x86/percpu, sched/fair: Avoid local_clock() [Linux 5.3]

This Linux kernel change "x86/percpu, sched/fair: Avoid local_clock()" is included in the Linux 5.3 release. This change is authored by Peter Zijlstra <peterz [at] infradead.org> on Wed Feb 27 10:27:58 2019 +0100. The commit for this change in Linux stable tree is 8dc2d99 (patch).

x86/percpu, sched/fair: Avoid local_clock()

Nadav reported that code-gen changed because of the this_cpu_*()
constraints, avoid this for select_idle_cpu() because that runs with
preemption (and IRQs) disabled anyway.

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>

There are 5 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 kernel/sched/fair.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f35930f..8591529e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6189,6 +6189,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
    u64 time, cost;
    s64 delta;
    int cpu, nr = INT_MAX;
+   int this = smp_processor_id();

    this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
    if (!this_sd)
@@ -6212,7 +6213,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
            nr = 4;
    }

-   time = local_clock();
+   time = cpu_clock(this);

    for_each_cpu_wrap(cpu, sched_domain_span(sd), target) {
        if (!--nr)
@@ -6223,7 +6224,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
            break;
    }

-   time = local_clock() - time;
+   time = cpu_clock(this) - time;
    cost = this_sd->avg_scan_cost;
    delta = (s64)(time - cost) / 8;
    this_sd->avg_scan_cost += delta;

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