linux-kernel - Re: [PATCH] CPU hotplug: Slow down hotplug operations

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Message-Id: <20140507130655.9f7c46de8cb39726079458fb@linux-foundation.org>
Date:	Wed, 7 May 2014 13:06:55 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mike Galbraith <mgalbraith@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: Re: [PATCH] CPU hotplug: Slow down hotplug operations

On Wed,  7 May 2014 21:57:41 +0200 Borislav Petkov <bp@...en8.de> wrote:

> We have all those eager tester dudes which scratch up a dirty script to
> pound on CPU hotplug senselessly and then report bugs they've managed to
> trigger.
> 
> Well, first of all, most, if not all, bugs they trigger are CPU hotplug
> related anyway. But we know hotplug is full of duct tape and brown
> paper bags. So we end up clearly wasting too much time dealing with a
> mechanism we know it is b0rked in the first place.
> 
> Oh, and I would understand if that pounding were close to some real
> usage patterns but I've yet to receive a justification for toggling
> cores on- and offline senselessly.
> 
> In any case, before this gets rewritten properly (I'm being told we
> might get lucky after all) let's slow down hotplugging on purpose and
> thus make it uninteresting, as a temporary brown paper bag solution
> until the real thing gets done.
> 
> This way we'll save us a lot of time and efforts in chasing the wrong
> bugs.

Well, I only yesterday merged Srivatsa's `CPU hotplug, stop-machine:
plug race-window that leads to "IPI-to-offline-CPU"' bugfix.  That bug
presumably wouldn't have been fixed if this patch was in place.
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