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From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
Cc: Tejun Heo <tj@kernel.org>, Li Zefan <lizefan@huawei.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Ingo Molnar <mingo@redhat.com>,
	cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-doc@vger.kernel.org, kernel-team@fb.com, pjt@google.com,
	luto@amacapital.net, Mike Galbraith <efault@gmx.de>,
	torvalds@linux-foundation.org, Roman Gushchin <guro@fb.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Patrick Bellasi <patrick.bellasi@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v9 3/7] cpuset: Add cpuset.sched.load_balance flag to v2
Date: Thu, 31 May 2018 17:20:50 +0200	[thread overview]
Message-ID: <20180531152050.GK12180@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <42cc1f44-2355-1c0c-b575-49c863303c42@redhat.com>

On Thu, May 31, 2018 at 09:54:27AM -0400, Waiman Long wrote:
> On 05/31/2018 08:26 AM, Peter Zijlstra wrote:

> > I still find all that a bit weird.
> >
> > So load_balance=0 basically changes a partition into a
> > 'fully-partitioned partition' with the seemingly random side-effect that
> > now sub-partitions are allowed to consume all CPUs.
> 
> Are you suggesting that we should allow sub-partition to consume all the
> CPUs no matter the load balance state? I can live with that if you think
> it is more logical.

I'm on the fence myself; the only thing I'm fairly sure of is that tying
this particular behaviour to the load-balance knob seems off.

> > The rationale, only given in the Changelog above, seems to be to allow
> > 'easy' emulation of isolcpus.
> >
> > I'm still not convinced this is a useful knob to have. You can do
> > fully-partitioned by simply creating a lot of 1 cpu parititions.
> 
> That is certainly true. However, I think there are some additional
> overhead in the scheduler side in maintaining those 1-cpu partitions. Right?

cpuset-controller as such doesn't have much overhead scheduler wise,
cpu-controller OTOH does, and there depth is the predominant factor, so
many sibling groups should not matter there either.

> > So this one knob does two separate things, both of which seem, to me,
> > redundant.
> >
> > Can we please get better rationale for this?
> 
> I am fine getting rid of the load_balance flag if this is the consensus.
> However, we do need to come up with a good migration story for those
> users that need the isolcpus capability. I think Mike was the one asking
> for supporting isolcpus. So Mike, what is your take on that.

So I don't strictly mind having a knob that does the 'fully-partitioned
partition' thing -- however odd that sounds -- but I feel we should have
a solid use-case for it.

I also think we should not mix the 'consume all' thing with the
'fully-partitioned' thing, as they are otherwise unrelated.

WARNING: multiple messages have this Message-ID (diff)
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
Cc: Tejun Heo <tj@kernel.org>, Li Zefan <lizefan@huawei.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Ingo Molnar <mingo@redhat.com>,
	cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-doc@vger.kernel.org, kernel-team@fb.com, pjt@google.com,
	luto@amacapital.net, Mike Galbraith <efault@gmx.de>,
	torvalds@linux-foundation.org, Roman Gushchin <guro@fb.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Patrick Bellasi <patrick.bellasi@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v9 3/7] cpuset: Add cpuset.sched.load_balance flag to v2
Date: Thu, 31 May 2018 17:20:50 +0200	[thread overview]
Message-ID: <20180531152050.GK12180@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <42cc1f44-2355-1c0c-b575-49c863303c42@redhat.com>

On Thu, May 31, 2018 at 09:54:27AM -0400, Waiman Long wrote:
> On 05/31/2018 08:26 AM, Peter Zijlstra wrote:

> > I still find all that a bit weird.
> >
> > So load_balance=0 basically changes a partition into a
> > 'fully-partitioned partition' with the seemingly random side-effect that
> > now sub-partitions are allowed to consume all CPUs.
> 
> Are you suggesting that we should allow sub-partition to consume all the
> CPUs no matter the load balance state? I can live with that if you think
> it is more logical.

I'm on the fence myself; the only thing I'm fairly sure of is that tying
this particular behaviour to the load-balance knob seems off.

> > The rationale, only given in the Changelog above, seems to be to allow
> > 'easy' emulation of isolcpus.
> >
> > I'm still not convinced this is a useful knob to have. You can do
> > fully-partitioned by simply creating a lot of 1 cpu parititions.
> 
> That is certainly true. However, I think there are some additional
> overhead in the scheduler side in maintaining those 1-cpu partitions. Right?

cpuset-controller as such doesn't have much overhead scheduler wise,
cpu-controller OTOH does, and there depth is the predominant factor, so
many sibling groups should not matter there either.

