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From: Aaron Lu <aaron.lu@linux.alibaba.com>
To: Vineeth Remanan Pillai <vpillai@digitalocean.com>
Cc: Nishanth Aravamudan <naravamudan@digitalocean.com>,
	Julien Desfossez <jdesfossez@digitalocean.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	mingo@kernel.org, tglx@linutronix.de, pjt@google.com,
	torvalds@linux-foundation.org, linux-kernel@vger.kernel.org,
	subhra.mazumdar@oracle.com, fweisbec@gmail.com,
	keescook@chromium.org, kerrnel@google.com,
	Phil Auld <pauld@redhat.com>, Aaron Lu <aaron.lwe@gmail.com>,
	Aubrey Li <aubrey.intel@gmail.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Mel Gorman <mgorman@techsingularity.net>,
	Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
	Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [RFC PATCH v2 11/17] sched: Basic tracking of matching tasks
Date: Mon, 29 Apr 2019 11:36:22 +0800	[thread overview]
Message-ID: <20190429033620.GA128241@aaronlu> (raw)
In-Reply-To: <2364f2b65bf50826d881c84d7634b6565dfee527.1556025155.git.vpillai@digitalocean.com>

On Tue, Apr 23, 2019 at 04:18:16PM +0000, Vineeth Remanan Pillai wrote:
> +/*
> + * l(a,b)
> + * le(a,b) := !l(b,a)
> + * g(a,b)  := l(b,a)
> + * ge(a,b) := !l(a,b)
> + */
> +
> +/* real prio, less is less */
> +static inline bool __prio_less(struct task_struct *a, struct task_struct *b, bool core_cmp)
> +{
> +	u64 vruntime;
> +
> +	int pa = __task_prio(a), pb = __task_prio(b);
> +
> +	if (-pa < -pb)
> +		return true;
> +
> +	if (-pb < -pa)
> +		return false;
> +
> +	if (pa == -1) /* dl_prio() doesn't work because of stop_class above */
> +		return !dl_time_before(a->dl.deadline, b->dl.deadline);
> +
> +	vruntime = b->se.vruntime;
> +	if (core_cmp) {
> +		vruntime -= task_cfs_rq(b)->min_vruntime;
> +		vruntime += task_cfs_rq(a)->min_vruntime;
> +	}
> +	if (pa == MAX_RT_PRIO + MAX_NICE) /* fair */
> +		return !((s64)(a->se.vruntime - vruntime) <= 0);
> +
> +	return false;
> +}

This unfortunately still doesn't work.

Consider the following task layout on two sibling CPUs(cpu0 and cpu1):

    rq0.cfs_rq    rq1.cfs_rq
        |             |
     se_bash        se_hog

se_hog is the sched_entity for a cpu intensive task and se_bash is the
sched_entity for bash.

There are two problems:
1 SCHED_DEBIT
when user execute some commands through bash, say ls, bash will fork.
The newly forked ls' vruntime is set in the future due to SCHED_DEBIT.
This made 'ls' lose in __prio_less() when compared with hog, whose
vruntime may very likely be the same as its cfs_rq's min_vruntime.

This is OK since we do not want forked process to starve already running
ones. The problem is, since hog keeps running, its vruntime will always
sync with its cfs_rq's min_vruntime. OTOH, 'ls' can not run, its
cfs_rq's min_vruntime doesn't proceed, making 'ls' always lose to hog.

2 who schedules, who wins
so I disabled SCHED_DEBIT, for testing's purpose. When cpu0 schedules,
ls could win where both sched_entity's vruntime is the same as their
cfs_rqs' min_vruntime. So does hog: when cpu1 schedules, hog can preempt
ls in the same way. The end result is, interactive task can lose to cpu
intensive task and ls can feel "dead".

