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    Subject[tip: sched/core] sched/fair: Add NOHZ balancer flag for nohz.next_balance updates
    The following commit has been merged into the sched/core branch of tip:

    Commit-ID: efd984c481abb516fab8bafb25bf41fd9397a43c
    Gitweb: https://git.kernel.org/tip/efd984c481abb516fab8bafb25bf41fd9397a43c
    Author: Valentin Schneider <valentin.schneider@arm.com>
    AuthorDate: Mon, 23 Aug 2021 12:16:59 +01:00
    Committer: Peter Zijlstra <peterz@infradead.org>
    CommitterDate: Tue, 05 Oct 2021 15:51:30 +02:00

    sched/fair: Add NOHZ balancer flag for nohz.next_balance updates

    A following patch will trigger NOHZ idle balances as a means to update
    nohz.next_balance. Vincent noted that blocked load updates can have
    non-negligible overhead, which should be avoided if the intent is to only
    update nohz.next_balance.

    Add a new NOHZ balance kick flag, NOHZ_NEXT_KICK. Gate NOHZ blocked load
    update by the presence of NOHZ_STATS_KICK - currently all NOHZ balance
    kicks will have the NOHZ_STATS_KICK flag set, so no change in behaviour is
    expected.

    Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
    Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
    Link: https://lkml.kernel.org/r/20210823111700.2842997-2-valentin.schneider@arm.com
    ---
    kernel/sched/fair.c | 24 ++++++++++++++----------
    kernel/sched/sched.h | 8 +++++++-
    2 files changed, 21 insertions(+), 11 deletions(-)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index f6a05d9..f4de7f5 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -10375,7 +10375,7 @@ static void nohz_balancer_kick(struct rq *rq)
    goto out;

    if (rq->nr_running >= 2) {
    - flags = NOHZ_KICK_MASK;
    + flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
    goto out;
    }

    @@ -10389,7 +10389,7 @@ static void nohz_balancer_kick(struct rq *rq)
    * on.
    */
    if (rq->cfs.h_nr_running >= 1 && check_cpu_capacity(rq, sd)) {
    - flags = NOHZ_KICK_MASK;
    + flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
    goto unlock;
    }
    }
    @@ -10403,7 +10403,7 @@ static void nohz_balancer_kick(struct rq *rq)
    */
    for_each_cpu_and(i, sched_domain_span(sd), nohz.idle_cpus_mask) {
    if (sched_asym_prefer(i, cpu)) {
    - flags = NOHZ_KICK_MASK;
    + flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
    goto unlock;
    }
    }
    @@ -10416,7 +10416,7 @@ static void nohz_balancer_kick(struct rq *rq)
    * to run the misfit task on.
    */
    if (check_misfit_status(rq, sd)) {
    - flags = NOHZ_KICK_MASK;
    + flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
    goto unlock;
    }

    @@ -10443,7 +10443,7 @@ static void nohz_balancer_kick(struct rq *rq)
    */
    nr_busy = atomic_read(&sds->nr_busy_cpus);
    if (nr_busy > 1) {
    - flags = NOHZ_KICK_MASK;
    + flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
    goto unlock;
    }
    }
    @@ -10605,7 +10605,8 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
    * setting the flag, we are sure to not clear the state and not
    * check the load of an idle cpu.
    */
    - WRITE_ONCE(nohz.has_blocked, 0);
    + if (flags & NOHZ_STATS_KICK)
    + WRITE_ONCE(nohz.has_blocked, 0);

    /*
    * Ensures that if we miss the CPU, we must see the has_blocked
    @@ -10627,13 +10628,15 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
    * balancing owner will pick it up.
    */
    if (need_resched()) {
    - has_blocked_load = true;
    + if (flags & NOHZ_STATS_KICK)
    + has_blocked_load = true;
    goto abort;
    }

    rq = cpu_rq(balance_cpu);

    - has_blocked_load |= update_nohz_stats(rq);
    + if (flags & NOHZ_STATS_KICK)
    + has_blocked_load |= update_nohz_stats(rq);

    /*
    * If time for next balance is due,
    @@ -10664,8 +10667,9 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
    if (likely(update_next_balance))
    nohz.next_balance = next_balance;

    - WRITE_ONCE(nohz.next_blocked,
    - now + msecs_to_jiffies(LOAD_AVG_PERIOD));
    + if (flags & NOHZ_STATS_KICK)
    + WRITE_ONCE(nohz.next_blocked,
    + now + msecs_to_jiffies(LOAD_AVG_PERIOD));

    abort:
    /* There is still blocked load, enable periodic update */
    diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
    index 3d3e579..1fec313 100644
    --- a/kernel/sched/sched.h
    +++ b/kernel/sched/sched.h
    @@ -2709,12 +2709,18 @@ extern void cfs_bandwidth_usage_dec(void);
    #define NOHZ_BALANCE_KICK_BIT 0
    #define NOHZ_STATS_KICK_BIT 1
    #define NOHZ_NEWILB_KICK_BIT 2
    +#define NOHZ_NEXT_KICK_BIT 3

    +/* Run rebalance_domains() */
    #define NOHZ_BALANCE_KICK BIT(NOHZ_BALANCE_KICK_BIT)
    +/* Update blocked load */
    #define NOHZ_STATS_KICK BIT(NOHZ_STATS_KICK_BIT)
    +/* Update blocked load when entering idle */
    #define NOHZ_NEWILB_KICK BIT(NOHZ_NEWILB_KICK_BIT)
    +/* Update nohz.next_balance */
    +#define NOHZ_NEXT_KICK BIT(NOHZ_NEXT_KICK_BIT)

    -#define NOHZ_KICK_MASK (NOHZ_BALANCE_KICK | NOHZ_STATS_KICK)
    +#define NOHZ_KICK_MASK (NOHZ_BALANCE_KICK | NOHZ_STATS_KICK | NOHZ_NEXT_KICK)

    #define nohz_flags(cpu) (&cpu_rq(cpu)->nohz_flags)

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