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    Subject[PATCH 5.15 105/135] rcu: Apply callbacks processing time limit only on softirq
    Date
    From: Frederic Weisbecker <frederic@kernel.org>

    commit a554ba288845fd3f6f12311fd76a51694233458a upstream.

    Time limit only makes sense when callbacks are serviced in softirq mode
    because:

    _ In case we need to get back to the scheduler,
    cond_resched_tasks_rcu_qs() is called after each callback.

    _ In case some other softirq vector needs the CPU, the call to
    local_bh_enable() before cond_resched_tasks_rcu_qs() takes care about
    them via a call to do_softirq().

    Therefore, make sure the time limit only applies to softirq mode.

    Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
    Tested-by: Valentin Schneider <valentin.schneider@arm.com>
    Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
    Cc: Valentin Schneider <valentin.schneider@arm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Cc: Josh Triplett <josh@joshtriplett.org>
    Cc: Joel Fernandes <joel@joelfernandes.org>
    Cc: Boqun Feng <boqun.feng@gmail.com>
    Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
    Cc: Uladzislau Rezki <urezki@gmail.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
    [UR: backport to 5.15-stable]
    Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    kernel/rcu/tree.c | 26 +++++++++++++-------------
    1 file changed, 13 insertions(+), 13 deletions(-)

    --- a/kernel/rcu/tree.c
    +++ b/kernel/rcu/tree.c
    @@ -2476,7 +2476,7 @@ static void rcu_do_batch(struct rcu_data
    div = READ_ONCE(rcu_divisor);
    div = div < 0 ? 7 : div > sizeof(long) * 8 - 2 ? sizeof(long) * 8 - 2 : div;
    bl = max(rdp->blimit, pending >> div);
    - if (unlikely(bl > 100)) {
    + if (in_serving_softirq() && unlikely(bl > 100)) {
    long rrn = READ_ONCE(rcu_resched_ns);

    rrn = rrn < NSEC_PER_MSEC ? NSEC_PER_MSEC : rrn > NSEC_PER_SEC ? NSEC_PER_SEC : rrn;
    @@ -2517,6 +2517,18 @@ static void rcu_do_batch(struct rcu_data
    if (count >= bl && (need_resched() ||
    (!is_idle_task(current) && !rcu_is_callbacks_kthread())))
    break;
    +
    + /*
    + * Make sure we don't spend too much time here and deprive other
    + * softirq vectors of CPU cycles.
    + */
    + if (unlikely(tlimit)) {
    + /* only call local_clock() every 32 callbacks */
    + if (likely((count & 31) || local_clock() < tlimit))
    + continue;
    + /* Exceeded the time limit, so leave. */
    + break;
    + }
    } else {
    local_bh_enable();
    lockdep_assert_irqs_enabled();
    @@ -2524,18 +2536,6 @@ static void rcu_do_batch(struct rcu_data
    lockdep_assert_irqs_enabled();
    local_bh_disable();
    }
    -
    - /*
    - * Make sure we don't spend too much time here and deprive other
    - * softirq vectors of CPU cycles.
    - */
    - if (unlikely(tlimit)) {
    - /* only call local_clock() every 32 callbacks */
    - if (likely((count & 31) || local_clock() < tlimit))
    - continue;
    - /* Exceeded the time limit, so leave. */
    - break;
    - }
    }

    local_irq_save(flags);

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