Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks

From: Heiko Carstens <heiko.carstens@de.ibm.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
	linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	Greg KH <gregkh@linuxfoundation.org>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	David Rientjes <rientjes@google.com>,
	Daniel Kiper <daniel.kiper@oracle.com>,
	linux-api@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	linux-s390@vger.kernel.org, xen-devel@lists.xenproject.org,
	linux-acpi@vger.kernel.org
Subject: Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks
Date: Tue, 28 Feb 2017 11:21:05 +0100	[thread overview]
Message-ID: <20170228102105.GA13872@osiris> (raw)
In-Reply-To: <20170227154304.GK26504@dhcp22.suse.cz>

On Mon, Feb 27, 2017 at 04:43:04PM +0100, Michal Hocko wrote:
> On Mon 27-02-17 12:25:10, Heiko Carstens wrote:
> > On Mon, Feb 27, 2017 at 11:02:09AM +0100, Vitaly Kuznetsov wrote:
> > > A couple of other thoughts:
> > > 1) Having all newly added memory online ASAP is probably what people
> > > want for all virtual machines.
> > 
> > This is not true for s390. On s390 we have "standby" memory that a guest
> > sees and potentially may use if it sets it online. Every guest that sets
> > memory offline contributes to the hypervisor's standby memory pool, while
> > onlining standby memory takes memory away from the standby pool.
> > 
> > The use-case is that a system administrator in advance knows the maximum
> > size a guest will ever have and also defines how much memory should be used
> > at boot time. The difference is standby memory.
> > 
> > Auto-onlining of standby memory is the last thing we want.
> > 
> > > Unfortunately, we have additional complexity with memory zones
> > > (ZONE_NORMAL, ZONE_MOVABLE) and in some cases manual intervention is
> > > required. Especially, when further unplug is expected.
> > 
> > This also is a reason why auto-onlining doesn't seem be the best way.
> 
> Can you imagine any situation when somebody actually might want to have
> this knob enabled? From what I understand it doesn't seem to be the
> case.

I can only speak for s390, and at least here I think auto-online is always
wrong, especially if you consider the added complexity that you may want to
online memory sometimes to ZONE_NORMAL and sometimes to ZONE_MOVABLE.