Received-SPF: pass (google.com: domain of linux-kernel+bounces-60277-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:40f1:3f00::1 as permitted sender) client-ip=2604:1380:40f1:3f00::1;
Message-ID: <b6e6454a-d08f-4a71-b546-16cf3e2b88a5@redhat.com>
Date: Fri, 9 Feb 2024 23:19:58 -0500
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [RFC PATCH 0/8] cgroup/cpuset: Support RCU_NOCB on isolated
 partitions
Content-Language: en-US
To: Frederic Weisbecker <frederic@kernel.org>
Cc: Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
 Johannes Weiner <hannes@cmpxchg.org>, Jonathan Corbet <corbet@lwn.net>,
 "Paul E. McKenney" <paulmck@kernel.org>,
 Neeraj Upadhyay <quic_neeraju@quicinc.com>,
 Joel Fernandes <joel@joelfernandes.org>,
 Josh Triplett <josh@joshtriplett.org>, Boqun Feng <boqun.feng@gmail.com>,
 Steven Rostedt <rostedt@goodmis.org>,
 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
 Lai Jiangshan <jiangshanlai@gmail.com>, Zqiang <qiang.zhang1211@gmail.com>,
 Davidlohr Bueso <dave@stgolabs.net>, Shuah Khan <shuah@kernel.org>,
 cgroups@vger.kernel.org, linux-doc@vger.kernel.org,
 linux-kernel@vger.kernel.org, rcu@vger.kernel.org,
 linux-kselftest@vger.kernel.org, Mrunal Patel <mpatel@redhat.com>,
 Ryan Phillips <rphillips@redhat.com>, Brent Rowsell <browsell@redhat.com>,
 Peter Hunt <pehunt@redhat.com>, Cestmir Kalina <ckalina@redhat.com>,
 Nicolas Saenz Julienne <nsaenz@kernel.org>,
 Alex Gladkov <agladkov@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>,
 Phil Auld <pauld@redhat.com>, Paul Gortmaker <paul.gortmaker@windriver.com>,
 Daniel Bristot de Oliveira <bristot@kernel.org>,
 Juri Lelli <juri.lelli@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
 Costa Shulyupin <cshulyup@redhat.com>
References: <20240117163511.88173-1-longman@redhat.com>
 <ZagJPoEsLZ6Dg-NG@mtj.duckdns.org>
 <5ee5bf79-6cdc-4d1b-a19f-f0d5165a5f16@redhat.com>
 <ZcIsd6fjgmsb2dxr@localhost.localdomain>
From: Waiman Long <longman@redhat.com>
In-Reply-To: <ZcIsd6fjgmsb2dxr@localhost.localdomain>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit

On 2/6/24 07:56, Frederic Weisbecker wrote:
> Le Wed, Jan 17, 2024 at 12:15:07PM -0500, Waiman Long a écrit :
>> On 1/17/24 12:07, Tejun Heo wrote:
>>> Hello,
>>>
>>> On Wed, Jan 17, 2024 at 11:35:03AM -0500, Waiman Long wrote:
>>>> The first 2 patches are adopted from Federic with minor twists to fix
>>>> merge conflicts and compilation issue. The rests are for implementing
>>>> the new cpuset.cpus.isolation_full interface which is essentially a flag
>>>> to globally enable or disable full CPU isolation on isolated partitions.
>>> I think the interface is a bit premature. The cpuset partition feature is
>>> already pretty restrictive and makes it really clear that it's to isolate
>>> the CPUs. I think it'd be better to just enable all the isolation features
>>> by default. If there are valid use cases which can't be served without
>>> disabling some isolation features, we can worry about adding the interface
>>> at that point.
>> My current thought is to make isolated partitions act like isolcpus=domain,
>> additional CPU isolation capabilities are optional and can be turned on
>> using isolation_full. However, I am fine with making all these turned on by
>> default if it is the consensus.
> Right it was the consensus last time I tried. Along with the fact that mutating
> this isolation_full set has to be done on offline CPUs to simplify the whole
> picture.
>
> So lemme try to summarize what needs to be done:
>
> 1) An all-isolation feature file (that is, all the HK_TYPE_* things) on/off for
>    now. And if it ever proves needed, provide a way later for more finegrained
>    tuning.
That is more or less the current plan. As detailed below, HK_TYPE_DOMAIN 
& HK_TYPE_WQ isolation are included in the isolated partitions by 
default. I am also thinking about including other relatively cheap 
isolation flags by default. The expensive ones will have to be enabled 
via isolation_full.
>
> 2) This file must only apply to offline CPUs because it avoids migrations and
>    stuff.
Well, the process of first moving the CPUs offline first is rather 
expensive. I won't mind doing some partial offlining based on the 
existing set of teardown and bringup callbacks, but I would try to avoid 
fully offlining the CPUs first.
>
> 3) I need to make RCU NOCB tunable only on offline CPUs, which isn't that much
>     changes.
>
> 4) HK_TYPE_TIMER:
>     * Wrt. timers in general, not much needs to be done, the CPUs are
>       offline. But:
>     * arch/x86/kvm/x86.c does something weird
>     * drivers/char/random.c might need some care
>     * watchdog needs to be (de-)activated
>     
> 5) HK_TYPE_DOMAIN:
>     * This one I fear is not mutable, this is isolcpus...

HK_TYPE_DOMAIN is already available via the current cpuset isolated 
partition functionality. What I am currently doing is to extend that to 
other HK_TYPE* flags.


>
> 6) HK_TYPE_MANAGED_IRQ:
>     * I prefer not to think about it :-)
>
> 7) HK_TYPE_TICK:
>     * Maybe some tiny ticks internals to revisit, I'll check that.
>     * There is a remote tick to take into consideration, but again the
>       CPUs are offline so it shouldn't be too complicated.
>
> 8) HK_TYPE_WQ:
>     * Fortunately we already have all the mutable interface in place.
>       But we must make it live nicely with the sysfs workqueue affinity
>       files.

HK_TYPE_WQ is basically done and it is going to work properly with the 
workqueue affinity sysfs files. From the workqueue of view, HK_TYPE_WQ 
is currently treated the same as HK_TYPE_DOMAIN.

>
> 9) HK_FLAG_SCHED:
>     * Oops, this one is ignored by nohz_full/isolcpus, isn't it?
>     Should be removed?
I don't think HK_FLAG_SCHED is being used at all. So I believe we should 
remove it to avoid confusion.
>
> 10) HK_TYPE_RCU:
>      * That's point 3) and also some kthreads to affine, which leads us
>       to the following in HK_TYPE_KTHREAD:
>
> 11) HK_FLAG_KTHREAD:
>      * I'm guessing it's fine as long as isolation_full is also an
>        isolated partition. Then unbound kthreads shouldn't run there.

Yes, isolation_full applies only to isolated partitions. It extends the 
amount of CPU isolation by enabling all the other CPU available 
isolation flags.

Cheers,
Longman