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    SubjectRe: [PATCH] x86,sched: Fix sched_smt_power_savings totally broken
    * Peter Zijlstra <a.p.zijlstra@chello.nl> [2012-01-09 12:05:56]:

    > On Mon, 2012-01-09 at 19:14 -0500, Youquan Song wrote:
    > > Fine, I will base your suggestion to develop another patch soon.
    > >
    >
    > >
    > > @@ -3923,6 +3923,10 @@ static inline void update_sg_lb_stats(struct
    > > sched_domain *sd,
    > > SCHED_POWER_SCALE);
    > > if (!sgs->group_capacity)
    > > sgs->group_capacity = fix_small_capacity(sd, group);
    > > +
    > > + if (sched_smt_power_savings)
    > > + sgs->group_capacity *= 2;
    >
    > Note, this has the hard-coded assumption you only have 2 threads per
    > core, which while true for intel, isn't true in general. I think you
    > meant to write *= group->group_weight or somesuch.
    >
    > Also, you forgot to limit this to the SD_SHARE_CPUPOWER domain, you're
    > now doubling the capacity for all domains.
    >
    > Furthermore, have a look at the SD_PREFER_SIBLING logic and make sure
    > you're not fighting that.

    Hi Peter,

    I think I had proposed the following fix earlier. Can we revisit this
    now? This works, but it is not the best solution.

    sched: Fix group_capacity for sched_smt_powersavings

    sched_smt_powersavings for threaded systems need this fix for
    consolidation to sibling threads to work. Since threads have
    fractional capacity, group_capacity will turn out to be one
    always and not accommodate another task in the sibling thread.

    This fix makes group_capacity a function of cpumask_weight that
    will enable the power saving load balancer to pack tasks among
    sibling threads and keep more cores idle.

    Signed-off-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 8e42de9..77ac4ca 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -4010,6 +4010,21 @@ static inline void update_sd_lb_stats(struct sched_domain *sd, int this_cpu,
    */
    if (prefer_sibling && !local_group && sds->this_has_capacity)
    sgs.group_capacity = min(sgs.group_capacity, 1UL);
    + /*
    + * If power savings balance is set at this domain, then
    + * make capacity equal to number of hardware threads to
    + * accommodate more tasks until capacity is reached.
    + */
    + else if (sd->flags & SD_POWERSAVINGS_BALANCE)
    + sgs.group_capacity =
    + cpumask_weight(sched_group_cpus(sg));
    +
    + /*
    + * The default group_capacity is rounded from sum of
    + * fractional cpu_powers of sibling hardware threads
    + * in order to enable fair use of available hardware
    + * resources.
    + */

    if (local_group) {
    sds->this_load = sgs.avg_load;
    @@ -4432,7 +4447,8 @@ static int need_active_balance(struct sched_domain *sd, int idle,
    * move_tasks() will succeed. ld_moved will be true and this
    * active balance code will not be triggered.
    */
    - if (sched_mc_power_savings < POWERSAVINGS_BALANCE_WAKEUP)
    + if (sched_mc_power_savings < POWERSAVINGS_BALANCE_WAKEUP &&
    + sched_smt_power_savings < POWERSAVINGS_BALANCE_WAKEUP)
    return 0;
    }



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