Re: [PATCH 00/18] multiple preferred nodes

From: Ben Widawsky <ben.widawsky@intel.com>
To: Michal Hocko <mhocko@kernel.org>
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 13:01:40 -0700	[thread overview]
Message-ID: <20200624200140.dypw6snshshzlbwa@intel.com> (raw)
In-Reply-To: <20200624195158.GX1320@dhcp22.suse.cz>

On 20-06-24 21:51:58, Michal Hocko wrote:
> On Wed 24-06-20 12:37:33, Ben Widawsky wrote:
> > On 20-06-24 20:39:17, Michal Hocko wrote:
> > > 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.
> > 
> > Ah, gotcha. Enabling independent masks seemed useful. Some bad decisions got me
> > to that point. UAPI cannot get independent masks, and callers of these functions
> > don't yet use them.
> > 
> > So let me ask before I actually type it up and find it's much much simpler, is
> > there not some perceived benefit to having both masks being independent?
> 
> I am not sure I follow. Which two masks do you have in mind? zonelist
> and user provided nodemask?

Internally, a nodemask_t for preferred node, and a nodemask_t for bound nodes.

> 
> -- 
> Michal Hocko
> SUSE Labs
> 