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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 09:52:16 +0200
Message-ID: <20200624075216.GC1320@dhcp22.suse.cz> (raw)
In-Reply-To: <20200623161211.qjup5km5eiisy5wy@intel.com>

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?
-- 
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 [this message]
2020-06-24 16:16         ` Ben Widawsky
2020-06-24 18:39           ` Michal Hocko
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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