From: Michal Hocko <mhocko@kernel.org>
To: Igor Mammedov <imammedo@redhat.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>,
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: Fri, 3 Mar 2017 09:27:23 +0100 [thread overview]
Message-ID: <20170303082723.GB31499@dhcp22.suse.cz> (raw)
In-Reply-To: <20170302180315.78975d4b@nial.brq.redhat.com>
On Thu 02-03-17 18:03:15, Igor Mammedov wrote:
> On Thu, 2 Mar 2017 15:28:16 +0100
> Michal Hocko <mhocko@kernel.org> wrote:
>
> > On Thu 02-03-17 14:53:48, Igor Mammedov wrote:
> > [...]
> > > When trying to support memory unplug on guest side in RHEL7,
> > > experience shows otherwise. Simplistic udev rule which onlines
> > > added block doesn't work in case one wants to online it as movable.
> > >
> > > Hotplugged blocks in current kernel should be onlined in reverse
> > > order to online blocks as movable depending on adjacent blocks zone.
> >
> > Could you be more specific please? Setting online_movable from the udev
> > rule should just work regardless of the ordering or the state of other
> > memblocks. If that doesn't work I would call it a bug.
> It's rather an implementation constrain than a bug
> for details and workaround patch see
> [1] https://bugzilla.redhat.com/show_bug.cgi?id=1314306#c7
"You are not authorized to access bug #1314306"
could you paste the reasoning here please?
> patch attached there is limited by another memory hotplug
> issue, which is NORMAL/MOVABLE zone balance, if kernel runs
> on configuration where the most of memory is hot-removable
> kernel might experience lack of memory in zone NORMAL.
yes and that is an inherent problem of movable memory.
> > > Which means simple udev rule isn't usable since it gets event from
> > > the first to the last hotplugged block order. So now we would have
> > > to write a daemon that would
> > > - watch for all blocks in hotplugged memory appear (how would it know)
> > > - online them in right order (order might also be different depending
> > > on kernel version)
> > > -- it becomes even more complicated in NUMA case when there are
> > > multiple zones and kernel would have to provide user-space
> > > with information about zone maps
> > >
> > > In short current experience shows that userspace approach
> > > - doesn't solve issues that Vitaly has been fixing (i.e. onlining
> > > fast and/or under memory pressure) when udev (or something else
> > > might be killed)
> >
> > yeah and that is why the patch does the onlining from the kernel.
> onlining in this patch is limited to hyperv and patch breaks
> auto-online on x86 kvm/vmware/baremetal as they reuse the same
> hotplug path.
Those can use the udev or do you see any reason why they couldn't?
> > > > 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.
> > > For x86:
> > > * this config option is enabled by default in recent Fedora,
> >
> > How do you want to support usecases which really want to online memory
> > as movable? Do you expect those users to disable the option because
> > unless I am missing something the in kernel auto onlining only supporst
> > regular onlining.
>
> current auto onlining config option does what it's been designed for,
> i.e. it onlines hotplugged memory.
> It's possible for non average Fedora user to override default
> (commit 86dd995d6) if she/he needs non default behavior
> (i.e. user knows how to online manually and/or can write
> a daemon that would handle all of nuances of kernel in use).
>
> For the rest when Fedora is used in cloud and user increases memory
> via management interface of whatever cloud she/he uses, it just works.
>
> So it's choice of distribution to pick its own default that makes
> majority of user-base happy and this patch removes it without taking
> that in consideration.
You still can have a udev rule to achive the same thing for
non-ballooning based hotplug.
> How to online memory is different issue not related to this patch,
> current default onlining as ZONE_NORMAL works well for scaling
> up VMs.
>
> Memory unplug is rather new and it doesn't work reliably so far,
> moving onlining to user-space won't really help. Further work
> is need to be done so that it would work reliably.
The main problem I have with this is that this is a limited usecase
driven configuration knob which doesn't work properly for other usecases
(namely movable online once your distribution choses to set the config
option to auto online). There is a userspace solution for this so this
shouldn't have been merged in the first place! It sneaked a proper review
process (linux-api wasn't CC to get a broader attenttion) which is
really sad.
So unless this causes a major regression which would be hard to fix I
will submit the patch for inclusion.
--
Michal Hocko
SUSE Labs
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next prev parent reply other threads:[~2017-03-03 8:27 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-02-27 9:28 [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Michal Hocko
2017-02-27 10:02 ` Vitaly Kuznetsov
2017-02-27 10:21 ` Michal Hocko
2017-02-27 10:49 ` Vitaly Kuznetsov
2017-02-27 12:56 ` Michal Hocko
2017-02-27 13:17 ` Vitaly Kuznetsov
2017-02-27 11:25 ` Heiko Carstens
2017-02-27 11:50 ` Vitaly Kuznetsov
2017-02-27 15:43 ` Michal Hocko
2017-02-28 10:21 ` Heiko Carstens
2017-03-02 13:53 ` Igor Mammedov
2017-03-02 14:28 ` Michal Hocko
2017-03-02 17:03 ` Igor Mammedov
2017-03-03 8:27 ` Michal Hocko [this message]
2017-03-03 17:34 ` Igor Mammedov
2017-03-06 14:54 ` Michal Hocko
2017-03-07 12:40 ` Igor Mammedov
2017-03-09 12:54 ` Michal Hocko
2017-03-10 13:58 ` WTH is going on with memory hotplug sysf interface (was: Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks) Michal Hocko
2017-03-10 15:53 ` Michal Hocko
2017-03-10 19:00 ` Reza Arbab
2017-03-13 9:21 ` Michal Hocko
2017-03-13 14:58 ` Reza Arbab
2017-03-14 19:35 ` Andrea Arcangeli
2017-03-15 7:57 ` Michal Hocko
2017-03-13 15:11 ` Michal Hocko
2017-03-13 23:16 ` Andi Kleen
2017-03-10 17:39 ` WTH is going on with memory hotplug sysf interface Yasuaki Ishimatsu
2017-03-13 9:19 ` Michal Hocko
2017-03-14 16:05 ` YASUAKI ISHIMATSU
2017-03-14 16:20 ` Michal Hocko
2017-03-13 10:31 ` WTH is going on with memory hotplug sysf interface (was: Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks) Igor Mammedov
2017-03-13 10:43 ` Michal Hocko
2017-03-13 13:57 ` Igor Mammedov
2017-03-13 14:36 ` Michal Hocko
2017-03-13 10:55 ` [RFC PATCH] mm, hotplug: get rid of auto_online_blocks Igor Mammedov
2017-03-13 12:28 ` Michal Hocko
2017-03-13 12:54 ` Vitaly Kuznetsov
2017-03-13 13:19 ` Michal Hocko
2017-03-13 13:42 ` Vitaly Kuznetsov
2017-03-13 14:32 ` Michal Hocko
2017-03-13 15:10 ` Vitaly Kuznetsov
2017-03-14 13:20 ` Igor Mammedov
2017-03-15 7:53 ` Michal Hocko
2017-03-10 22:00 ` Daniel Kiper
2017-02-27 17:28 ` Reza Arbab
2017-02-27 17:34 ` Michal Hocko
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