Re: [PATCH] CPU hotplug: Slow down hotplug operations

[Posted May 13, 2014 by corbet]

From:		Andrew Morton <akpm-AT-linux-foundation.org>
To:		Borislav Petkov <bp-AT-alien8.de>
Subject:		Re: [PATCH] CPU hotplug: Slow down hotplug operations
Date:		Wed, 7 May 2014 13:06:55 -0700
Message-ID:		<20140507130655.9f7c46de8cb39726079458fb@linux-foundation.org>
Cc:		LKML <linux-kernel-AT-vger.kernel.org>, "H. Peter Anvin" <hpa-AT-zytor.com>, Ingo Molnar <mingo-AT-kernel.org>, Thomas Gleixner <tglx-AT-linutronix.de>, Peter Zijlstra <peterz-AT-infradead.org>, Mel Gorman <mgorman-AT-suse.com>, Steven Rostedt <rostedt-AT-goodmis.org>, Mike Galbraith <mgalbraith-AT-suse.de>, Linus Torvalds <torvalds-AT-linux-foundation.org>, "Srivatsa S. Bhat" <srivatsa.bhat-AT-linux.vnet.ibm.com>
Archive‑link:		Article

On Wed,  7 May 2014 21:57:41 +0200 Borislav Petkov <bp@alien8.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.