I haven't figured out a way to solve this yet. A core wide cfs_rq's
min_vruntime can probably solve this. Your suggestions are appreciated.

  parent reply	other threads:[~2019-04-29  3:36 UTC|newest]

Thread overview: 109+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-23 16:18 [RFC PATCH v2 00/17] Core scheduling v2 Vineeth Remanan Pillai
2019-04-23 16:18 ` [RFC PATCH v2 01/17] stop_machine: Fix stop_cpus_in_progress ordering Vineeth Remanan Pillai
2019-04-23 16:18 ` [RFC PATCH v2 02/17] sched: Fix kerneldoc comment for ia64_set_curr_task Vineeth Remanan Pillai
2019-04-23 16:18 ` [RFC PATCH v2 03/17] sched: Wrap rq::lock access Vineeth Remanan Pillai
2019-04-23 16:18 ` [RFC PATCH v2 04/17] sched/{rt,deadline}: Fix set_next_task vs pick_next_task Vineeth Remanan Pillai
2019-04-23 16:18 ` [RFC PATCH v2 05/17] sched: Add task_struct pointer to sched_class::set_curr_task Vineeth Remanan Pillai
2019-04-23 16:18 ` [RFC PATCH v2 06/17] sched/fair: Export newidle_balance() Vineeth Remanan Pillai
2019-04-23 16:18 ` [RFC PATCH v2 07/17] sched: Allow put_prev_task() to drop rq->lock Vineeth Remanan Pillai
2019-04-23 16:18 ` [RFC PATCH v2 08/17] sched: Rework pick_next_task() slow-path Vineeth Remanan Pillai
2019-04-23 16:18 ` [RFC PATCH v2 09/17] sched: Introduce sched_class::pick_task() Vineeth Remanan Pillai
2019-04-26 14:02   ` Peter Zijlstra
2019-04-26 16:10     ` Vineeth Remanan Pillai
2019-04-29  5:38   ` Aaron Lu
2019-04-23 16:18 ` [RFC PATCH v2 10/17] sched: Core-wide rq->lock Vineeth Remanan Pillai
2019-04-23 16:18 ` [RFC PATCH v2 11/17] sched: Basic tracking of matching tasks Vineeth Remanan Pillai
2019-04-24  0:08   ` Tim Chen
2019-04-24 20:43     ` Vineeth Remanan Pillai
2019-04-24 22:12       ` Tim Chen
2019-04-25 14:35       ` Phil Auld
2019-05-22 19:52         ` Vineeth Remanan Pillai
2019-04-24  0:17   ` Tim Chen
2019-04-24 20:43     ` Vineeth Remanan Pillai
2019-04-29  3:36   ` Aaron Lu [this message]
2019-05-10 13:06     ` Peter Zijlstra
2019-04-29  6:15   ` Aaron Lu
2019-05-01 23:27     ` Tim Chen
2019-05-03  0:06       ` Tim Chen
2019-05-08 15:49         ` Aubrey Li
2019-05-08 18:19           ` Subhra Mazumdar
2019-05-08 18:37             ` Subhra Mazumdar
2019-05-09  0:01               ` Aubrey Li
2019-05-09  0:25                 ` Subhra Mazumdar
2019-05-09  1:38                   ` Aubrey Li
2019-05-09  2:14                     ` Subhra Mazumdar
2019-05-09 15:10                       ` Aubrey Li
2019-05-09 17:50                         ` Subhra Mazumdar
2019-05-10  0:09                           ` Tim Chen
2019-04-23 16:18 ` [RFC PATCH v2 12/17] sched: A quick and dirty cgroup tagging interface Vineeth Remanan Pillai
2019-04-25 14:26   ` Phil Auld
2019-04-26 14:13     ` Peter Zijlstra
2019-04-26 14:19       ` Phil Auld
2019-05-10 15:12   ` Julien Desfossez
2019-04-23 16:18 ` [RFC PATCH v2 13/17] sched: Add core wide task selection and scheduling Vineeth Remanan Pillai
2019-04-29  7:13   ` Aaron Lu
2019-05-18 15:37   ` Aubrey Li
2019-05-20 13:04     ` Phil Auld
2019-05-20 14:04       ` Vineeth Pillai
