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From: Michal Hocko <mhocko@kernel.org>
To: Ben Widawsky <ben.widawsky@intel.com>
Cc: linux-mm <linux-mm@kvack.org>, Andi Kleen <ak@linux.intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Christoph Lameter <cl@linux.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	David Hildenbrand <david@redhat.com>,
	David Rientjes <rientjes@google.com>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Jonathan Corbet <corbet@lwn.net>,
	Kuppuswamy Sathyanarayanan 
	<sathyanarayanan.kuppuswamy@linux.intel.com>,
	Lee Schermerhorn <lee.schermerhorn@hp.com>,
	Li Xinhai <lixinhai.lxh@gmail.com>,
	Mel Gorman <mgorman@techsingularity.net>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Mina Almasry <almasrymina@google.com>, Tejun Heo <tj@kernel.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	linux-api@vger.kernel.org
Subject: Re: [PATCH 00/18] multiple preferred nodes
Date: Wed, 24 Jun 2020 20:39:17 +0200
Message-ID: <20200624183917.GW1320@dhcp22.suse.cz> (raw)
In-Reply-To: <20200624161643.75fkkvsxlmp3bf2e@intel.com>

On Wed 24-06-20 09:16:43, Ben Widawsky wrote:
> On 20-06-24 09:52:16, Michal Hocko wrote:
> > On Tue 23-06-20 09:12:11, Ben Widawsky wrote:
> > > On 20-06-23 13:20:48, Michal Hocko wrote:
> > [...]
> > > > It would be also great to provide a high level semantic description
> > > > here. I have very quickly glanced through patches and they are not
> > > > really trivial to follow with many incremental steps so the higher level
> > > > intention is lost easily.
> > > > 
> > > > Do I get it right that the default semantic is essentially
> > > > 	- allocate page from the given nodemask (with __GFP_RETRY_MAYFAIL
> > > > 	  semantic)
> > > > 	- fallback to numa unrestricted allocation with the default
> > > > 	  numa policy on the failure
> > > > 
> > > > Or are there any usecases to modify how hard to keep the preference over
> > > > the fallback?
> > > 
> > > tl;dr is: yes, and no usecases.
> > 
> > OK, then I am wondering why the change has to be so involved. Except for
> > syscall plumbing the only real change to the allocator path would be
> > something like
> > 
> > static nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *policy)
> > {
> > 	/* Lower zones don't get a nodemask applied for MPOL_BIND */
> > 	if (unlikely(policy->mode == MPOL_BIND || 
> > 	   	     policy->mode == MPOL_PREFERED_MANY) &&
> > 			apply_policy_zone(policy, gfp_zone(gfp)) &&
> > 			cpuset_nodemask_valid_mems_allowed(&policy->v.nodes))
> > 		return &policy->v.nodes;
> > 
> > 	return NULL;
> > }
> > 
> > alloc_pages_current
> > 
> > 	if (pol->mode == MPOL_INTERLEAVE)
> > 		page = alloc_page_interleave(gfp, order, interleave_nodes(pol));
> > 	else {
> > 		gfp_t gfp_attempt = gfp;
> > 
> > 		/*
> > 		 * Make sure the first allocation attempt will try hard
> > 		 * but eventually fail without OOM killer or other
> > 		 * disruption before falling back to the full nodemask
> > 		 */
> > 		if (pol->mode == MPOL_PREFERED_MANY)
> > 			gfp_attempt |= __GFP_RETRY_MAYFAIL;	
> > 
> > 		page = __alloc_pages_nodemask(gfp_attempt, order,
> > 				policy_node(gfp, pol, numa_node_id()),
> > 				policy_nodemask(gfp, pol));
> > 		if (!page && pol->mode == MPOL_PREFERED_MANY)
> > 			page = __alloc_pages_nodemask(gfp, order,
> > 				numa_node_id(), NULL);
> > 	}
> > 
> > 	return page;
> > 
> > similar (well slightly more hairy) in alloc_pages_vma
> > 
> > Or do I miss something that really requires more involved approach like
> > building custom zonelists and other larger changes to the allocator?
> 
> I think I'm missing how this allows selecting from multiple preferred nodes. In
> this case when you try to get the page from the freelist, you'll get the
> zonelist of the preferred node, and when you actually scan through on page
> allocation, you have no way to filter out the non-preferred nodes. I think the
> plumbing of multiple nodes has to go all the way through
> __alloc_pages_nodemask(). But it's possible I've missed the point.

policy_nodemask() will provide the nodemask which will be used as a
filter on the policy_node.
-- 
Michal Hocko
SUSE Labs

  reply index

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20200619162425.1052382-1-ben.widawsky@intel.com>
2020-06-22  7:09 ` Michal Hocko
2020-06-23 11:20   ` Michal Hocko
2020-06-23 16:12     ` Ben Widawsky
2020-06-24  7:52       ` Michal Hocko
2020-06-24 16:16         ` Ben Widawsky
2020-06-24 18:39           ` Michal Hocko [this message]
2020-06-24 19:37             ` Ben Widawsky
2020-06-24 19:51               ` Michal Hocko
2020-06-24 20:01                 ` Ben Widawsky
2020-06-24 20:07                   ` Michal Hocko
2020-06-24 20:23                     ` Ben Widawsky
2020-06-24 20:42                       ` Michal Hocko
2020-06-24 20:55                         ` Ben Widawsky
2020-06-25  6:28                           ` Michal Hocko
2020-06-26 21:39         ` Ben Widawsky
2020-06-29 10:16           ` Michal Hocko

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