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

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Date:   Fri, 30 Jul 2021 11:05:02 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Michal Hocko <mhocko@...e.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 Thu, Jul 29, 2021 at 06:21:19PM +0200, Michal Hocko wrote:
> On Thu 29-07-21 23:12:42, Feng Tang wrote:
> > On Thu, Jul 29, 2021 at 03:38:44PM +0200, Michal Hocko wrote:
> [...]
> > > Also the
> > > semantic to give nodes some ordering based on their numbers sounds
> > > rather weird to me.
> > 
> > I agree, and as I admitted in the first reply, this need to be fixed.
> 
> OK. I was not really clear that we are on the same page here.
> 
> > > The semantic I am proposing is to allocate from prefered nodes in
> > > distance order starting from the local node.
> > 
> > So the plan is:
> > * if the local node is set in 'prefer-many's nodemask, then chose
> > * otherwise chose the node with the shortest distance to local node
> > ?
> 
> Yes and what I am trying to say is that you will achieve that simply by
> doing the following in policy_node:
> 	if (policy->mode == MPOL_PREFERRED_MANY)
> 		return nd;

One thing is, it's possible that 'nd' is not set in the preferred
nodemask. 

For policy_node(), most of its caller use the local node id as 'nd'
parameter. For HBM and PMEM memory nodes, they are cpuless nodes,
so they will not be a 'local node', but some use cases only prefer
these nodes.

Thanks,
Feng

> -- 
> Michal Hocko
> SUSE Labs