Re: [PATCH v1 4/4] mm/mempolicy: kill MPOL_F_LOCAL bit

From: Michal Hocko <mhocko@suse.com>
To: Feng Tang <feng.tang@intel.com>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	David Rientjes <rientjes@google.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Ben Widawsky <ben.widawsky@intel.com>,
	linux-kernel@vger.kernel.org,
	Andrea Arcangeli <aarcange@redhat.com>,
	Mel Gorman <mgorman@techsingularity.net>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	Vlastimil Babka <vbabka@suse.cz>, Andi Kleen <ak@linux.intel.com>,
	Dan Williams <dan.j.williams@intel.com>,
	ying.huang@intel.com
Subject: Re: [PATCH v1 4/4] mm/mempolicy: kill MPOL_F_LOCAL bit
Date: Thu, 27 May 2021 14:26:24 +0200	[thread overview]
Message-ID: <YK+P8GDH2kn4FDsA@dhcp22.suse.cz> (raw)
In-Reply-To: <20210527121041.GA7743@shbuild999.sh.intel.com>

On Thu 27-05-21 20:10:41, Feng Tang wrote:
> On Thu, May 27, 2021 at 10:20:08AM +0200, Michal Hocko wrote:
> > On Wed 26-05-21 13:01:42, Feng Tang wrote:
> > > Now the only remaining case of a real 'local' policy faked by
> > > 'prefer' policy plus MPOL_F_LOCAL bit is:
> > > 
> > > A valid 'prefer' policy with a valid 'preferred' node is 'rebind'
> > > to a nodemask which doesn't contains the 'preferred' node, then it
> > > will handle allocation with 'local' policy.
> > > 
> > > Add a new 'MPOL_F_LOCAL_TEMP' bit for this case, and kill the
> > > MPOL_F_LOCAL bit, which could simplify the code much.
> > 
> > As I've pointed out in the reply to the previous patch. It would have
> > been much better if most of the MPOL_F_LOCAL usage was gone by this
> > patch.
> > 
> > I also dislike a new MPOL_F_LOCAL_TEMP. This smells like sneaking the
> > hack back in after you have painstakingly removed it. So this looks like
> > a step backwards to me. I also do not understand why do we need the
> > rebind callback for local policy at all. There is no node mask for local
> > so what is going on here?
> 
> This is the special case 4 for 'perfer' policy with MPOL_F_STATIC_NODES
> flag set, say it prefer node 1, when it is later 'refind' to a new
> nodemask node 2-3, according to current code it will be add the
> MPOL_F_LOCAL bit and performs 'local' policy acctually. And in future
> it is 'rebind' again with a nodemask 1-2, it will be restored back
> to 'prefer' policy with preferred node 1.

Honestly I still do not follow the actual problem. A preferred node is a
_hint_. If you rebind the task to a different cpuset then why should we
actually care? The allocator will fallback to the closest node according
to the distance metric. Maybe the original code was trying to handle
that in some way but I really do fail to understand that code and I
strongly suspect it is more likely to overengineered rather than backed
by a real usecase. I might be wrong here but then this is an excellent
opportunity to clarify all those subtleties.
-- 
Michal Hocko
SUSE Labs