linux-kernel - Re: [PATCH 0/2 v3] cpu hotplug: Preserve topology directory after soft remove event

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Message-ID: <20160927115700.GN5016@twins.programming.kicks-ass.net>
Date:   Tue, 27 Sep 2016 13:57:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Prarit Bhargava <prarit@...hat.com>
Cc:     Borislav Petkov <bp@...e.de>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Len Brown <len.brown@...el.com>,
        Andi Kleen <ak@...ux.intel.com>, Jiri Olsa <jolsa@...hat.com>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH 0/2 v3] cpu hotplug: Preserve topology directory after
 soft remove event

On Tue, Sep 27, 2016 at 07:47:56AM -0400, Prarit Bhargava wrote:

> There's a difference between soft remove (via sysfs) and a true hot remove
> operation (where the whole thing is physically removed).  Soft remove only
> results in the processor being made "not available" to the scheduler.

How is it different? We do _ONE_ CPU unplug operation. We do not touch
the thing anymore after that. It _can_ be taken out after that.

Therefore hotplug wipes the topology information and clears the CPU from
relevant bitmasks.

>From the kernel's POV there really is no distinction.