __LINUX_PERCPU_H */
> 
> -- 

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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