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Date: Mon, 14 Jan 2013 17:03:11 +0800
From: li guang <lig.fnst@...fujitsu.com>
To: Alex Shi <alex.shi@...el.com>
Cc: Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
Morten Rasmussen <Morten.Rasmussen@....com>,
"mingo@...hat.com" <mingo@...hat.com>,
"peterz@...radead.org" <peterz@...radead.org>,
"tglx@...utronix.de" <tglx@...utronix.de>,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
"arjan@...ux.intel.com" <arjan@...ux.intel.com>,
"bp@...en8.de" <bp@...en8.de>, "pjt@...gle.com" <pjt@...gle.com>,
"namhyung@...nel.org" <namhyung@...nel.org>,
"efault@....de" <efault@....de>,
"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 05/22] sched: remove domain iterations in
fork/exec/wake
在 2013-01-11五的 22:56 +0800，Alex Shi写道：
> On 01/11/2013 04:01 PM, li guang wrote:
> > 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@...el.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.
> >
>
> Sure. but seems it is a bit hard to go down the idlest group.
>
> and the old logical is real cost too much, on my 2 socket NHM/SNB
> server, hackbench can increase 2~5% performance. and no clean
> performance on kbuild/aim7 etc.
what about remove local group bias?
--
regards!
li guang
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