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    SubjectRe: [PATCH Resend v5] sched: fix init NOHZ_IDLE flag
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    On 11 April 2013 16:56, Frederic Weisbecker <fweisbec@gmail.com> wrote:
    > On Wed, Apr 03, 2013 at 03:37:43PM +0200, Vincent Guittot wrote:
    >> On my smp platform which is made of 5 cores in 2 clusters, I have the
    >> nr_busy_cpu field of sched_group_power struct that is not null when the
    >> platform is fully idle. The root cause is:
    >> During the boot sequence, some CPUs reach the idle loop and set their
    >> NOHZ_IDLE flag while waiting for others CPUs to boot. But the nr_busy_cpus
    >> field is initialized later with the assumption that all CPUs are in the busy
    >> state whereas some CPUs have already set their NOHZ_IDLE flag.
    >>
    >> More generally, the NOHZ_IDLE flag must be initialized when new sched_domains
    >> are created in order to ensure that NOHZ_IDLE and nr_busy_cpus are aligned.
    >>
    >> This condition can be ensured by adding a synchronize_rcu between the
    >> destruction of old sched_domains and the creation of new ones so the NOHZ_IDLE
    >> flag will not be updated with old sched_domain once it has been initialized.
    >> But this solution introduces a additionnal latency in the rebuild sequence
    >> that is called during cpu hotplug.
    >>
    >> As suggested by Frederic Weisbecker, another solution is to have the same
    >> rcu lifecycle for both NOHZ_IDLE and sched_domain struct. I have introduce
    >> a new sched_domain_rq struct that is the entry point for both sched_domains
    >> and objects that must follow the same lifecycle like NOHZ_IDLE flags. They
    >> will share the same RCU lifecycle and will be always synchronized.
    >>
    >> The synchronization is done at the cost of :
    >> - an additional indirection for accessing the first sched_domain level
    >> - an additional indirection and a rcu_dereference before accessing to the
    >> NOHZ_IDLE flag.
    >>
    >> Change since v4:
    >> - link both sched_domain and NOHZ_IDLE flag in one RCU object so
    >> their states are always synchronized.
    >>
    >> Change since V3;
    >> - NOHZ flag is not cleared if a NULL domain is attached to the CPU
    >> - Remove patch 2/2 which becomes useless with latest modifications
    >>
    >> Change since V2:
    >> - change the initialization to idle state instead of busy state so a CPU that
    >> enters idle during the build of the sched_domain will not corrupt the
    >> initialization state
    >>
    >> Change since V1:
    >> - remove the patch for SCHED softirq on an idle core use case as it was
    >> a side effect of the other use cases.
    >>
    >> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
    >
    > Ok, the lockless scheme involving nr_buzy_cpus and rq flags seem to be correct now.
    > We can hope somebody will come up with a less complicated solution. But for now that's
    > the best fix I've seen.

    Thanks

    >
    > I just have a few comments on details.
    >
    >> ---
    >> include/linux/sched.h | 6 +++
    >> kernel/sched/core.c | 105 ++++++++++++++++++++++++++++++++++++++++++++-----
    >> kernel/sched/fair.c | 35 +++++++++++------
    >> kernel/sched/sched.h | 24 +++++++++--
    >> 4 files changed, 145 insertions(+), 25 deletions(-)
    >>
    >> diff --git a/include/linux/sched.h b/include/linux/sched.h
    >> index d35d2b6..2a52188 100644
    >> --- a/include/linux/sched.h
    >> +++ b/include/linux/sched.h
    >> @@ -959,6 +959,12 @@ struct sched_domain {
    >> unsigned long span[0];
    >> };
    >>
    >> +struct sched_domain_rq {
    >> + struct sched_domain *sd;
    >> + unsigned long flags;
    >> + struct rcu_head rcu; /* used during destruction */
    >> +};
    >
    > So the reason for this level of indirection won't be intuitive for those
    > who read that code. Please add some comments that explain why we need
    > that. ie: because we need the object lifecycle of sched_power and flags to be the
    > same for the lockless scheme to work.

