Date:   Thu, 16 Feb 2023 09:37:34 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Qi Zheng <zhengqi.arch@...edance.com>
Cc:     akpm@...ux-foundation.org, vbabka@...e.cz, david@...hat.com,
        rppt@...nel.org, willy@...radead.org, mgorman@...hsingularity.net,
        osalvador@...e.de, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Muchun Song <muchun.song@...ux.dev>
Subject: Re: [PATCH 0/2] handle memoryless nodes more appropriately

On Thu 16-02-23 16:21:54, Qi Zheng wrote:
> 
> 
> On 2023/2/16 15:51, Michal Hocko wrote:
> > On Thu 16-02-23 07:11:19, Qi Zheng wrote:
> > > 
> > > 
> > > On 2023/2/16 00:36, Michal Hocko wrote:
> > > > On Wed 15-02-23 23:24:10, Qi Zheng wrote:
> > > > > Hi all,
> > > > > 
> > > > > Currently, in the process of initialization or offline memory, memoryless
> > > > > nodes will still be built into the fallback list of itself or other nodes.
> > > > > 
> > > > > This is not what we expected, so this patch series removes memoryless
> > > > > nodes from the fallback list entirely.
> > > > > 
> > > > > Comments and suggestions are welcome.
> > > 
> > > Hi Michal,
> > > 
> > > > 
> > > > This is a tricky area full of surprises and it is really easy to
> > > 
> > > Would you mind giving an example of a "new problem"?
> > 
> > The initialization is spread over several places and it is quite easy to
> > introduce bugs because it is hard to review this area. Been there done
> > that. Just look into the git log.
> 
> I understand your concern, but should we therefore reject all revisions
> to this?

No, but either somebode is willing to invest a non-trivial amount of
time and unify the NUMA initialization code that is spread over arch
specific code in different places or we should just focus on addressing
bugs.

> > > > introduce new problems. What kind of problem/issue are you trying to
> > > > solve/handle by these changes?
> > > 
> > > IIUC, I think there are two reasons:
> > > 
> > > Firstly, as mentioned in commit message, the memoryless node has no
> > > memory to allocate (If it can be allocated, it may also cause the panic
> > > I mentioned in [1]), so we should not continue to traverse it when
> > > allocating memory at runtime, which will have a certain overhead.
> > 
> > Sure that is not the most optimal implementation but does this matter in
> > practice? Can you observe any actual measurable performance penalty?
> 
> No, and the original reason for noticing this place was the panic I
> mentioned in [1] (< NODE_MIN_SIZE). And if we had handled the memoryless
> node's zonelist correctly before, we wouldn't have had that panic at
> all.

Yes, this is another good example of how subtle the code is. Mike has
posted a patch that simply drops the NODE_MIN_SIZE constrain and I
believe that is the right thing to do at this stage. There is a non-zero
risk of regression but at least we will be forced to fix the original
problem properly or at least document is properly.

> > Currently we are just sacrificing some tiny performance for a
> > simplicity.
> Hmm, I don't think my modification complicates the code.
> 
> > > Secondly, from the perspective of semantic correctness, why do we remove
> > > the memoryless node from the fallback list of other normal nodes
> > > (N_MEMORY), but not from its own fallback list (PATCH[1/2])? Why should
> > > an upcoming memoryless node continue exist in the fallback list of
> > > itself and other normal nodes (PATCH[2/2])?
> > 
> > I am not sure I follow. What is the semantic correctness issue?
> 
> Sorry for the ambiguity, what I meant was that memoryless nodes should
> never have been built into any fallback list, not just for performance
> optimizations.

Well, I am not 100% sure I agree with you here. The performance would be
the only reason why to drop those nodes from zonelists. Other than that
zonelists are a useful abstraction for the node distance ordering. Even
if those nodes do not have any memory at all in principle there is no
big difference from depleted nodes.
-- 
Michal Hocko
SUSE Labs

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