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().

While this does reduce text size somewhat, this mostly results in code
movement to .text.unlikely as a result of more/larger .cold.
subfunctions. Less text on the hotpath is good for I$.

  $ ./compare.sh defconfig-build1 defconfig-build2 vmlinux.o
  setup_APIC_ibs                                             90         98   -12,+20
  force_ibs_eilvt_setup                                     400        413   -57,+70
  pci_serr_error                                            109        104   -54,+49
  pci_serr_error                                            109        104   -54,+49
  unknown_nmi_error                                         125        120   -76,+71
  unknown_nmi_error                                         125        120   -76,+71
  io_check_error                                            125        132   -97,+104
  intel_thermal_interrupt                                   730        822   +92,+0
  intel_init_thermal                                        951        945   -6,+0
  generic_get_mtrr                                          301        294   -7,+0
  generic_get_mtrr                                          301        294   -7,+0
  generic_set_all                                           749        754   -44,+49
  get_fixed_ranges                                          352        360   -41,+49
  x86_acpi_suspend_lowlevel                                 369        363   -6,+0
  check_tsc_sync_source                                     412        412   -71,+71
  irq_migrate_all_off_this_cpu                              662        674   -14,+26
  clocksource_watchdog                                      748        748   -113,+113
  __perf_event_account_interrupt                            204        197   -7,+0
  attempt_merge                                            1748       1741   -7,+0
  intel_guc_send_ct                                        1424       1409   -15,+0
  __fini_doorbell                                           235        231   -4,+0
  bdw_set_cdclk                                             928        923   -5,+0
  gen11_dsi_disable                                        1571       1556   -15,+0
  gmbus_wait                                                493        488   -5,+0
  md_make_request                                           376        369   -7,+0
  __split_and_process_bio                                   543        536   -7,+0
  delay_tsc                                                  96         89   -7,+0
  hsw_disable_pc8                                           696        691   -5,+0
  tsc_verify_tsc_adjust                                     215        228   -22,+35
  cpuidle_driver_unref                                       56         49   -7,+0
  blk_account_io_completion                                 159        148   -11,+0
  mtrr_wrmsr                                                 95         99   -29,+33
  __intel_wait_for_register_fw                              401        419   +18,+0
  cpuidle_driver_ref                                         43         36   -7,+0
  cpuidle_get_driver                                         15          8   -7,+0
  blk_account_io_done                                       535        528   -7,+0
  irq_migrate_all_off_this_cpu                              662        674   -14,+26
  check_tsc_sync_source                                     412        412   -71,+71
  irq_wait_for_poll                                         170        163   -7,+0
  generic_end_io_acct                                       329        322   -7,+0
  x86_acpi_suspend_lowlevel                                 369        363   -6,+0
  nohz_balance_enter_idle                                   198        191   -7,+0
  generic_start_io_acct                                     254        247   -7,+0
  blk_account_io_start                                      341        334   -7,+0
  perf_event_task_tick                                      682        675   -7,+0
  intel_init_thermal                                        951        945   -6,+0
  amd_e400_c1e_apic_setup                                    47         51   -28,+32
  setup_APIC_eilvt                                          350        328   -22,+0
  hsw_enable_pc8                                           1611       1605   -6,+0
                                               total   12985947   12985892   -994,+939
Reported-by: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/smp.h

@@ -162,7 +162,8 @@ __visible void smp_call_function_single_interrupt(struct pt_regs *r);
  * 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);

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.
+ *
+ * The CPU id is stable when:
  *
- * 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.)
+ *  - 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()
 
 /*