2019-05-21  8:19         ` Aubrey Li
2019-05-21 13:24           ` Vineeth Pillai
2019-04-23 16:18 ` [RFC PATCH v2 14/17] sched/fair: Add a few assertions Vineeth Remanan Pillai
2019-04-23 16:18 ` [RFC PATCH v2 15/17] sched: Trivial forced-newidle balancer Vineeth Remanan Pillai
2019-04-23 23:46   ` Aubrey Li
2019-04-24 14:03     ` Vineeth Remanan Pillai
2019-04-24 14:05     ` Vineeth Remanan Pillai
2019-04-23 16:18 ` [RFC PATCH v2 16/17] sched: Wake up sibling if it has something to run Vineeth Remanan Pillai
2019-04-26 15:03   ` Peter Zijlstra
2019-04-29 12:36     ` Julien Desfossez
2019-04-23 16:18 ` [RFC PATCH v2 17/17] sched: Debug bits Vineeth Remanan Pillai
2019-05-17 17:18   ` Aubrey Li
2019-04-23 18:02 ` [RFC PATCH v2 00/17] Core scheduling v2 Phil Auld
2019-04-23 18:45   ` Vineeth Remanan Pillai
2019-04-29  3:53     ` Aaron Lu
2019-05-06 19:39       ` Julien Desfossez
2019-05-08  2:30         ` Aaron Lu
2019-05-08 17:49           ` Julien Desfossez
2019-05-09  2:11             ` Aaron Lu
2019-05-15 21:36               ` Vineeth Remanan Pillai
2019-04-23 23:25 ` Aubrey Li
2019-04-24 11:19   ` Vineeth Remanan Pillai
2019-05-15 21:39     ` Vineeth Remanan Pillai
2019-04-24 13:13 ` Aubrey Li
2019-04-24 14:00   ` Julien Desfossez
2019-04-25  3:15     ` Aubrey Li
2019-04-25  9:55       ` Ingo Molnar
2019-04-25 14:46         ` Mel Gorman
2019-04-25 18:53           ` Ingo Molnar
2019-04-25 18:59             ` Thomas Gleixner
2019-04-25 19:34               ` Ingo Molnar
2019-04-25 21:31             ` Mel Gorman
2019-04-26  8:42               ` Ingo Molnar
2019-04-26 10:43                 ` Mel Gorman
2019-04-26 18:37                   ` Subhra Mazumdar
2019-04-26 19:49                     ` Mel Gorman
2019-04-26  9:45               ` Ingo Molnar
2019-04-26 10:19                 ` Mel Gorman
2019-04-27  9:06                   ` Ingo Molnar
2019-04-26  9:51               ` Ingo Molnar
2019-04-26 14:15             ` Phil Auld
2019-04-26  2:18         ` Aubrey Li
2019-04-26  9:51           ` Ingo Molnar
2019-04-27  3:51         ` Aubrey Li
2019-04-27  9:17           ` Ingo Molnar
2019-04-27 14:04             ` Aubrey Li
2019-04-27 14:21               ` Ingo Molnar
2019-04-27 15:54                 ` Aubrey Li
2019-04-28  9:33                   ` Ingo Molnar
2019-04-28 10:29                     ` Aubrey Li
2019-04-28 12:17                       ` Ingo Molnar
2019-04-29  2:17                         ` Li, Aubrey
2019-04-29  6:14                           ` Ingo Molnar
2019-04-29 13:25                             ` Li, Aubrey
2019-04-29 15:39                               ` Phil Auld
2019-04-30  1:24                                 ` Aubrey Li
2019-04-29 16:00                               ` Ingo Molnar
2019-04-30  1:34                                 ` Aubrey Li
2019-04-30  4:42                                   ` Ingo Molnar
2019-05-18  0:58                                     ` Li, Aubrey
2019-05-18  1:08                                       ` Li, Aubrey
2019-04-25 14:36 ` Julien Desfossez

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