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SubjectRe: [PATCH v3 05/22] sched: remove domain iterations in fork/exec/wake
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在 2013-01-11五的 10:26 +0530,Preeti U Murthy写道:
> Hi Morten,Alex
>
> On 01/09/2013 11:51 PM, Morten Rasmussen wrote:
> > On Sat, Jan 05, 2013 at 08:37:34AM +0000, Alex Shi wrote:
> >> Guess the search cpu from bottom to up in domain tree come from
> >> commit 3dbd5342074a1e sched: multilevel sbe sbf, the purpose is
> >> balancing over tasks on all level domains.
> >>
> >> This balancing cost much if there has many domain/groups in a large
> >> system. And force spreading task among different domains may cause
> >> performance issue due to bad locality.
> >>
> >> If we remove this code, we will get quick fork/exec/wake, plus better
> >> balancing among whole system, that also reduce migrations in future
> >> load balancing.
> >>
> >> This patch increases 10+% performance of hackbench on my 4 sockets
> >> NHM and SNB machines.
> >>
> >> Signed-off-by: Alex Shi <alex.shi@intel.com>
> >> ---
> >> kernel/sched/fair.c | 20 +-------------------
> >> 1 file changed, 1 insertion(+), 19 deletions(-)
> >>
> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> index ecfbf8e..895a3f4 100644
> >> --- a/kernel/sched/fair.c
> >> +++ b/kernel/sched/fair.c
> >> @@ -3364,15 +3364,9 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
> >> goto unlock;
> >> }
> >>
> >> - while (sd) {
> >> + if (sd) {
> >> int load_idx = sd->forkexec_idx;
> >> struct sched_group *group;
> >> - int weight;
> >> -
> >> - if (!(sd->flags & sd_flag)) {
> >> - sd = sd->child;
> >> - continue;
> >> - }
> >>
> >> if (sd_flag & SD_BALANCE_WAKE)
> >> load_idx = sd->wake_idx;
> >> @@ -3382,18 +3376,6 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
> >> goto unlock;
> >>
> >> new_cpu = find_idlest_cpu(group, p, cpu);
> >> -
> >> - /* Now try balancing at a lower domain level of new_cpu */
> >> - cpu = new_cpu;
> >> - weight = sd->span_weight;
> >> - sd = NULL;
> >> - for_each_domain(cpu, tmp) {
> >> - if (weight <= tmp->span_weight)
> >> - break;
> >> - if (tmp->flags & sd_flag)
> >> - sd = tmp;
> >> - }
> >> - /* while loop will break here if sd == NULL */
> >
> > I agree that this should be a major optimization. I just can't figure
> > out why the existing recursive search for an idle cpu switches to the
> > new cpu near the end and then starts a search for an idle cpu in the new
> > cpu's domain. Is this to handle some exotic sched domain configurations?
> > If so, they probably wouldn't work with your optimizations.
>
> Let me explain my understanding of why the recursive search is the way
> it is.
>
> _________________________ sd0
> | |
> | ___sd1__ ___sd2__ |
> | | | | | |
> | | sgx | | sga | |
> | | sgy | | sgb | |
> | |________| |________| |
> |_________________________|
>
> What the current recursive search is doing is (assuming we start with
> sd0-the top level sched domain whose flags are rightly set). we find
> that sd1 is the idlest group,and a cpux1 in sgx is the idlest cpu.
>
> We could have ideally stopped the search here.But the problem with this
> is that there is a possibility that sgx is more loaded than sgy; meaning
> the cpus in sgx are heavily imbalanced;say there are two cpus cpux1 and
> cpux2 in sgx,where cpux2 is heavily loaded and cpux1 has recently gotten
> idle and load balancing has not come to its rescue yet.According to the
> search above, cpux1 is idle,but is *not the right candidate for
> scheduling forked task,it is the right candidate for relieving the load
> from cpux2* due to cache locality etc.

This corner case may occur after "[PATCH v3 03/22] sched: fix
find_idlest_group mess logical" brought in the local sched_group bias,
and assume balancing runs on cpux2.
ideally, find_idlest_group should find the real idlest(this case: sgy),
then, this patch is reasonable.

>
> Therefore in the next recursive search we go one step inside sd1-the
> chosen idlest group candidate,which also happens to be the *next level
> sched domain for cpux1-the chosen idle cpu*. It then returns sgy as the
> idlest perhaps,if the situation happens to be better than what i have
> described for sgx and an appropriate cpu there is chosen.
>
> So in short a bird's eye view of a large sched domain to choose the cpu
> would be very short sighted,we could end up creating imbalances within
> lower level sched domains.To avoid this the recursive search plays safe
> and chooses the best idle group after viewing the large sched domain in
> detail.
>
> Therefore even i feel that this patch should be implemented after
> thorough tests.
>
>
>
> > Morten
>
> Regards
> Preeti U Murthy
>
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--
regards!
li guang

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