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s390/idle: Slightly optimize idle time accounting

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Slightly optimize account_idle_time_irq() and update_timer_idle():

- Use fast single instruction __atomic64() primitives to update per
  cpu idle_time and idle_count, instead of READ_ONCE() / WRITE_ONCE()
  pairs

- stcctm() is an inline assembly with a full memory barrier. This
  leads to a not necessary extra dereference of smp_cpu_mtid in
  update_timer_idle(). Avoid this and read smp_cpu_mtid into a
  variable

- Use __this_cpu_add() instead of this_cpu_add() to avoid disabling /
  enabling of preemption several times in a loop in update_timer_idle().

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
This commit is contained in:
Heiko Carstens 2026-02-18 15:20:07 +01:00 committed by Vasily Gorbik
parent aefa6ec890
commit 00d8b035eb
1 changed files with 8 additions and 7 deletions
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15
arch/s390/kernel/idle.c
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@ -26,12 +26,13 @@ void update_timer_idle(void)
struct s390_idle_data *idle = this_cpu_ptr(&s390_idle);
struct lowcore *lc = get_lowcore();
u64 cycles_new[8];
int i;
int i, mtid;
if (smp_cpu_mtid) {
stcctm(MT_DIAG, smp_cpu_mtid, cycles_new);
for (i = 0; i < smp_cpu_mtid; i++)
this_cpu_add(mt_cycles[i], cycles_new[i] - idle->mt_cycles_enter[i]);
mtid = smp_cpu_mtid;
if (mtid) {
stcctm(MT_DIAG, mtid, cycles_new);
for (i = 0; i < mtid; i++)
__this_cpu_add(mt_cycles[i], cycles_new[i] - idle->mt_cycles_enter[i]);
}
/*
@ -58,8 +59,8 @@ void account_idle_time_irq(void)
idle_time = get_lowcore()->int_clock - idle->clock_idle_enter;
/* Account time spent with enabled wait psw loaded as idle time. */
WRITE_ONCE(idle->idle_time, READ_ONCE(idle->idle_time) + idle_time);
WRITE_ONCE(idle->idle_count, READ_ONCE(idle->idle_count) + 1);
__atomic64_add(idle_time, &idle->idle_time);
__atomic64_add_const(1, &idle->idle_count);
account_idle_time(cputime_to_nsecs(idle_time));
}