linux-kernel - Re: [PATCH v2] isolcpus: affine kernel threads to specified cpumask

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Message-ID: <20200326165239.GD3946@lenoir>
Date:   Thu, 26 Mar 2020 17:52:40 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Chris Friesen <chris.friesen@...driver.com>,
        linux-kernel@...r.kernel.org, Christoph Lameter <cl@...ux.com>,
        Jim Somerville <Jim.Somerville@...driver.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] isolcpus: affine kernel threads to specified cpumask

On Thu, Mar 26, 2020 at 05:20:05PM +0100, Frederic Weisbecker wrote:
> On Wed, Mar 25, 2020 at 08:47:36AM -0300, Marcelo Tosatti wrote:
> > 
> > Hi Frederic,
> > 
> > On Wed, Mar 25, 2020 at 01:30:00AM +0100, Frederic Weisbecker wrote:
> > > On Tue, Mar 24, 2020 at 12:20:16PM -0300, Marcelo Tosatti wrote:
> > > > 
> > > > This is a kernel enhancement to configure the cpu affinity of kernel
> > > > threads via kernel boot option isolcpus=no_kthreads,<isolcpus_params>,<cpulist>
> > > > 
> > > > When this option is specified, the cpumask is immediately applied upon
> > > > thread launch. This does not affect kernel threads that specify cpu
> > > > and node.
> > > > 
> > > > This allows CPU isolation (that is not allowing certain threads
> > > > to execute on certain CPUs) without using the isolcpus=domain parameter,
> > > > making it possible to enable load balancing on such CPUs
> > > > during runtime (see
> > > > 
> > > > Note-1: this is based off on Wind River's patch at
> > > > https://github.com/starlingx-staging/stx-integ/blob/master/kernel/kernel-std/centos/patches/affine-compute-kernel-threads.patch
> > > > 
> > > > Difference being that this patch is limited to modifying
> > > > kernel thread cpumask: Behaviour of other threads can
> > > > be controlled via cgroups or sched_setaffinity.
> > > > 
> > > > Note-2: MontaVista's patch was based off Christoph Lameter's patch at
> > > > https://lwn.net/Articles/565932/ with the only difference being
> > > > the kernel parameter changed from kthread to kthread_cpus.
> > > > 
> > > > Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>
> > > 
> > > I'm wondering, why do you need such a boot shift at all when you
> > > can actually affine kthreads on runtime?
> > 
> > New, unbound kernel threads inherit the cpumask of kthreadd.
> > 
> > Therefore there is a race between kernel thread creation 
> > and affine.
> > 
> > If you know of a solution to that problem, that can be used instead.
> 
> Well, you could first set the affinity of kthreadd and only then the affinity
> of the others. But I can still imagine some tiny races with fork().

Ah forget that, I missed the part in kthread_create_on_node().

Thanks.