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

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Message-ID: <20171030165828.u77shgkuhzawxu4g@hirez.programming.kicks-ass.net>
Date:   Mon, 30 Oct 2017 17:58:28 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christopher Lameter <cl@...ux.com>
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, Oct 30, 2017 at 11:30:59AM -0500, Christopher Lameter wrote:
> 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.

The HPC workloads very much disabled load-balancing across most CPUs. If
you disable "sched_load_balance" the thing creates NULL sched_domains,
the exact thing isolcpus ends up doing.

This is something cpusets have done for a _long_ time, if not from the
very start.

Yes, you can also create smaller sched_domains which is useful for other
cases and you can even mix the lot, by creating a small set of
load-balanced CPUs for the system tasks while giving a bunch of
unbalanced CPUs to your application.