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Message-ID: <20080229090400.GD15328@linux.vnet.ibm.com>
Date: Fri, 29 Feb 2008 14:34:00 +0530
From: Dhaval Giani <dhaval@...ux.vnet.ibm.com>
To: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
Ingo Molnar <mingo@...e.hu>,
Sudhir Kumar <skumar@...ux.vnet.ibm.com>,
Balbir Singh <balbir@...ibm.com>,
Aneesh Kumar KV <aneesh.kumar@...ux.vnet.ibm.com>,
lkml <linux-kernel@...r.kernel.org>, vgoyal@...hat.com,
serue@...ibm.com, menage@...gle.com
Subject: Re: [RFC, PATCH 1/2] sched: change the fairness model of the CFS
group scheduler
On Thu, Feb 28, 2008 at 08:42:48PM +0100, Peter Zijlstra wrote:
>
> On Mon, 2008-02-25 at 19:46 +0530, Dhaval Giani wrote:
> > This patch allows tasks and groups to exist in the same cfs_rq. With this
> > change the CFS group scheduling follows a 1/(M+N) model from a 1/(1+N)
> > fairness model where M tasks and N groups exist at the cfs_rq level.
>
> Good. You know I agree with this direction.
>
> > Signed-off-by: Dhaval Giani <dhaval@...ux.vnet.ibm.com>
> > Signed-off-by: Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
> > ---
> > kernel/sched.c | 46 +++++++++++++++++++++
> > kernel/sched_fair.c | 113 +++++++++++++++++++++++++++++++++++++++++-----------
> > 2 files changed, 137 insertions(+), 22 deletions(-)
> >
> > Index: linux-2.6.25-rc2/kernel/sched.c
> > ===================================================================
> > --- linux-2.6.25-rc2.orig/kernel/sched.c
> > +++ linux-2.6.25-rc2/kernel/sched.c
> > @@ -224,10 +224,13 @@ struct task_group {
> > };
> >
> > #ifdef CONFIG_FAIR_GROUP_SCHED
> > +
> > +#ifdef CONFIG_USER_SCHED
> > /* Default task group's sched entity on each cpu */
> > static DEFINE_PER_CPU(struct sched_entity, init_sched_entity);
> > /* Default task group's cfs_rq on each cpu */
> > static DEFINE_PER_CPU(struct cfs_rq, init_cfs_rq) ____cacheline_aligned_in_smp;
> > +#endif
> >
> > static struct sched_entity *init_sched_entity_p[NR_CPUS];
> > static struct cfs_rq *init_cfs_rq_p[NR_CPUS];
> > @@ -7163,6 +7166,10 @@ static void init_tg_cfs_entry(struct rq
> > list_add(&cfs_rq->leaf_cfs_rq_list, &rq->leaf_cfs_rq_list);
> >
> > tg->se[cpu] = se;
> > + /* se could be NULL for init_task_group */
> > + if (!se)
> > + return;
> > +
> > se->cfs_rq = &rq->cfs;
> > se->my_q = cfs_rq;
> > se->load.weight = tg->shares;
> > @@ -7217,11 +7224,46 @@ void __init sched_init(void)
> > #ifdef CONFIG_FAIR_GROUP_SCHED
> > init_task_group.shares = init_task_group_load;
> > INIT_LIST_HEAD(&rq->leaf_cfs_rq_list);
> > +#ifdef CONFIG_CGROUP_SCHED
> > + /*
> > + * How much cpu bandwidth does init_task_group get?
> > + *
> > + * In case of task-groups formed thr' the cgroup filesystem, it
> > + * gets 100% of the cpu resources in the system. This overall
> > + * system cpu resource is divided among the tasks of
> > + * init_task_group and its child task-groups in a fair manner,
> > + * based on each entity's (task or task-group's) weight
> > + * (se->load.weight).
> > + *
> > + * In other words, if init_task_group has 10 tasks of weight
> > + * 1024) and two child groups A0 and A1 (of weight 1024 each),
> > + * then A0's share of the cpu resource is:
> > + *
> > + * A0's bandwidth = 1024 / (10*1024 + 1024 + 1024) = 8.33%
> > + *
> > + * We achieve this by letting init_task_group's tasks sit
> > + * directly in rq->cfs (i.e init_task_group->se[] = NULL).
> > + */
> > + init_tg_cfs_entry(rq, &init_task_group, &rq->cfs, NULL, i, 1);
> > + init_tg_rt_entry(rq, &init_task_group, &rq->rt, NULL, i, 1);
>
> That seems to agree with that.
>
> > +#elif defined CONFIG_USER_SCHED
> > + /*
> > + * In case of task-groups formed thr' the user id of tasks,
> > + * init_task_group represents tasks belonging to root user.
> > + * Hence it forms a sibling of all subsequent groups formed.
> > + * In this case, init_task_group gets only a fraction of overall
> > + * system cpu resource, based on the weight assigned to root
> > + * user's cpu share (INIT_TASK_GROUP_LOAD). This is accomplished
> > + * by letting tasks of init_task_group sit in a separate cfs_rq
> > + * (init_cfs_rq) and having one entity represent this group of
> > + * tasks in rq->cfs (i.e init_task_group->se[] != NULL).
> > + */
> > init_tg_cfs_entry(rq, &init_task_group,
> > &per_cpu(init_cfs_rq, i),
> > &per_cpu(init_sched_entity, i), i, 1);
>
> But I fail to parse this lengthy comment. What does it do:
>
> init_group
> / | \
> uid-0 uid-1000 uid-n
>
> or does it blend uid-0 into the init_group?
>
It blends uid-0 (root) into init_group.
<snip>
> > @@ -1100,6 +1127,27 @@ static void check_preempt_wakeup(struct
> > if (!sched_feat(WAKEUP_PREEMPT))
> > return;
> >
> > + /*
> > + * preemption test can be made between sibling entities who are in the
> > + * same cfs_rq i.e who have a common parent. Walk up the hierarchy of
> > + * both tasks untill we find their ancestors who are siblings of common
> > + * parent.
> > + */
> > +
> > + /* First walk up until both entities are at same depth */
> > + se_depth = depth_se(se);
> > + pse_depth = depth_se(pse);
> > +
> > + while (se_depth > pse_depth) {
> > + se_depth--;
> > + se = parent_entity(se);
> > + }
> > +
> > + while (pse_depth > se_depth) {
> > + pse_depth--;
> > + pse = parent_entity(pse);
> > + }
>
> Sad, but needed.. for now..
>
better ideas if any are welcome! Cannot think of anything right now :(
--
regards,
Dhaval
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