sched/membarrier: Fix redundant load of membarrier_state

JIRA: https://issues.redhat.com/browse/RHEL-96250

commit 7ab02bd
Author: Nysal Jan K.A. <nysal@linux.ibm.com>
Date:   Mon Mar 3 11:34:50 2025 +0530

    sched/membarrier: Fix redundant load of membarrier_state

    On architectures where ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
    is not selected, sync_core_before_usermode() is a no-op.
    In membarrier_mm_sync_core_before_usermode() the compiler does not
    eliminate redundant branches and load of mm->membarrier_state
    for this case as the atomic_read() cannot be optimized away.

    Here's a snippet of the code generated for finish_task_switch() on powerpc
    prior to this change:

      1b786c:   ld      r26,2624(r30)   # mm = rq->prev_mm;
      .......
      1b78c8:   cmpdi   cr7,r26,0
      1b78cc:   beq     cr7,1b78e4 <finish_task_switch+0xd0>
      1b78d0:   ld      r9,2312(r13)    # current
      1b78d4:   ld      r9,1888(r9)     # current->mm
      1b78d8:   cmpd    cr7,r26,r9
      1b78dc:   beq     cr7,1b7a70 <finish_task_switch+0x25c>
      1b78e0:   hwsync
      1b78e4:   cmplwi  cr7,r27,128
      .......
      1b7a70:   lwz     r9,176(r26)     # atomic_read(&mm->membarrier_state)
      1b7a74:   b       1b78e0 <finish_task_switch+0xcc>

    This was found while analyzing "perf c2c" reports on kernels prior
    to commit c1753fd ("mm: move mm_count into its own cache line")
    where mm_count was false sharing with membarrier_state.

    There is a minor improvement in the size of finish_task_switch().
    The following are results from bloat-o-meter for ppc64le:

      GCC 7.5.0
      ---------
      add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-32 (-32)
      Function                                     old     new   delta
      finish_task_switch                           884     852     -32

      GCC 12.2.1
      ----------
      add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-32 (-32)
      Function                                     old     new   delta
      finish_task_switch.isra                      852     820     -32

      LLVM 17.0.6
      -----------
      add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-36 (-36)
      Function                                     old     new   delta
      rt_mutex_schedule                            120     104     -16
      finish_task_switch                           792     772     -20

    Results on aarch64:

      GCC 14.1.1
      ----------
      add/remove: 0/2 grow/shrink: 1/1 up/down: 4/-60 (-56)
      Function                                     old     new   delta
      get_nohz_timer_target                        352     356      +4
      e843419@0b02_0000d7e7_408                      8       -      -8
      e843419@01bb_000021d2_868                      8       -      -8
      finish_task_switch.isra                      592     548     -44

    Signed-off-by: Nysal Jan K.A. <nysal@linux.ibm.com>
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Reviewed-by: Michael Ellerman <mpe@ellerman.id.au>
    Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
    Link: https://lore.kernel.org/r/20250303060457.531293-1-nysal@linux.ibm.com

Signed-off-by: Phil Auld <pauld@redhat.com>

Author: auldp

include/linux/sched/mm.h

@@ -466,6 +466,13 @@ enum {
 
 static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
 {
+	/*
+	 * The atomic_read() below prevents CSE. The following should
+	 * help the compiler generate more efficient code on architectures
+	 * where sync_core_before_usermode() is a no-op.
+	 */
+	if (!IS_ENABLED(CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE))
+		return;
 	if (current->mm != mm)
 		return;
 	if (likely(!(atomic_read(&mm->membarrier_state) &