Local atomic operations are fast and highly reentrant per CPU counters. Used for percpu variable updates. Local atomic operations only guarantee variable modification atomicity wrt the CPU which owns the data and these needs to be executed in a preemption safe way.
Here is the design of this patch. Since local_* operations are only need to be atomic to interrupts (IIUC), we have two options. Either replay the "op" if interrupted or replay the interrupt after the "op". Initial patchset posted was based on implementing local_* operation based on CR5 which replay's the "op". Patchset had issues in case of rewinding the address pointor from an array. This make the slow patch really slow. Since CR5 based implementation proposed using __ex_table to find the rewind addressr, this rasied concerns about size of __ex_table and vmlinux. https://lists.ozlabs.org/pipermail/linuxppc-dev/2014-December/123115.html But this patch uses Benjamin Herrenschmidt suggestion of arch_local_irq_disable_var() to soft_disable interrupts (including PMIs). After finishing the "op", arch_local_irq_restore() called and correspondingly interrupts are replayed if any occured. patch re-write the current local_* functions to use arch_local_irq_disbale. Base flow for each function is { arch_local_irq_disable_var(2) load .. store arch_local_irq_restore() } Currently only asm/local.h has been rewrite, and also the entire change is tested only in PPC64 (pseries guest) Reason for the approach is that, currently l[w/d]arx/st[w/d]cx. instruction pair is used for local_* operations, which are heavy on cycle count and they dont support a local variant. So to see whether the new implementation helps, used a modified version of Rusty's benchmark code on local_t. https://lkml.org/lkml/2008/12/16/450 Modifications to Rusty's benchmark code: - Executed only local_t test Here are the values with the patch. Time in ns per iteration Local_t Without Patch With Patch _inc 28 8 _add 28 8 _read 3 3 _add_return 28 7 Tested the patch in a - pSeries LPAR (with perf record) Signed-off-by: Madhavan Srinivasan <ma...@linux.vnet.ibm.com> --- arch/powerpc/include/asm/local.h | 91 +++++++++++++++++++++++++++------------- 1 file changed, 63 insertions(+), 28 deletions(-) diff --git a/arch/powerpc/include/asm/local.h b/arch/powerpc/include/asm/local.h index b8da91363864..afd3dabd92cb 100644 --- a/arch/powerpc/include/asm/local.h +++ b/arch/powerpc/include/asm/local.h @@ -14,24 +14,50 @@ typedef struct #define local_read(l) atomic_long_read(&(l)->a) #define local_set(l,i) atomic_long_set(&(l)->a, (i)) -#define local_add(i,l) atomic_long_add((i),(&(l)->a)) -#define local_sub(i,l) atomic_long_sub((i),(&(l)->a)) -#define local_inc(l) atomic_long_inc(&(l)->a) -#define local_dec(l) atomic_long_dec(&(l)->a) +static __inline__ void local_add(long i, local_t *l) +{ + long t; + unsigned long flags; + + flags = arch_local_irq_disable_var(2); + __asm__ __volatile__( + PPC_LL" %0,0(%2)\n\ + add %0,%1,%0\n" + PPC_STL" %0,0(%2)\n" + : "=&r" (t) + : "r" (i), "r" (&(l->a.counter))); + arch_local_irq_restore(flags); +} + +static __inline__ void local_sub(long i, local_t *l) +{ + long t; + unsigned long flags; + + flags = arch_local_irq_disable_var(2); + __asm__ __volatile__( + PPC_LL" %0,0(%2)\n\ + subf %0,%1,%0\n" + PPC_STL" %0,0(%2)\n" + : "=&r" (t) + : "r" (i), "r" (&(l->a.counter))); + arch_local_irq_restore(flags); +} static __inline__ long local_add_return(long a, local_t *l) { long t; + unsigned long flags; + flags = arch_local_irq_disable_var(2); __asm__ __volatile__( -"1:" PPC_LLARX(%0,0,%2,0) " # local_add_return\n\ + PPC_LL" %0,0(%2)\n\ add %0,%1,%0\n" - PPC405_ERR77(0,%2) - PPC_STLCX "%0,0,%2 \n\ - bne- 1b" + PPC_STL "%0,0(%2)\n" : "=&r" (t) : "r" (a), "r" (&(l->a.counter)) : "cc", "memory"); + arch_local_irq_restore(flags); return t; } @@ -41,16 +67,18 @@ static __inline__ long local_add_return(long a, local_t *l) static __inline__ long local_sub_return(long a, local_t *l) { long t; + unsigned long flags; + + flags = arch_local_irq_disable_var(2); __asm__ __volatile__( -"1:" PPC_LLARX(%0,0,%2,0) " # local_sub_return\n\ +"1:" PPC_LL" %0,0(%2)\n\ subf %0,%1,%0\n" - PPC405_ERR77(0,%2) - PPC_STLCX "%0,0,%2 \n\ - bne- 1b" + PPC_STL "%0,0(%2)\n" : "=&r" (t) : "r" (a), "r" (&(l->a.counter)) : "cc", "memory"); + arch_local_irq_restore(flags); return t; } @@ -58,16 +86,17 @@ static __inline__ long local_sub_return(long a, local_t *l) static __inline__ long local_inc_return(local_t *l) { long t; + unsigned long flags; + flags = arch_local_irq_disable_var(2); __asm__ __volatile__( -"1:" PPC_LLARX(%0,0,%1,0) " # local_inc_return\n\ +"1:" PPC_LL" %0,0(%1)\n\ addic %0,%0,1\n" - PPC405_ERR77(0,%1) - PPC_STLCX "%0,0,%1 \n\ - bne- 1b" + PPC_STL "%0,0(%1)\n" : "=&r" (t) : "r" (&(l->a.counter)) : "cc", "xer", "memory"); + arch_local_irq_restore(flags); return t; } @@ -85,20 +114,24 @@ static __inline__ long local_inc_return(local_t *l) static __inline__ long local_dec_return(local_t *l) { long t; + unsigned long flags; + flags = arch_local_irq_disable_var(2); __asm__ __volatile__( -"1:" PPC_LLARX(%0,0,%1,0) " # local_dec_return\n\ + PPC_LL" %0,0(%1)\n\ addic %0,%0,-1\n" - PPC405_ERR77(0,%1) - PPC_STLCX "%0,0,%1\n\ - bne- 1b" + PPC_STL "%0,0(%1)\n" : "=&r" (t) : "r" (&(l->a.counter)) : "cc", "xer", "memory"); + arch_local_irq_restore(flags); return t; } +#define local_inc(l) local_inc_return(l) +#define local_dec(l) local_dec_return(l) + #define local_cmpxchg(l, o, n) \ (cmpxchg_local(&((l)->a.counter), (o), (n))) #define local_xchg(l, n) (xchg_local(&((l)->a.counter), (n))) @@ -115,20 +148,21 @@ static __inline__ long local_dec_return(local_t *l) static __inline__ int local_add_unless(local_t *l, long a, long u) { long t; + unsigned long flags; + flags = arch_local_irq_disable_var(2); __asm__ __volatile__ ( -"1:" PPC_LLARX(%0,0,%1,0) " # local_add_unless\n\ + PPC_LL" %0,0(%1)\n\ cmpw 0,%0,%3 \n\ beq- 2f \n\ add %0,%2,%0 \n" - PPC405_ERR77(0,%2) - PPC_STLCX "%0,0,%1 \n\ - bne- 1b \n" + PPC_STL" %0,0(%1) \n" " subf %0,%2,%0 \n\ 2:" : "=&r" (t) : "r" (&(l->a.counter)), "r" (a), "r" (u) : "cc", "memory"); + arch_local_irq_restore(flags); return t != u; } @@ -145,19 +179,20 @@ static __inline__ int local_add_unless(local_t *l, long a, long u) static __inline__ long local_dec_if_positive(local_t *l) { long t; + unsigned long flags; + flags = arch_local_irq_disable_var(2); __asm__ __volatile__( -"1:" PPC_LLARX(%0,0,%1,0) " # local_dec_if_positive\n\ + PPC_LL" %0,0(%1)\n\ cmpwi %0,1\n\ addi %0,%0,-1\n\ blt- 2f\n" - PPC405_ERR77(0,%1) - PPC_STLCX "%0,0,%1\n\ - bne- 1b" + PPC_STL "%0,0(%1)\n" "\n\ 2:" : "=&b" (t) : "r" (&(l->a.counter)) : "cc", "memory"); + arch_local_irq_restore(flags); return t; } -- 2.7.4 _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
