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6cd74d2b61
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>
149 lines
3.2 KiB
C
149 lines
3.2 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _ARCH_POWERPC_LOCAL_H
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#define _ARCH_POWERPC_LOCAL_H
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#ifdef CONFIG_PPC_BOOK3S_64
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#include <linux/percpu.h>
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#include <linux/atomic.h>
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#include <linux/irqflags.h>
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#include <asm/hw_irq.h>
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typedef struct
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{
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long v;
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} local_t;
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#define LOCAL_INIT(i) { (i) }
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static __inline__ long local_read(local_t *l)
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{
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return READ_ONCE(l->v);
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}
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static __inline__ void local_set(local_t *l, long i)
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{
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WRITE_ONCE(l->v, i);
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}
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#define LOCAL_OP(op, c_op) \
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static __inline__ void local_##op(long i, local_t *l) \
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{ \
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unsigned long flags; \
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\
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powerpc_local_irq_pmu_save(flags); \
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l->v c_op i; \
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powerpc_local_irq_pmu_restore(flags); \
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}
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#define LOCAL_OP_RETURN(op, c_op) \
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static __inline__ long local_##op##_return(long a, local_t *l) \
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{ \
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long t; \
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unsigned long flags; \
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\
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powerpc_local_irq_pmu_save(flags); \
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t = (l->v c_op a); \
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powerpc_local_irq_pmu_restore(flags); \
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\
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return t; \
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}
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#define LOCAL_OPS(op, c_op) \
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LOCAL_OP(op, c_op) \
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LOCAL_OP_RETURN(op, c_op)
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LOCAL_OPS(add, +=)
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LOCAL_OPS(sub, -=)
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#define local_add_negative(a, l) (local_add_return((a), (l)) < 0)
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#define local_inc_return(l) local_add_return(1LL, l)
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#define local_inc(l) local_inc_return(l)
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/*
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* local_inc_and_test - increment and test
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* @l: pointer of type local_t
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*
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* Atomically increments @l by 1
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* and returns true if the result is zero, or false for all
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* other cases.
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*/
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#define local_inc_and_test(l) (local_inc_return(l) == 0)
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#define local_dec_return(l) local_sub_return(1LL, l)
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#define local_dec(l) local_dec_return(l)
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#define local_sub_and_test(a, l) (local_sub_return((a), (l)) == 0)
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#define local_dec_and_test(l) (local_dec_return((l)) == 0)
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static __inline__ long local_cmpxchg(local_t *l, long o, long n)
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{
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long t;
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unsigned long flags;
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powerpc_local_irq_pmu_save(flags);
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t = l->v;
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if (t == o)
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l->v = n;
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powerpc_local_irq_pmu_restore(flags);
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return t;
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}
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static __inline__ long local_xchg(local_t *l, long n)
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{
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long t;
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unsigned long flags;
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powerpc_local_irq_pmu_save(flags);
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t = l->v;
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l->v = n;
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powerpc_local_irq_pmu_restore(flags);
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return t;
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}
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/**
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* local_add_unless - add unless the number is a given value
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* @l: pointer of type local_t
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* @a: the amount to add to v...
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* @u: ...unless v is equal to u.
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*
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* Atomically adds @a to @l, so long as it was not @u.
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* Returns non-zero if @l was not @u, and zero otherwise.
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*/
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static __inline__ int local_add_unless(local_t *l, long a, long u)
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{
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unsigned long flags;
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int ret = 0;
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powerpc_local_irq_pmu_save(flags);
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if (l->v != u) {
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l->v += a;
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ret = 1;
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}
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powerpc_local_irq_pmu_restore(flags);
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return ret;
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}
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#define local_inc_not_zero(l) local_add_unless((l), 1, 0)
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/* Use these for per-cpu local_t variables: on some archs they are
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* much more efficient than these naive implementations. Note they take
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* a variable, not an address.
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*/
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#define __local_inc(l) ((l)->v++)
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#define __local_dec(l) ((l)->v++)
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#define __local_add(i,l) ((l)->v+=(i))
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#define __local_sub(i,l) ((l)->v-=(i))
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#else /* CONFIG_PPC64 */
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#include <asm-generic/local.h>
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#endif /* CONFIG_PPC64 */
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#endif /* _ARCH_POWERPC_LOCAL_H */
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