Message-ID: <CAAa6QmQRNGiHPzCtGem9HuCqUBbzUDYDibrw=-iZ=MeEWtOnCw@mail.gmail.com>
Date:   Wed, 2 Nov 2022 13:21:58 -0700
From:   "Zach O'Keefe" <zokeefe@...gle.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     Michal Hocko <mhocko@...e.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Davidoff <davidoff@...mf.net>,
        Bob Liu <lliubbo@...il.com>
Subject: Re: [PATCH] mm: don't warn if the node is offlined

On Wed, Nov 2, 2022 at 1:08 PM Yang Shi <shy828301@...il.com> wrote:
>
> On Wed, Nov 2, 2022 at 11:59 AM Zach O'Keefe <zokeefe@...gle.com> wrote:
> >
> > On Wed, Nov 2, 2022 at 11:18 AM Yang Shi <shy828301@...il.com> wrote:
> > >
> > > On Wed, Nov 2, 2022 at 10:47 AM Michal Hocko <mhocko@...e.com> wrote:
> > > >
> > > > On Wed 02-11-22 10:36:07, Yang Shi wrote:
> > > > > On Wed, Nov 2, 2022 at 9:15 AM Michal Hocko <mhocko@...e.com> wrote:
> > > > > >
> > > > > > On Wed 02-11-22 09:03:57, Yang Shi wrote:
> > > > > > > On Wed, Nov 2, 2022 at 12:39 AM Michal Hocko <mhocko@...e.com> wrote:
> > > > > > > >
> > > > > > > > On Tue 01-11-22 12:13:35, Zach O'Keefe wrote:
> > > > > > > > [...]
> > > > > > > > > This is slightly tangential - but I don't want to send a new mail
> > > > > > > > > about it -- but I wonder if we should be doing __GFP_THISNODE +
> > > > > > > > > explicit node vs having hpage_collapse_find_target_node() set a
> > > > > > > > > nodemask. We could then provide fallback nodes for ties, or if some
> > > > > > > > > node contained > some threshold number of pages.
> > > > > > > >
> > > > > > > > I would simply go with something like this (not even compile tested):
> > > > > > >
> > > > > > > Thanks, Michal. It is definitely an option. As I talked with Zach, I'm
> > > > > > > not sure whether it is worth making the code more complicated for such
> > > > > > > micro optimization or not. Removing __GFP_THISNODE or even removing
> > > > > > > the node balance code should be fine too IMHO. TBH I doubt there would
> > > > > > > be any noticeable difference.
> > > > > >
> > > > > > I do agree that an explicit nodes (quasi)round robin sounds over
> > > > > > engineered. It makes some sense to try to target the prevalent node
> > > > > > though because this code can be executed from khugepaged and therefore
> > > > > > allocating with a completely different affinity than the original fault.
> > > > >
> > > > > Yeah, the corner case comes from the node balance code, it just tries
> > > > > to balance between multiple prevalent nodes, so you agree to remove it
> > > > > IIRC?
> > > >
> > > > Yeah, let's just collect all good nodes into a nodemask and keep
> > > > __GFP_THISNODE in place. You can consider having the nodemask per collapse_control
> > > > so that you allocate it only once in the struct lifetime.
> > >
> > > Actually my intention is more aggressive, just remove that node balance code.
> > >
> >
> > The balancing code dates back to 2013 commit 9f1b868a13ac ("mm: thp:
> > khugepaged: add policy for finding target node") where it was made to
> > satisfy "numactl --interleave=all". I don't know why any real
> > workloads would want this -- but there very well could be a valid use
> > case. If not, I think it could be removed independent of what we do
> > with __GFP_THISNODE and nodemask.
>
> Hmm... if the code is used for interleave, I don't think nodemask
> could preserve the behavior IIUC. The nodemask also tries to allocate
> memory from the preferred node, and fallback to the allowed nodes from
> nodemask when the allocation fails on the preferred node. But the
> round robin style node balance tries to distribute the THP on the
> nodes evenly.

Ya, I don't think this has anything to do with nodemask -- I think I
inadvertently started a discussion about it and we now have 2 threads
merged into one :)

> And I just thought of __GFP_THISNODE + nodemask should not be the
> right combination IIUC, right? __GFP_THISNODE does disallow any
> fallback, so nodemask is actually useless.

Ya I was confused when I read this the first time -- thanks for
clarifying my understanding.

> So I think we narrowed down to two options:
> 1. Preserve the interleave behavior but bail out if the target node is
> not online (it is also racy, but doesn't hurt)
> 2. Remove the node balance code entirely
>

Agreed. Really comes down to if we care about that "numactl
--interleave" use case. My inclination would be to just remove it --
if we didn't have that code today, and someone raised this use case
and asked for the code to be added, I'm not sure it'd be approved.

> >
> > Balancing aside -- I haven't fully thought through what an ideal (and
> > further overengineered) solution would be for numa, but one (perceived
> > - not measured) issue that khugepaged might have (MADV_COLLAPSE
> > doesn't have the choice) is on systems with many, many nodes with
> > source pages sprinkled across all of them. Should we collapse these
> > pages into a single THP from the node with the most (but could still
> > be a small %) pages? Probably there are better candidates. So, maybe a
> > khugepaged-only check for max_value > (HPAGE_PMD_NR >> 1) or something
> > makes sense.
>
> Anyway you have to allocate a THP on one node, I don't think of a
> better idea to make the node selection fairer. But I'd prefer to wait
> for real life usecase surfaces.

So, the thought here is that we don't _have_ to allocate a THP. We can
bail-out, just as we do with max_ptes_*, when we think allocating a
THP isn't beneficial. As mentioned, MADV_COLLAPSE still has to
allocate a THP -- but khugepaged need not. I'm fine waiting on this
until needed, however.

> >
> > > >
> > > > And as mentioned in other reply it would be really nice to hide this
> > > > under CONFIG_NUMA (in a standalong follow up of course).
> > >
> > > The hpage_collapse_find_target_node() function itself is defined under
> > > CONFIG_NUMA.
> > >
> > > >
> > > > --
> > > > Michal Hocko
> > > > SUSE Labs

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