linux-kernel - Re: [tip:sched/core] sched/isolation: Document the isolcpus= flags

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Message-ID: <alpine.DEB.2.20.1710301125060.1182@nuc-kabylake>
Date:   Mon, 30 Oct 2017 11:30:59 -0500 (CDT)
From:   Christopher Lameter <cl@...ux.com>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     cmetcalf@...lanox.com, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, torvalds@...ux-foundation.org, hpa@...or.com,
        riel@...hat.com, mingo@...nel.org, efault@....de,
        frederic@...nel.org, kernellwp@...il.com,
        paulmck@...ux.vnet.ibm.com, lcapitulino@...hat.com,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched/isolation: Document the isolcpus= flags

On Mon, 30 Oct 2017, Peter Zijlstra wrote:

> > isolcpus is the *right* approach here because you are micromanaging the OS
> > and are putting dedicated pieces of software on each core.
>
> That is what you want, and cpusets should allow for that just fine.

Well yes a cpuset of one processor I guess.

> > A cgroup suggests that threads would be scheduled over multiple cores
> > which is *not* what you want.
>
> No, that suggestion is false. cpusets should allow you to isolate
> individual CPUs just fine.
>
> > cgroup has to do something with containers
>
> Sod containers. That's just modern group think. cpusets existed long
> before all that wankery and it should very well retain the original use
> cases.

Historically cpusets were not used for cpu isolation. They were used to
restrict applications threads to sets of cpus for performance reasons. And
we are here dealing with individual processors.

> That said, I know there's problems with cpusets, and those should be
> fixed. But that doesn't mean isolcpus is anything other than a vile
> hack.

Controlling the way an individual processor works would be best done with
some kind of configuration in sysfs. I.e. /sys/cpu/<xx>/no_sched or so.

In lieu of that I think isolcpus is definitely better than a cpuset and it
is the only way that traditionally processor handling of the OS has been
restricted.