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    SubjectRe: [patch] sched: fix SMT scheduler regression in find_busiest_queue()
    * Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com> [2010-02-16 23:55:30]:

    > * Peter Zijlstra <peterz@infradead.org> [2010-02-16 18:28:44]:
    >
    > > On Tue, 2010-02-16 at 21:29 +0530, Vaidyanathan Srinivasan wrote:
    > > > Agreed. Placement control should be handled by SD_PREFER_SIBLING
    > > > and SD_POWER_SAVINGS flags.
    > > >
    > > > --Vaidy
    > > >
    > > > ---
    > > >
    > > > 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 522cf0e..ec3a5c5 100644
    > > > --- a/kernel/sched_fair.c
    > > > +++ b/kernel/sched_fair.c
    > > > @@ -2538,9 +2538,17 @@ static inline void update_sd_lb_stats(struct sched_domain *sd, int this_cpu,
    > > > * In case the child domain prefers tasks go to siblings
    > > > * first, lower the group capacity to one so that we'll try
    > > > * and move all the excess tasks away.
    > >
    > > I prefer a blank line in between two paragraphs, but even better would
    > > be to place this comment at the else if site.
    > >
    > > > + * If power savings balance is set at this domain, then
    > > > + * make capacity equal to number of hardware threads to
    > > > + * accomodate more tasks until capacity is reached. The
    > >
    > > my spell checker seems to prefer: accommodate
    >
    > ok, will fix the comment.

    Thanks for the review, here is the updated patch:
    ---
    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 522cf0e..4466144 100644
    --- a/kernel/sched_fair.c
    +++ b/kernel/sched_fair.c
    @@ -2541,6 +2541,21 @@ static inline void update_sd_lb_stats(struct sched_domain *sd, int this_cpu,
    */
    if (prefer_sibling)
    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(group));
    +
    + /*
    + * 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;
    @@ -2855,7 +2870,8 @@ static int need_active_balance(struct sched_domain *sd, int sd_idle, int idle)
    !test_sd_parent(sd, SD_POWERSAVINGS_BALANCE))
    return 0;

    - 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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