6.6-stable review patch. If anyone has any objections, please let me know.
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From: Finn Thain fthain@linux-m68k.org
[ Upstream commit 87c3a5893e865739ce78aa7192d36011022e0af7 ]
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.
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.
The performance issue addressed here is minor on uniprocessor m68k. I got a 0.01% improvement from this patch for a simple "find /sys -false" benchmark. For architectures and workloads susceptible to cache line bounce the improvement is expected to be larger. The only SMP architecture I have is x86, and as x86 unaffected I have not done any further measurements.
Fixes: 15115830c887 ("preempt: Cleanup the macro maze a bit") Signed-off-by: Finn Thain fthain@linux-m68k.org Signed-off-by: Ingo Molnar mingo@kernel.org Link: https://lore.kernel.org/r/0a403120a682a525e6db2d81d1a3ffcc137c3742.169475683... Signed-off-by: Sasha Levin sashal@kernel.org --- 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 1424670df161d..9aa6358a1a16b 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; }
+/* + * These macro definitions avoid redundant invocations of preempt_count() + * because such invocations would result in redundant loads given that + * preempt_count() is commonly implemented with READ_ONCE(). + */ + #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: