From: Rik van Riel <riel@surriel.com>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
Kernel Team <kernel-team@fb.com>, Paul Turner <pjt@google.com>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Morten Rasmussen <morten.rasmussen@arm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Mel Gorman <mgorman@techsingularity.net>
Subject: Re: [PATCH 08/15] sched,fair: simplify timeslice length code
Date: Thu, 29 Aug 2019 12:00:31 -0400 [thread overview]
Message-ID: <2a87463e8a51c34733e9c1fcf63380f9caa7afc4.camel@surriel.com> (raw)
In-Reply-To: <CAKfTPtDX+keNfNxf78yMoF3QaXSG_fZHJ_nqCFKYDMYGa84A6Q@mail.gmail.com>
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On Thu, 2019-08-29 at 16:02 +0200, Vincent Guittot wrote:
> On Thu, 29 Aug 2019 at 01:19, Rik van Riel <riel@surriel.com> wrote:
>
> > What am I overlooking?
>
> My point is more for task that runs several ticks in a row. Their
> sched_slice will be shorter in some cases with your changes so they
> can be preempted earlier by other runnable tasks with a lower
> vruntime
> and there will be more context switch
I can think of exactly one case where the time slice
will be shorter with my new code than with the old code,
and that is the case where:
- A CPU has nr_running > sched_nr_latency
- __sched_period returns a value larger than sysctl_sched_latency
- one of the tasks is much higher priority than the others
- that one task alone gets a timeslice larger than sysctl_sched_latency
With the new code, that high priority task will get a time
slice that is a (large) fraction of sysctl_sched_latency,
while the other (lower priority) tasks get their time slices
rounded up to sysctl_sched_min_granularity.
When tasks get their timeslice rounded up, that will increase
the total sched period in a similar way the old code did by
returning a longer period from __sched_period.
If a CPU is faced with a large number of equal priority tasks,
both the old code and the new code would end up giving each
task a timeslice length of sysctl_sched_min_granularity.
What am I missing?
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next prev parent reply other threads:[~2019-08-29 16:00 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-22 2:17 [PATCH RFC v4 0/15] sched,fair: flatten CPU controller runqueues Rik van Riel
2019-08-22 2:17 ` [PATCH 01/15] sched: introduce task_se_h_load helper Rik van Riel
2019-08-23 18:13 ` Dietmar Eggemann
2019-08-24 0:05 ` Rik van Riel
2019-08-22 2:17 ` [PATCH 02/15] sched: change /proc/sched_debug fields Rik van Riel
2019-08-22 2:17 ` [PATCH 03/15] sched,fair: redefine runnable_load_avg as the sum of task_h_load Rik van Riel
2019-08-28 13:50 ` Vincent Guittot
2019-08-28 14:47 ` Rik van Riel
2019-08-28 15:02 ` Vincent Guittot
2019-08-22 2:17 ` [PATCH 04/15] sched,fair: move runnable_load_avg to cfs_rq Rik van Riel
2019-08-22 2:17 ` [PATCH 05/15] sched,fair: remove cfs_rqs from leaf_cfs_rq_list bottom up Rik van Riel
2019-08-28 14:09 ` Vincent Guittot
2019-08-22 2:17 ` [PATCH 06/15] sched,cfs: use explicit cfs_rq of parent se helper Rik van Riel
2019-08-28 13:53 ` Vincent Guittot
2019-08-28 15:28 ` Rik van Riel
2019-08-28 16:34 ` Vincent Guittot
2019-08-22 2:17 ` [PATCH 07/15] sched,cfs: fix zero length timeslice calculation Rik van Riel
2019-08-28 16:59 ` Vincent Guittot
2019-08-22 2:17 ` [PATCH 08/15] sched,fair: simplify timeslice length code Rik van Riel
2019-08-28 17:32 ` Vincent Guittot
2019-08-28 23:18 ` Rik van Riel
2019-08-29 14:02 ` Vincent Guittot
2019-08-29 16:00 ` Rik van Riel [this message]
2019-08-30 6:41 ` Vincent Guittot
2019-08-30 15:01 ` Rik van Riel
2019-09-02 7:51 ` Vincent Guittot
2019-09-02 17:47 ` Rik van Riel
2019-08-22 2:17 ` [PATCH 09/15] sched,fair: refactor enqueue/dequeue_entity Rik van Riel
2019-09-03 15:38 ` Vincent Guittot
2019-09-03 20:27 ` Rik van Riel
2019-09-04 6:44 ` Vincent Guittot
2019-08-22 2:17 ` [PATCH 10/15] sched,fair: add helper functions for flattened runqueue Rik van Riel
2019-08-22 2:17 ` [PATCH 11/15] sched,fair: flatten hierarchical runqueues Rik van Riel
2019-08-23 18:14 ` Dietmar Eggemann
2019-08-24 1:16 ` Rik van Riel
2019-08-22 2:17 ` [PATCH 12/15] sched,fair: flatten update_curr functionality Rik van Riel
2019-08-27 10:37 ` Dietmar Eggemann
2019-08-22 2:17 ` [PATCH 13/15] sched,fair: propagate sum_exec_runtime up the hierarchy Rik van Riel
2019-08-28 7:51 ` Dietmar Eggemann
2019-08-28 13:14 ` Rik van Riel
2019-08-29 17:20 ` Dietmar Eggemann
2019-08-29 18:06 ` Rik van Riel
2019-08-22 2:17 ` [PATCH 14/15] sched,fair: ramp up task_se_h_weight quickly Rik van Riel
2019-08-22 2:17 ` [PATCH 15/15] sched,fair: scale vdiff in wakeup_preempt_entity Rik van Riel
2019-09-02 10:53 ` [PATCH RFC v4 0/15] sched,fair: flatten CPU controller runqueues Dietmar Eggemann
2019-09-03 1:44 ` Rik van Riel
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