[PATCH] convert cnt32_to_63 to inline

Mathieu Desnoyers
2008-11-11T18:28:05Z

* Nicolas Pitre (nico@cam.org) wrote:
> On Mon, 10 Nov 2008, Andrew Morton wrote:
>
> > On Mon, 10 Nov 2008 18:15:32 -0500 (EST)
> > Nicolas Pitre <nico@cam.org> wrote:
> >
> > > On Mon, 10 Nov 2008, Andrew Morton wrote:
> > >
> > > > This references its second argument twice, which can cause correctness
> > > > or efficiency problems.
> > > >
> > > > There is no reason that this had to be implemented in cpp.
> > > > Implementing it in C will fix the above problem.
> > >
> > > No, it won't, for correctness and efficiency reasons.
> > >
> > > And I've explained why already.
> >
> > I'd be very surprised if you've really found a case where a macro is
> > faster than an inlined function. I don't think that has happened
> > before.
>
> That hasn't anything to do with "a macro is faster" at all. It's all
> about the order used to evaluate provided arguments. And the first one
> might be anything like a memory value, an IO operation, an expression,
> etc. An inline function would work correctly with pointers only and
> therefore totally break apart on x86 for example.
>
>
> Nicolas

Let's see what it gives once implemented. Only compile-tested. Assumes
pxa, sa110 and mn10300 are all UP-only. Correct smp_rmb() are used for
arm versatile.

Turn cnt32_to_63 into an inline function.
Change all callers to new API.
Document barrier usage.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
CC: Nicolas Pitre <nico@cam.org>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: torvalds@linux-foundation.org
CC: rmk+lkml@arm.linux.org.uk
CC: dhowells@redhat.com
CC: paulus@samba.org
CC: a.p.zijlstra@chello.nl
CC: mingo@elte.hu
CC: benh@kernel.crashing.org
CC: rostedt@goodmis.org
CC: tglx@linutronix.de
CC: davem@davemloft.net
CC: ralf@linux-mips.org
---
arch/arm/mach-pxa/time.c | 14 ++++++++++++-
arch/arm/mach-sa1100/generic.c | 15 +++++++++++++-
arch/arm/mach-versatile/core.c | 12 ++++++++++-
arch/mn10300/kernel/time.c | 19 +++++++++++++-----
include/linux/cnt32_to_63.h | 42 +++++++++++++++++++++++++++++------------
5 files changed, 82 insertions(+), 20 deletions(-)

Index: linux.trees.git/arch/arm/mach-pxa/time.c
===================================================================
--- linux.trees.git.orig/arch/arm/mach-pxa/time.c 2008-11-11 12:20:42.000000000 -0500
+++ linux.trees.git/arch/arm/mach-pxa/time.c 2008-11-11 13:05:01.000000000 -0500
@@ -37,6 +37,10 @@
#define OSCR2NS_SCALE_FACTOR 10

static unsigned long oscr2ns_scale;
+static u32 sched_clock_cnt_hi; /*
+ * Shared cnt_hi OK with cycle counter only
+ * for UP systems.
+ */

static void __init set_oscr2ns_scale(unsigned long oscr_rate)
{
@@ -54,7 +58,15 @@ static void __init set_oscr2ns_scale(uns

unsigned long long sched_clock(void)
{
- unsigned long long v = cnt32_to_63(OSCR);
+ u32 cnt_lo, cnt_hi;
+ unsigned long long v;
+
+ preempt_disable_notrace();
+ cnt_hi = sched_clock_cnt_hi;
+ barrier(); /* read cnt_hi before cnt_lo */
+ cnt_lo = OSCR;
+ v = cnt32_to_63(cnt_hi, cnt_lo, &sched_clock_cnt_hi);
+ preempt_enable_notrace();
return (v * oscr2ns_scale) >> OSCR2NS_SCALE_FACTOR;
}

