linux-kernel - Re: [PATCH v6 1/6] mm/mempolicy: Add MPOL_PREFERRED

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Date:   Wed, 28 Jul 2021 18:12:21 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ben Widawsky <ben.widawsky@...el.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andi Kleen <ak@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>, ying.huang@...el.com,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v6 1/6] mm/mempolicy: Add MPOL_PREFERRED_MANY for
 multiple preferred nodes

On Wed 28-07-21 22:11:56, Feng Tang wrote:
> On Wed, Jul 28, 2021 at 02:31:03PM +0200, Michal Hocko wrote:
> > [Sorry for a late review]
> 
> Not at all. Thank you for all your reviews and suggestions from v1
> to v6!
> 
> > On Mon 12-07-21 16:09:29, Feng Tang wrote:
> > [...]
> > > @@ -1887,7 +1909,8 @@ nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *policy)
> > >  /* Return the node id preferred by the given mempolicy, or the given id */
> > >  static int policy_node(gfp_t gfp, struct mempolicy *policy, int nd)
> > >  {
> > > -	if (policy->mode == MPOL_PREFERRED) {
> > > +	if (policy->mode == MPOL_PREFERRED ||
> > > +	    policy->mode == MPOL_PREFERRED_MANY) {
> > >  		nd = first_node(policy->nodes);
> > >  	} else {
> > >  		/*
> > 
> > Do we really want to have the preferred node to be always the first node
> > in the node mask? Shouldn't that strive for a locality as well? Existing
> > callers already prefer numa_node_id() - aka local node - and I belive we
> > shouldn't just throw that away here.
>  
> I think it's about the difference of 'local' and 'prefer/perfer-many'
> policy. There are different kinds of memory HW: HBM(High Bandwidth
> Memory), normal DRAM, PMEM (Persistent Memory), which have different
> price, bandwidth, speed etc. A platform may have two, or all three of
> these types, and there are real use case which want memory comes
> 'preferred' node/nodes than the local node.
> 
> And good point for 'local node', if the 'prefer-many' policy's
> nodemask has local node set, we should pick it han this
> 'first_node', and the same semantic also applies to the other
> several places you pointed out. Or do I misunderstand you point?

Yeah. Essentially what I am trying to tell is that for
MPOL_PREFERRED_MANY you simply want to return the given node without any
alternation. That node will be used for the fallback zonelist and the
nodemask would make sure we won't get out of the policy.
-- 
Michal Hocko
SUSE Labs