> > So this one knob does two separate things, both of which seem, to me,
> > redundant.
> >
> > Can we please get better rationale for this?
> 
> I am fine getting rid of the load_balance flag if this is the consensus.
> However, we do need to come up with a good migration story for those
> users that need the isolcpus capability. I think Mike was the one asking
> for supporting isolcpus. So Mike, what is your take on that.

So I don't strictly mind having a knob that does the 'fully-partitioned
partition' thing -- however odd that sounds -- but I feel we should have
a solid use-case for it.

I also think we should not mix the 'consume all' thing with the
'fully-partitioned' thing, as they are otherwise unrelated.
--
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  reply	other threads:[~2018-05-31 15:21 UTC|newest]

Thread overview: 54+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-29 13:41 [PATCH v9 0/7] Enable cpuset controller in default hierarchy Waiman Long
2018-05-29 13:41 ` Waiman Long
2018-05-29 13:41 ` [PATCH v9 1/7] cpuset: " Waiman Long
2018-05-29 13:41   ` Waiman Long
2018-05-29 13:41 ` [PATCH v9 2/7] cpuset: Add new v2 cpuset.sched.domain_root flag Waiman Long
2018-05-29 13:41   ` Waiman Long
2018-05-30 14:18   ` Juri Lelli
2018-05-30 14:18     ` Juri Lelli
2018-05-30 14:57     ` Waiman Long
2018-05-30 14:57       ` Waiman Long
2018-05-31  9:49   ` Peter Zijlstra
2018-05-31  9:49     ` Peter Zijlstra
2018-05-29 13:41 ` [PATCH v9 3/7] cpuset: Add cpuset.sched.load_balance flag to v2 Waiman Long
2018-05-29 13:41   ` Waiman Long
2018-05-31 10:44   ` Peter Zijlstra
2018-05-31 10:44     ` Peter Zijlstra
2018-05-31 10:54   ` Peter Zijlstra
2018-05-31 10:54     ` Peter Zijlstra
2018-05-31 13:36     ` Waiman Long
2018-05-31 13:36       ` Waiman Long
2018-05-31 12:26   ` Peter Zijlstra
2018-05-31 12:26     ` Peter Zijlstra
2018-05-31 13:54     ` Waiman Long
2018-05-31 13:54       ` Waiman Long
2018-05-31 15:20       ` Peter Zijlstra [this message]
2018-05-31 15:20         ` Peter Zijlstra
2018-05-31 15:36         ` Waiman Long
2018-05-31 15:36           ` Waiman Long
2018-05-31 16:08           ` Peter Zijlstra
2018-05-31 16:08             ` Peter Zijlstra
2018-05-31 16:42             ` Waiman Long
2018-05-31 16:42               ` Waiman Long
2018-06-20 14:46               ` Peter Zijlstra
2018-06-20 14:46                 ` Peter Zijlstra
2018-06-21  7:40                 ` Waiman Long
2018-06-21  7:40                   ` Waiman Long
2018-05-29 13:41 ` [PATCH v9 4/7] cpuset: Make generate_sched_domains() recognize isolated_cpus Waiman Long
2018-05-29 13:41   ` Waiman Long
2018-05-29 13:41 ` [PATCH v9 5/7] cpuset: Expose cpus.effective and mems.effective on cgroup v2 root Waiman Long
2018-05-29 13:41   ` Waiman Long
2018-05-29 13:41 ` [PATCH v9 6/7] cpuset: Don't rebuild sched domains if cpu changes in non-domain root Waiman Long
2018-05-29 13:41   ` Waiman Long
2018-05-29 13:41 ` [PATCH v9 7/7] cpuset: Allow reporting of sched domain generation info Waiman Long
2018-05-29 13:41   ` Waiman Long
2018-05-30 10:13 ` [PATCH v9 0/7] Enable cpuset controller in default hierarchy Juri Lelli
2018-05-30 10:13   ` Juri Lelli
2018-05-30 12:56   ` Waiman Long
2018-05-30 12:56     ` Waiman Long
2018-05-30 13:05     ` Juri Lelli
2018-05-30 13:05       ` Juri Lelli
2018-05-30 13:47       ` Waiman Long
2018-05-30 13:47         ` Waiman Long
2018-05-30 13:52         ` Juri Lelli
2018-05-30 13:52           ` Juri Lelli

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