linux-kernel - [PATCH 2/5] x86/percpu: Relax smp_processor

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Date:   Wed, 27 Feb 2019 11:12:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     torvalds@...ux-foundation.org, mingo@...nel.org, bp@...en8.de,
        tglx@...utronix.de, luto@...nel.org, namit@...are.com,
        peterz@...radead.org
Cc:     linux-kernel@...r.kernel.org, Nadav Amit <nadav.amit@...il.com>
Subject: [PATCH 2/5] x86/percpu: Relax smp_processor_id()

Nadav reported that since this_cpu_read() became asm-volatile, many
smp_processor_id() users generated worse code due to the extra
constraints.

However since smp_processor_id() is reading a stable value, we can use
__this_cpu_read().

Reported-by: Nadav Amit <nadav.amit@...il.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/include/asm/smp.h |    3 ++-
 include/linux/smp.h        |   45 +++++++++++++++++++++++++++++++--------------
 2 files changed, 33 insertions(+), 15 deletions(-)

--- a/arch/x86/include/asm/smp.h
+++ b/arch/x86/include/asm/smp.h
@@ -162,7 +162,8 @@ __visible void smp_call_function_single_
  * from the initial startup. We map APIC_BASE very early in page_setup(),
  * so this is correct in the x86 case.
  */
-#define raw_smp_processor_id() (this_cpu_read(cpu_number))
+#define raw_smp_processor_id()  this_cpu_read(cpu_number)
+#define __smp_processor_id() __this_cpu_read(cpu_number)
 
 #ifdef CONFIG_X86_32
 extern int safe_smp_processor_id(void);
--- a/include/linux/smp.h
+++ b/include/linux/smp.h
@@ -181,29 +181,46 @@ static inline int get_boot_cpu_id(void)
 
 #endif /* !SMP */
 
-/*
- * smp_processor_id(): get the current CPU ID.
+/**
+ * raw_processor_id() - get the current (unstable) CPU id
+ *
+ * For then you know what you are doing and need an unstable
+ * CPU id.
+ */
+
+/**
+ * smp_processor_id() - get the current (stable) CPU id
+ *
+ * This is the normal accessor to the CPU id and should be used
+ * whenever possible.
  *
- * if DEBUG_PREEMPT is enabled then we check whether it is
- * used in a preemption-safe way. (smp_processor_id() is safe
- * if it's used in a preemption-off critical section, or in
- * a thread that is bound to the current CPU.)
+ * The CPU id is stable when:
+ *
+ *  - IRQs are disabled;
+ *  - preemption is disabled;
+ *  - the task is CPU affine.
  *
- * NOTE: raw_smp_processor_id() is for internal use only
- * (smp_processor_id() is the preferred variant), but in rare
- * instances it might also be used to turn off false positives
- * (i.e. smp_processor_id() use that the debugging code reports but
- * which use for some reason is legal). Don't use this to hack around
- * the warning message, as your code might not work under PREEMPT.
+ * When CONFIG_DEBUG_PREEMPT; we verify these assumption and WARN
+ * when smp_processor_id() is used when the CPU id is not stable.
  */
+
+/*
+ * Allow the architecture to differentiate between a stable and unstable read.
+ * For example, x86 uses an IRQ-safe asm-volatile read for the unstable but a
+ * regular asm read for the stable.
+ */
+#ifndef __smp_processor_id
+#define __smp_processor_id(x) raw_smp_processor_id(x)
+#endif
+
 #ifdef CONFIG_DEBUG_PREEMPT
   extern unsigned int debug_smp_processor_id(void);
 # define smp_processor_id() debug_smp_processor_id()
 #else
-# define smp_processor_id() raw_smp_processor_id()
+# define smp_processor_id() __smp_processor_id()
 #endif
 
-#define get_cpu()		({ preempt_disable(); smp_processor_id(); })
+#define get_cpu()		({ preempt_disable(); __smp_processor_id(); })
 #define put_cpu()		preempt_enable()
 
 /*