Commit 6cd74d2b authored by Madhavan Srinivasan's avatar Madhavan Srinivasan Committed by Michael Ellerman
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powerpc/64s: Implement local_t using irq soft masking 

local_t is used for atomic modifications for per-CPU data, versus
re-entrant modifications via interrupts.

local_t read-modify-write atomic operations are currently implemented
with hardware atomics (larx/stcx), which are quite slow. This patch
implements them by masking all types of interrupts that may do local_t
operations ("standard" and perf interrupts).

Rusty's benchmark (https://lkml.org/lkml/2008/12/16/450) gives the
following timings for the local_t test, in nanoseconds per iteration:

             larx/stcx   irq+pmu disable
_inc                38                10
_add                38                10
_read                4                 4
_add_return         38                10

There are still some interrupt types (system reset, machine check, and
watchdog), which can not safely use local_t operations, because they
are not masked.

An alternative approach was proposed, using a CR bit to mark a critical
section, which is tested in the interrupt return path, and would then
branch to a fixup handler (similar to exception fixups), which re-starts
the operation. The problem with this was the complexity of the fixup
handler and the latency of the slow path.

https://lists.ozlabs.org/pipermail/linuxppc-dev/2014-November/123024.html



Signed-off-by: default avatarMadhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
parent 3b7e3020

arch/powerpc/include/asm/local.h

+141 −0
Original line number Diff line number Diff line
@@ -2,6 +2,147 @@ 
#ifndef _ARCH_POWERPC_LOCAL_H

#define _ARCH_POWERPC_LOCAL_H



#ifdef CONFIG_PPC_BOOK3S_64



#include <linux/percpu.h>

#include <linux/atomic.h>

#include <linux/irqflags.h>



#include <asm/hw_irq.h>



typedef struct

{

	long v;

} local_t;



#define LOCAL_INIT(i)	{ (i) }



static __inline__ long local_read(local_t *l)

{

	return READ_ONCE(l->v);

}



static __inline__ void local_set(local_t *l, long i)

{

	WRITE_ONCE(l->v, i);

}



#define LOCAL_OP(op, c_op)						\

static __inline__ void local_##op(long i, local_t *l)			\

{									\

	unsigned long flags;						\

									\

	powerpc_local_irq_pmu_save(flags);				\

	l->v c_op i;						\

	powerpc_local_irq_pmu_restore(flags);				\

}



#define LOCAL_OP_RETURN(op, c_op)					\

static __inline__ long local_##op##_return(long a, local_t *l)		\

{									\

	long t;								\

	unsigned long flags;						\

									\

	powerpc_local_irq_pmu_save(flags);				\

	t = (l->v c_op a);						\

	powerpc_local_irq_pmu_restore(flags);				\

									\

	return t;							\

}



#define LOCAL_OPS(op, c_op)		\

	LOCAL_OP(op, c_op)		\

	LOCAL_OP_RETURN(op, c_op)



LOCAL_OPS(add, +=)

LOCAL_OPS(sub, -=)



#define local_add_negative(a, l)	(local_add_return((a), (l)) < 0)

#define local_inc_return(l)		local_add_return(1LL, l)

#define local_inc(l)			local_inc_return(l)



/*

 * local_inc_and_test - increment and test

 * @l: pointer of type local_t

 *

 * Atomically increments @l by 1

 * and returns true if the result is zero, or false for all

 * other cases.

 */

#define local_inc_and_test(l)		(local_inc_return(l) == 0)



#define local_dec_return(l)		local_sub_return(1LL, l)

#define local_dec(l)			local_dec_return(l)

#define local_sub_and_test(a, l)	(local_sub_return((a), (l)) == 0)

#define local_dec_and_test(l)		(local_dec_return((l)) == 0)



static __inline__ long local_cmpxchg(local_t *l, long o, long n)

{

	long t;

	unsigned long flags;



	powerpc_local_irq_pmu_save(flags);

	t = l->v;

	if (t == o)

		l->v = n;

	powerpc_local_irq_pmu_restore(flags);



	return t;

}



static __inline__ long local_xchg(local_t *l, long n)

{

	long t;

	unsigned long flags;



	powerpc_local_irq_pmu_save(flags);

	t = l->v;

	l->v = n;

	powerpc_local_irq_pmu_restore(flags);



	return t;

}



/**

 * local_add_unless - add unless the number is a given value

 * @l: pointer of type local_t

 * @a: the amount to add to v...

 * @u: ...unless v is equal to u.

 *

 * Atomically adds @a to @l, so long as it was not @u.

 * Returns non-zero if @l was not @u, and zero otherwise.

 */

static __inline__ int local_add_unless(local_t *l, long a, long u)

{

	unsigned long flags;

	int ret = 0;



	powerpc_local_irq_pmu_save(flags);

	if (l->v != u) {

		l->v += a;

		ret = 1;

	}

	powerpc_local_irq_pmu_restore(flags);



	return ret;

}



#define local_inc_not_zero(l)		local_add_unless((l), 1, 0)



/* Use these for per-cpu local_t variables: on some archs they are

 * much more efficient than these naive implementations.  Note they take

 * a variable, not an address.

 */



#define __local_inc(l)		((l)->v++)

#define __local_dec(l)		((l)->v++)

#define __local_add(i,l)	((l)->v+=(i))

#define __local_sub(i,l)	((l)->v-=(i))



#else /* CONFIG_PPC64 */



#include <asm-generic/local.h>



#endif /* CONFIG_PPC64 */



#endif /* _ARCH_POWERPC_LOCAL_H */

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