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powerpc/64s: Implement local_t using irq soft masking

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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: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Madhavan Srinivasan
2017-12-20 09:25:57 +05:30
committed by Michael Ellerman
parent 3b7e302086
commit 6cd74d2b61
1 changed files with 141 additions and 0 deletions
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141
arch/powerpc/include/asm/local.h
View File

@@ -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 */