    ok, i will add a comment

    >
    >> +
    >> static inline struct cpumask *sched_domain_span(struct sched_domain *sd)
    >> {
    >> return to_cpumask(sd->span);
    >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    >> index 7f12624..69e2313 100644
    >> --- a/kernel/sched/core.c
    >> +++ b/kernel/sched/core.c
    >> @@ -5602,6 +5602,15 @@ static void destroy_sched_domains(struct sched_domain *sd, int cpu)
    >> destroy_sched_domain(sd, cpu);
    >> }
    >>
    >> +static void destroy_sched_domain_rq(struct sched_domain_rq *sd_rq, int cpu)
    >> +{
    >> + if (!sd_rq)
    >> + return;
    >> +
    >> + destroy_sched_domains(sd_rq->sd, cpu);
    >> + kfree_rcu(sd_rq, rcu);
    >> +}
    >> +
    >> /*
    >> * Keep a special pointer to the highest sched_domain that has
    >> * SD_SHARE_PKG_RESOURCE set (Last Level Cache Domain) for this
    >> @@ -5632,10 +5641,23 @@ static void update_top_cache_domain(int cpu)
    >> * hold the hotplug lock.
    >> */
    >> static void
    >> -cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
    >> +cpu_attach_domain(struct sched_domain_rq *sd_rq, struct root_domain *rd,
    >> + int cpu)
    >> {
    >> struct rq *rq = cpu_rq(cpu);
    >> - struct sched_domain *tmp;
    >> + struct sched_domain_rq *tmp_rq;
    >
    > old_sd_rq would be a clearer name.

    ok

    >
    >> + struct sched_domain *tmp, *sd = NULL;
    >> +
    >> + /*
    >> + * If we don't have any sched_domain and associated object, we can
    >> + * directly jump to the attach sequence otherwise we try to degenerate
    >> + * the sched_domain
    >> + */
    >> + if (!sd_rq)
    >> + goto attach;
    >> +
    >> + /* Get a pointer to the 1st sched_domain */
    >> + sd = sd_rq->sd;
    >>
    >> /* Remove the sched domains which do not contribute to scheduling. */
    >> for (tmp = sd; tmp; ) {
    >> @@ -5658,14 +5680,17 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
    >> destroy_sched_domain(tmp, cpu);
    >> if (sd)
    >> sd->child = NULL;
    >> + /* update sched_domain_rq */
    >> + sd_rq->sd = sd;
    >> }
    >>
    >> +attach:
    >> sched_domain_debug(sd, cpu);
    >>
    >> rq_attach_root(rq, rd);
    >> - tmp = rq->sd;
    >> - rcu_assign_pointer(rq->sd, sd);
    >> - destroy_sched_domains(tmp, cpu);
    >> + tmp_rq = rq->sd_rq;
    >> + rcu_assign_pointer(rq->sd_rq, sd_rq);
    >> + destroy_sched_domain_rq(tmp_rq, cpu);
    >>
    >> update_top_cache_domain(cpu);
    >
    > [...]
    >> +static void __sdrq_free(const struct cpumask *cpu_map, struct s_data *d)
    >> +{
    >> + int j;
    >> +
    >> + for_each_cpu(j, cpu_map)
    >> + if (*per_cpu_ptr(d->sd_rq, j))
    >> + kfree(*per_cpu_ptr(d->sd_rq, j));
    >
    > kfree(NULL) works.

    thanks

    >
    >> +}
    >> +
    >> +static void build_sched_domain_rq(struct s_data *d, int cpu)
    >> +{
    >> + struct sched_domain_rq *sd_rq;
    >> + struct sched_domain *sd;
    >> +
    >> + /* Attach sched_domain to sched_domain_rq */
    >> + sd = *per_cpu_ptr(d->sd, cpu);
    >> + sd_rq = *per_cpu_ptr(d->sd_rq, cpu);
    >> + sd_rq->sd = sd;
    >> + /* Init flags */
    >> + set_bit(NOHZ_IDLE, sched_rq_flags(sd_rq));
    >> +}
    >> +
    >> struct sched_domain *build_sched_domain(struct sched_domain_topology_level *tl,
    >> struct s_data *d, const struct cpumask *cpu_map,
    >> struct sched_domain_attr *attr, struct sched_domain *child,
    >> @@ -6495,6 +6572,7 @@ static int build_sched_domains(const struct cpumask *cpu_map,
    >> struct sched_domain_attr *attr)
    >> {
    >> enum s_alloc alloc_state = sa_none;
    >> + struct sched_domain_rq *sd_rq;
    >> struct sched_domain *sd;
    >> struct s_data d;
    >> int i, ret = -ENOMEM;
    >> @@ -6547,11 +6625,18 @@ static int build_sched_domains(const struct cpumask *cpu_map,
    >> }
    >> }
    >>
    >> + /* Init objects that must follow the sched_domain lifecycle */
    >> + for_each_cpu(i, cpu_map) {
    >> + build_sched_domain_rq(&d, i);
    >> + }
    >
    > I suggest you put the above domain_rq initialization before the domain
    > initialization.

    I can't because build_sched_domain will init struct s_data d that is
    used in build_sched_domain_rq

    >
    > So that we have that more intuitively ordered initialization:
    >
    > * set up domains_rq
    > * set up domains
    > * set up sched groups
    > * set up sched groups power
    >
    >> +
    >> /* Attach the domains */
    >> rcu_read_lock();
    >> for_each_cpu(i, cpu_map) {
    >> - sd = *per_cpu_ptr(d.sd, i);
    >> - cpu_attach_domain(sd, d.rd, i);
    >> + sd_rq = *per_cpu_ptr(d.sd_rq, i);
    >> + cpu_attach_domain(sd_rq, d.rd, i);
    >> + /* claim allocation of sched_domain_rq object */
    >> + *per_cpu_ptr(d.sd_rq, i) = NULL;
    >> }
    >> rcu_read_unlock();
    > [...]
    >> @@ -5673,7 +5681,12 @@ static void run_rebalance_domains(struct softirq_action *h)
    >>
    >> static inline int on_null_domain(int cpu)
    >> {
    >> - return !rcu_dereference_sched(cpu_rq(cpu)->sd);
    >> + struct sched_domain_rq *sd_rq =
    >> + rcu_dereference_sched(cpu_rq(cpu)->sd_rq);
    >> + struct sched_domain *sd = NULL;
    >> + if (sd_rq)
    >> + sd = sd_rq->sd;
    >
    > Is it possible to have sd_rq->sd == NULL ?

    I think so with the isolcpus as an example

    >
    >> + return !sd;
    >> }
    >>
    >> /*
    >> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
    >> index cc03cfd..f589306 100644
    >> --- a/kernel/sched/sched.h
    >> +++ b/kernel/sched/sched.h
    >> @@ -417,7 +417,7 @@ struct rq {
    >>
    >> #ifdef CONFIG_SMP
    >> struct root_domain *rd;
    >> - struct sched_domain *sd;
    >> + struct sched_domain_rq *sd_rq;
    >>
    >> unsigned long cpu_power;
    >>
    >> @@ -505,21 +505,37 @@ DECLARE_PER_CPU(struct rq, runqueues);
    >>
    >> #ifdef CONFIG_SMP
    >>
    >> -#define rcu_dereference_check_sched_domain(p) \
    >> +#define rcu_dereference_check_sched_domain_rq(p) \
    >> rcu_dereference_check((p), \
    >> lockdep_is_held(&sched_domains_mutex))
    >>
    >> +#define get_sched_domain_rq(cpu) \
    >> + rcu_dereference_check_sched_domain_rq(cpu_rq(cpu)->sd_rq)
    >
    > How about rcu_dereference_domain_rq()? It seems important to me that
    > we keep the rcu_dereference_*() naming so that we don't hide what really
    > happens there behind a more opaque naming.

    you're right

    >
    > I mean RCU is tricky to deal with and it's important not to obfuscate its use.
    >
    >> +
    >> +#define rcu_dereference_check_sched_domain(cpu) ({ \
    >> + struct sched_domain_rq *__sd_rq = get_sched_domain_rq(cpu); \
    >> + struct sched_domain *__sd = NULL; \
    >> + if (__sd_rq) \
    >> + __sd = __sd_rq->sd; \
    >
    > Same question for the NULL sd.
    >
    >> + __sd; \
    >> +})
    >> +
    >> +#define sched_rq_flags(sd_rq) (&sd_rq->flags)
    >
    > Hm, why this "sched_" prefix? Maybe rq_domain_flags() ?

    ok i'm going to change the name

    >
    > Thanks.


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