Except on x86, preempt_count is always accessed with READ_ONCE.
Repeated invocations in macros like irq_count() produce repeated loads.
These redundant instructions appear in various fast paths. In the one
shown below, for example, irq_count() is evaluated during kernel entry
if !tick_nohz_full_cpu(smp_processor_id()).
0001ed0a <irq_enter_rcu>:
1ed0a: 4e56 0000 linkw %fp,#0
1ed0e: 200f movel %sp,%d0
1ed10: 0280 ffff e000 andil #-8192,%d0
1ed16: 2040 moveal %d0,%a0
1ed18: 2028 0008 movel %a0@(8),%d0
1ed1c: 0680 0001 0000 addil #65536,%d0
1ed22: 2140 0008 movel %d0,%a0@(8)
1ed26: 082a 0001 000f btst #1,%a2@(15)
1ed2c: 670c beqs 1ed3a <irq_enter_rcu+0x30>
1ed2e: 2028 0008 movel %a0@(8),%d0
1ed32: 2028 0008 movel %a0@(8),%d0
1ed36: 2028 0008 movel %a0@(8),%d0
1ed3a: 4e5e unlk %fp
1ed3c: 4e75 rts
This patch doesn't prevent the pointless btst and beqs instructions
above, but it does eliminate 2 of the 3 pointless move instructions
here and elsewhere.
On x86, preempt_count is per-cpu data and the problem does not arise
presumably because the compiler is free to optimize more effectively.
Cc: Thomas Gleixner <[email protected]>
Fixes: 15115830c887 ("preempt: Cleanup the macro maze a bit")
Signed-off-by: Finn Thain <[email protected]>
---
This patch was tested on m68k and x86. I was expecting no changes
to object code for x86 and mostly that's what I saw. However, there
were a few places where code generation was perturbed for some reason.
Changed since v1:
- Added a comment that was requested by Frederic.
---
include/linux/preempt.h | 15 +++++++++++++--
1 file changed, 13 insertions(+), 2 deletions(-)
diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index 1424670df161..d2ac07abf225 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -99,14 +99,21 @@ static __always_inline unsigned char interrupt_context_level(void)
return level;
}
+/*
+ * Redundant invocations of preempt_count() are to be avoided because that
+ * function is commonly implemented using READ_ONCE. Hence, the optimal
+ * definition of irq_count() or in_task() depends on CONFIG_PREEMPT_RT.
+ */
+
#define nmi_count() (preempt_count() & NMI_MASK)
#define hardirq_count() (preempt_count() & HARDIRQ_MASK)
#ifdef CONFIG_PREEMPT_RT
# define softirq_count() (current->softirq_disable_cnt & SOFTIRQ_MASK)
+# define irq_count() ((preempt_count() & (NMI_MASK | HARDIRQ_MASK)) | softirq_count())
#else
# define softirq_count() (preempt_count() & SOFTIRQ_MASK)
+# define irq_count() (preempt_count() & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_MASK))
#endif
-#define irq_count() (nmi_count() | hardirq_count() | softirq_count())
/*
* Macros to retrieve the current execution context:
@@ -119,7 +126,11 @@ static __always_inline unsigned char interrupt_context_level(void)
#define in_nmi() (nmi_count())
#define in_hardirq() (hardirq_count())
#define in_serving_softirq() (softirq_count() & SOFTIRQ_OFFSET)
-#define in_task() (!(in_nmi() | in_hardirq() | in_serving_softirq()))
+#ifdef CONFIG_PREEMPT_RT
+# define in_task() (!((preempt_count() & (NMI_MASK | HARDIRQ_MASK)) | in_serving_softirq()))
+#else
+# define in_task() (!(preempt_count() & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))
+#endif
/*
* The following macros are deprecated and should not be used in new code:
--
2.39.3