Index: linux.trees.git/include/linux/cnt32_to_63.h
===================================================================
--- linux.trees.git.orig/include/linux/cnt32_to_63.h 2008-11-11 12:20:17.000000000 -0500
+++ linux.trees.git/include/linux/cnt32_to_63.h 2008-11-11 13:10:44.000000000 -0500
@@ -32,7 +32,9 @@ union cnt32_to_63 {

/**
* cnt32_to_63 - Expand a 32-bit counter to a 63-bit counter
- * @cnt_lo: The low part of the counter
+ * @cnt_hi: The high part of the counter (read first)
+ * @cnt_lo: The low part of the counter (read after cnt_hi)
+ * @cnt_hi_ptr: Pointer to high part of the counter
*
* Many hardware clock counters are only 32 bits wide and therefore have
* a relatively short period making wrap-arounds rather frequent. This
@@ -57,7 +59,10 @@ union cnt32_to_63 {
* code must be executed at least once per each half period of the 32-bit
* counter to properly update the state bit in memory. This is usually not a
* problem in practice, but if it is then a kernel timer could be scheduled
- * to manage for this code to be executed often enough.
+ * to manage for this code to be executed often enough. If a per-cpu cnt_hi is
+ * used, the value must be updated at least once per 32-bits half-period on each
+ * CPU. If cnt_hi is shared between CPUs, it suffice to update it once per
+ * 32-bits half-period on any CPU.
*
* Note that the top bit (bit 63) in the returned value should be considered
* as garbage. It is not cleared here because callers are likely to use a
@@ -65,16 +70,29 @@ union cnt32_to_63 {
* implicitly by making the multiplier even, therefore saving on a runtime
* clear-bit instruction. Otherwise caller must remember to clear the top
* bit explicitly.
+ *
+ * Preemption must be disabled when reading the cnt_hi and cnt_lo values and
+ * calling this function.
+ *
+ * The cnt_hi parameter _must_ be read before cnt_lo. This implies using the
+ * proper barriers :
+ * - smp_rmb() if cnt_lo is read from mmio and the cnt_hi variable is shared
+ * across CPUs.
+ * - use a per-cpu variable for cnt_high if cnt_lo is read from per-cpu cycles
+ * counters or to read the counters with only a barrier().
*/
-#define cnt32_to_63(cnt_lo) \
-({ \
- static volatile u32 __m_cnt_hi; \
- union cnt32_to_63 __x; \
- __x.hi = __m_cnt_hi; \
- __x.lo = (cnt_lo); \
- if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \
- __m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \
- __x.val; \
-})
+static inline u64 cnt32_to_63(u32 cnt_hi, u32 cnt_lo, u32 *cnt_hi_ptr)
+{
+ union cnt32_to_63 __x = {
+ {
+ .hi = cnt_hi,
+ .lo = cnt_lo,
+ },
+ };
+
+ if (unlikely((s32)(__x.hi ^ __x.lo) < 0))
+ *cnt_hi_ptr = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31);
+ return __x.val; /* Remember to clear the top bit in the caller */
+}

#endif
Index: linux.trees.git/arch/arm/mach-sa1100/generic.c
===================================================================
--- linux.trees.git.orig/arch/arm/mach-sa1100/generic.c 2008-11-11 12:20:42.000000000 -0500
+++ linux.trees.git/arch/arm/mach-sa1100/generic.c 2008-11-11 13:05:10.000000000 -0500
@@ -34,6 +34,11 @@
unsigned int reset_status;
EXPORT_SYMBOL(reset_status);

+static u32 sched_clock_cnt_hi; /*
+ * Shared cnt_hi OK with cycle counter only
+ * for UP systems.
+ */
+
#define NR_FREQS 16

/*
@@ -133,7 +138,15 @@ EXPORT_SYMBOL(cpufreq_get);
*/
unsigned long long sched_clock(void)
{
- unsigned long long v = cnt32_to_63(OSCR);
+ u32 cnt_lo, cnt_hi;
+ unsigned long long v;
+
+ preempt_disable_notrace();
+ cnt_hi = sched_clock_cnt_hi;
+ barrier(); /* read cnt_hi before cnt_lo */
+ cnt_lo = OSCR;
+ v = cnt32_to_63(cnt_hi, cnt_lo, &sched_clock_cnt_hi);
+ preempt_enable_notrace();

