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From: Marcelo Tosatti <mtosatti@redhat.com>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>, Tejun Heo <tj@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Alex Belits <abelits@marvell.com>, Nitesh Lal <nilal@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Nicolas Saenz <nsaenzju@redhat.com>,
	Christoph Lameter <cl@gentwo.de>,
	Zefan Li <lizefan.x@bytedance.com>,
	cgroups@vger.kernel.org
Subject: Re: [RFC PATCH 6/6] cpuset: Add cpuset.isolation_mask file
Date: Mon, 19 Jul 2021 12:41:28 -0300	[thread overview]
Message-ID: <20210719154128.GB27911@fuller.cnet> (raw)
In-Reply-To: <20210719132649.GB116346@lothringen>

On Mon, Jul 19, 2021 at 03:26:49PM +0200, Frederic Weisbecker wrote:
> On Wed, Jul 14, 2021 at 01:31:57PM -0300, Marcelo Tosatti wrote:
> > On Wed, Jul 14, 2021 at 03:54:20PM +0200, Frederic Weisbecker wrote:
> > > Add a new cpuset.isolation_mask file in order to be able to modify the
> > > housekeeping cpumask for each individual isolation feature on runtime.
> > > In the future this will include nohz_full, unbound timers,
> > > unbound workqueues, unbound kthreads, managed irqs, etc...
> > > 
> > > Start with supporting domain exclusion and CPUs passed through
> > > "isolcpus=".
> > 
> > It is possible to just add return -ENOTSUPPORTED for the features 
> > whose support is not present?
> 
> Maybe, although that looks like a specialized error for corner cases.

Well, are you going to implement runtime enablement for all features,
including nohz_full, in the first patch set?

From my POV returning -ENOTSUPPORTED would allow for a gradual 
implementation of the features.

> > > CHECKME: Should we have individual cpuset.isolation.$feature files for
> > >          each isolation feature instead of a single mask file?
> > 
> > Yes, guess that is useful, for example due to the -ENOTSUPPORTED
> > comment above.
> > 
> > 
> > Guarantees on updates
> > =====================
> > 
> > Perhaps start with a document with:
> > 
> > On return to the write to the cpumask file, what are the guarantees?
> > 
> > For example, for kthread it is that any kernel threads from that point
> > on should start with the new mask. Therefore userspace should 
> > respect the order:
> > 
> > 1) Change kthread mask.
> > 2) Move threads.
> > 
> 
> Yep.
> 
> > Updates to interface
> > ====================
> > 
> > Also, thinking about updates to the interface (which today are one
> > cpumask per isolation feature) might be useful. What can happen:
> > 
> > 1) New isolation feature is added, feature name added to the interface.
> > 
> > Userspace must support new filename. If not there, then thats an 
> > old kernel without support for it.
> > 
> > 2) If an isolation feature is removed, a file will be gone. What should
> > be the behaviour there? Remove the file? (userspace should probably 
> > ignore the failure in that case?) (then features names should not be
> > reused, as that can confuse #1 above).
> 
> Heh, yeah that's complicated. I guess we should use one flag per file as that
> fits well within the current cpuset design. But we must carefully choose the new
> files to make sure they have the least chances to be useless in the long term.
> 
> > Or maybe have a versioned scheme?
> 
> I suspect we should avoid that at all costs :-)
> 
> Thanks!

Makes sense.


  reply	other threads:[~2021-07-19 18:27 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-14 13:54 [RFC PATCH 0/6] cpuset: Allow to modify isolcpus through cpuset Frederic Weisbecker
2021-07-14 13:54 ` [RFC PATCH 1/6] pci: Decouple HK_FLAG_WQ and HK_FLAG_DOMAIN cpumask fetch Frederic Weisbecker
2021-07-14 13:54 ` [RFC PATCH 2/6] workqueue: " Frederic Weisbecker
2021-07-14 13:54 ` [RFC PATCH 3/6] net: " Frederic Weisbecker
2021-07-14 13:54 ` [RFC PATCH 4/6] sched/isolation: Split domain housekeeping mask from the rest Frederic Weisbecker
2021-07-14 13:54 ` [RFC PATCH 5/6] sched/isolation: Make HK_FLAG_DOMAIN mutable Frederic Weisbecker
2021-07-21 14:28   ` Vincent Donnefort
2021-07-14 13:54 ` [RFC PATCH 6/6] cpuset: Add cpuset.isolation_mask file Frederic Weisbecker
2021-07-14 16:31   ` Marcelo Tosatti
2021-07-19 13:26     ` Frederic Weisbecker
2021-07-19 15:41       ` Marcelo Tosatti [this message]
2021-07-14 16:52   ` Peter Zijlstra
2021-07-14 23:13     ` Frederic Weisbecker
2021-07-14 23:44       ` Valentin Schneider
2021-07-15  0:07         ` Frederic Weisbecker
2021-07-15  9:04       ` Peter Zijlstra
2021-07-19 13:17         ` Frederic Weisbecker
2021-07-16 18:02 ` [RFC PATCH 0/6] cpuset: Allow to modify isolcpus through cpuset Waiman Long
2021-07-19 13:57   ` Frederic Weisbecker

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