linux-kernel - [PATCH] sched: Optimize in_task() and in

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Message-Id: <0a403120a682a525e6db2d81d1a3ffcc137c3742.1694756831.git.fthain@linux-m68k.org>
Date:   Fri, 15 Sep 2023 15:47:11 +1000
From:   Finn Thain <fthain@...ux-m68k.org>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>
Cc:     "Thomas Gleixner" <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] sched: Optimize in_task() and in_interrupt() a bit

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 <tglx@...utronix.de>
Fixes: 15115830c887 ("preempt: Cleanup the macro maze a bit")
Signed-off-by: Finn Thain <fthain@...ux-m68k.org>
---
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.

Changed since v2:
 - Clarify the comment about macros.

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..9aa6358a1a16 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:
-- 
2.39.3