/* the <<1 gets rid of the cnt_32_to_63 top bit saving on a bic insn */
v *= 78125<<1;
Index: linux.trees.git/arch/arm/mach-versatile/core.c
===================================================================
--- linux.trees.git.orig/arch/arm/mach-versatile/core.c 2008-11-11 12:20:42.000000000 -0500
+++ linux.trees.git/arch/arm/mach-versatile/core.c 2008-11-11 12:57:55.000000000 -0500
@@ -60,6 +60,8 @@
#define VA_VIC_BASE __io_address(VERSATILE_VIC_BASE)
#define VA_SIC_BASE __io_address(VERSATILE_SIC_BASE)

+static u32 sched_clock_cnt_hi;
+
static void sic_mask_irq(unsigned int irq)
{
irq -= IRQ_SIC_START;
@@ -238,7 +240,15 @@ void __init versatile_map_io(void)
*/
unsigned long long sched_clock(void)
{
- unsigned long long v = cnt32_to_63(readl(VERSATILE_REFCOUNTER));
+ u32 cnt_lo, cnt_hi;
+ unsigned long long v;
+
+ preempt_disable_notrace();
+ cnt_hi = sched_clock_cnt_hi;
+ smp_rmb();
+ cnt_lo = readl(VERSATILE_REFCOUNTER);
+ v = cnt32_to_63(cnt_hi, cnt_lo, &sched_clock_cnt_hi);
+ preempt_enable_notrace();

/* the <<1 gets rid of the cnt_32_to_63 top bit saving on a bic insn */
v *= 125<<1;
Index: linux.trees.git/arch/mn10300/kernel/time.c
===================================================================
--- linux.trees.git.orig/arch/mn10300/kernel/time.c 2008-11-11 12:41:42.000000000 -0500
+++ linux.trees.git/arch/mn10300/kernel/time.c 2008-11-11 13:04:42.000000000 -0500
@@ -29,6 +29,11 @@ unsigned long mn10300_iobclk; /* system
unsigned long mn10300_tsc_per_HZ; /* number of ioclks per jiffy */
#endif /* CONFIG_MN10300_RTC */

+static u32 sched_clock_cnt_hi; /*
+ * shared cnt_hi OK with cycle counter only
+ * for UP systems.
+ */
+
static unsigned long mn10300_last_tsc; /* time-stamp counter at last time
* interrupt occurred */

@@ -52,18 +57,22 @@ unsigned long long sched_clock(void)
unsigned long long ll;
unsigned l[2];
} tsc64, result;
- unsigned long tsc, tmp;
+ unsigned long tmp;
unsigned product[3]; /* 96-bit intermediate value */
+ u32 cnt_lo, cnt_hi;

- /* read the TSC value
- */
- tsc = 0 - get_cycles(); /* get_cycles() counts down */
+ preempt_disable_notrace();
+ cnt_hi = sched_clock_cnt_hi;
+ barrier(); /* read cnt_hi before cnt_lo */
+ cnt_lo = 0 - get_cycles(); /* get_cycles() counts down */

/* expand to 64-bits.
* - sched_clock() must be called once a minute or better or the
* following will go horribly wrong - see cnt32_to_63()
*/
- tsc64.ll = cnt32_to_63(tsc) & 0x7fffffffffffffffULL;
+ tsc64.ll = cnt32_to_63(cnt_hi, cnt_lo, &sched_clock_cnt_hi);
+ tsc64.ll &= 0x7fffffffffffffffULL;
+ preempt_enable_notrace();

/* scale the 64-bit TSC value to a nanosecond value via a 96-bit
* intermediate

--
Mathieu Desnoyers
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