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    SubjectRe: [PATCH 5/8] sched: Favour moving tasks towards the preferred node
    On Thu, Jun 27, 2013 at 06:11:27PM +0200, Peter Zijlstra wrote:
    > On Wed, Jun 26, 2013 at 03:38:04PM +0100, Mel Gorman wrote:
    > > +/* Returns true if the destination node has incurred more faults */
    > > +static bool migrate_improves_locality(struct task_struct *p, struct lb_env *env)
    > > +{
    > > + int src_nid, dst_nid;
    > > +
    > > + if (!p->numa_faults || !(env->sd->flags & SD_NUMA))
    > > + return false;
    > > +
    > > + src_nid = cpu_to_node(env->src_cpu);
    > > + dst_nid = cpu_to_node(env->dst_cpu);
    > > +
    > > + if (src_nid == dst_nid)
    > > + return false;
    > > +
    > > + if (p->numa_migrate_seq < sysctl_numa_balancing_settle_count &&
    > > + p->numa_preferred_nid == dst_nid)
    > > + return true;
    > > +
    > > + return false;
    > > +}
    >
    > Also, until I just actually _read_ that function; I assumed it would
    > compare p->numa_faults[src_nid] and p->numa_faults[dst_nid]. Because
    > even when the dst_nid isn't the preferred nid; it might still have more
    > pages than where we currently are.
    >

    I tested something like this and also tested it when only taking shared
    accesses into account but it performed badly in some cases. I've included
    the last patch I tested below for reference but dropped it until I figured
    out why it performed badly. I guessed it was due to increased bouncing
    due to shared faults but didn't prove it.

    > Idem with the proposed migrate_degrades_locality().
    >
    > Something like so I suppose
    >
    > ---
    > --- a/kernel/sched/fair.c
    > +++ b/kernel/sched/fair.c
    > @@ -3969,6 +3969,7 @@ task_hot(struct task_struct *p, u64 now,
    > return delta < (s64)sysctl_sched_migration_cost;
    > }
    >
    > +#ifdef CONFIG_NUMA_BALANCING
    > /* Returns true if the destination node has incurred more faults */
    > static bool migrate_improves_locality(struct task_struct *p, struct lb_env *env)
    > {
    > @@ -3983,13 +3984,50 @@ static bool migrate_improves_locality(st
    > if (src_nid == dst_nid)
    > return false;
    >
    > - if (p->numa_migrate_seq < sysctl_numa_balancing_settle_count &&
    > - p->numa_preferred_nid == dst_nid)
    > + if (p->numa_migrate_seq >= sysctl_numa_balancing_settle_count)
    > + return false;
    > +
    > + if (p->numa_preferred_nid == dst_nid)
    > + return true;
    > +
    > + if (p->numa_faults[src_nid] < p->numa_faults[dst_nid])
    > + return true;
    > +
    > + return false;
    > +}
    > +

    I tested something like this.

    > +static vool migrate_degrades_locality(struct task_struct *p, struct lb_env *env)
    > +{
    > + int src_nid, dst_nid;
    > +
    > + if (!p->numa_faults || !(env->sd->flags & SD_NUMA))
    > + return false;
    > +
    > + src_nid = cpu_to_node(env->src_cpu);
    > + dst_nid = cpu_to_node(env->dst_cpu);
    > +
    > + if (src_nid == dst_nid)
    > + return false;
    > +
    > + if (p->numa_faults[src_nid] > p->numa_faults[dst_nid])
    > return true;
    >
    > return false;
    > }

    But I had not tried this and it makes sense. I'll test it out and include
    it in the next revision if it looks good. Unless you object I'll add
    your signed-off because the version of the patch I'm about to test looks
    almost identical to this.

    >
    > +#else
    > +
    > +static inline bool migrate_improves_locality(struct task_struct *p, struct lb_env *env)
    > +{
    > + return false;
    > +}
    > +
    > +static inline bool migrate_degrades_locality(struct task_struct *p, struct lb_env *env)
    > +{
    > + return false;
    > +}
    > +
    > +#endif /* CONFIG_NUMA_BALANCING */
    >
    > /*
    > * can_migrate_task - may task p from runqueue rq be migrated to this_cpu?
    > @@ -4055,8 +4093,10 @@ int can_migrate_task(struct task_struct
    > return 1;
    >
    > tsk_cache_hot = task_hot(p, rq_clock_task(env->src_rq), env->sd);
    > + if (!tsk_cache_hot)
    > + tsk_cache_hot = migrate_degrades_locality(p, env);
    > if (!tsk_cache_hot ||
    > - env->sd->nr_balance_failed > env->sd->cache_nice_tries) {
    > + env->sd->nr_balance_failed > env->sd->cache_nice_tries) {
    >
    > if (tsk_cache_hot) {
    > schedstat_inc(env->sd, lb_hot_gained[env->idle]);
    >

    This is the last patch similar to this idea I tested.

    ---8<---
    sched: Favour moving tasks towards nodes that incurred more faults

    Signed-off-by: Mel Gorman <mgorman@suse.de>

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index e9bbb70..3379ca4 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -3980,9 +3980,18 @@ static bool migrate_improves_locality(struct task_struct *p, struct lb_env *env)
    if (src_nid == dst_nid)
    return false;

    - if (p->numa_migrate_seq < sysctl_numa_balancing_settle_count &&
    - p->numa_preferred_nid == dst_nid)
    - return true;
    + if (p->numa_migrate_seq < sysctl_numa_balancing_settle_count) {
    + if (p->numa_preferred_nid == dst_nid)
    + return true;
    +
    + /*
    + * Move towards node if there were a higher number of shared
    + * NUMA hinting faults
    + */
    + if (p->numa_faults[task_faults_idx(dst_nid, 0)] >
    + p->numa_faults[task_faults_idx(src_nid, 0)])
    + return true;
    + }

    return false;
    }

    --
    Mel Gorman
    SUSE Labs


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