linux-kernel - Re: [RFC v1 00/19] Modify zonelist to nodelist v1

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Date:   Fri, 22 Nov 2019 10:53:22 -0500
From:   Qian Cai <cai@....pw>
To:     Pengfei Li <fly@...nel.page>,
        "lixinhai.lxh@...il.com" <lixinhai.lxh@...il.com>
Cc:     akpm <akpm@...ux-foundation.org>,
        mgorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, cl <cl@...ux.com>,
        "iamjoonsoo.kim" <iamjoonsoo.kim@....com>, guro <guro@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [RFC v1 00/19] Modify zonelist to nodelist v1

On Fri, 2019-11-22 at 23:28 +0800, Pengfei Li wrote:
> On Fri, 22 Nov 2019 15:25:00 +0800
> "lixinhai.lxh@...il.com" <lixinhai.lxh@...il.com> wrote:
> 
> > On 2019-11-21 at 23:17 Pengfei Li wrote:
> > > Motivation
> > > ----------
> > > Currently if we want to iterate through all the nodes we have to
> > > traverse all the zones from the zonelist.
> > > 
> > > So in order to reduce the number of loops required to traverse node,
> > > this series of patches modified the zonelist to nodelist.
> > > 
> > > Two new macros have been introduced:
> > > 1) for_each_node_nlist
> > > 2) for_each_node_nlist_nodemask
> > > 
> > > 
> > > Benefit
> > > -------
> > > 1. For a NUMA system with N nodes, each node has M zones, the number
> > >    of loops is reduced from N*M times to N times when traversing
> > > node.
> > > 
> > 
> > It looks to me that we don't really have system which has N nodes and 
> > each node with M zones in its address range. 
> > We may have systems which has several nodes, but only the first node
> > has all zone types, other nodes only have NORMAL zone. (Evenly
> > distribute the !NORMAL zones on all nodes is not reasonable, as those
> > zones have limited size)
> > So iterate over zones to reach nodes should at N level, not M*N level.
> > 
> 
> Thanks for your comments.
> 
> In the case you said, the number of loops required to traverse all
> nodes is similar to traversing all zones.
> 
> I have two main reasons to explain that this series of patches is
> beneficial.
> 
> 1. When node has more than one zone, it will take fewer cycles to
> traverse all nodes. (for example, ZONE_MOVABLE?)

ZONE_MOVABLE is broken for ages (non-movable allocations are there all the time
last time I tried) which indicates there is very few people care about it, so it
is rather weak to use that as a justification for the churns it might cause.

> 
> 2. Using zonelist to traverse all nodes is inefficient, pgdat must be
> obtained indirectly via zone->zone_pgdat, and additional judgment must
> be made.
> 
> E.g
> 1) Using zonelist to traverse all nodes
> 
> 	last_pgdat = NULL;	
> 	for_each_zone_zonelist(zone, xxx) {
> 		pgdat = zone->zone_pgdat;
> 		if (pgdat == last_pgdat)
> 			continue;
> 
> 		last_pgdat = pgdat;
> 		do_something(pgdat);
> 	}
> 
> 2) Using nodelist to traverse all nodes
> 
> 	for_each_node_nodelist(node, xxx) {
> 		do_something(NODE_INFO(node));
> 	}
>