From: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Mel Gorman <mgorman@techsingularity.net>,
Rik van Riel <riel@surriel.com>,
Thomas Gleixner <tglx@linutronix.de>,
Valentin Schneider <valentin.schneider@arm.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
linuxppc-dev@lists.ozlabs.org,
Nathan Lynch <nathanl@linux.ibm.com>,
Michael Ellerman <mpe@ellerman.id.au>,
Scott Cheloha <cheloha@linux.ibm.com>,
Gautham R Shenoy <ego@linux.vnet.ibm.com>,
Geetika Moolchandani <Geetika.Moolchandani1@ibm.com>
Subject: Re: [PATCH 1/3] sched/topology: Allow archs to populate distance map
Date: Fri, 21 May 2021 14:58:30 +0530 [thread overview]
Message-ID: <20210521092830.GF2633526@linux.vnet.ibm.com> (raw)
In-Reply-To: <YKdr0g6+eIHncqej@hirez.programming.kicks-ass.net>
* Peter Zijlstra <peterz@infradead.org> [2021-05-21 10:14:10]:
> On Fri, May 21, 2021 at 08:08:02AM +0530, Srikar Dronamraju wrote:
> > * Peter Zijlstra <peterz@infradead.org> [2021-05-20 20:56:31]:
> >
> > > On Thu, May 20, 2021 at 09:14:25PM +0530, Srikar Dronamraju wrote:
> > > > Currently scheduler populates the distance map by looking at distance
> > > > of each node from all other nodes. This should work for most
> > > > architectures and platforms.
> > > >
> > > > However there are some architectures like POWER that may not expose
> > > > the distance of nodes that are not yet onlined because those resources
> > > > are not yet allocated to the OS instance. Such architectures have
> > > > other means to provide valid distance data for the current platform.
> > > >
> > > > For example distance info from numactl from a fully populated 8 node
> > > > system at boot may look like this.
> > > >
> > > > node distances:
> > > > node 0 1 2 3 4 5 6 7
> > > > 0: 10 20 40 40 40 40 40 40
> > > > 1: 20 10 40 40 40 40 40 40
> > > > 2: 40 40 10 20 40 40 40 40
> > > > 3: 40 40 20 10 40 40 40 40
> > > > 4: 40 40 40 40 10 20 40 40
> > > > 5: 40 40 40 40 20 10 40 40
> > > > 6: 40 40 40 40 40 40 10 20
> > > > 7: 40 40 40 40 40 40 20 10
> > > >
> > > > However the same system when only two nodes are online at boot, then the
> > > > numa topology will look like
> > > > node distances:
> > > > node 0 1
> > > > 0: 10 20
> > > > 1: 20 10
> > > >
> > > > It may be implementation dependent on what node_distance(0,3) where
> > > > node 0 is online and node 3 is offline. In POWER case, it returns
> > > > LOCAL_DISTANCE(10). Here at boot the scheduler would assume that the max
> > > > distance between nodes is 20. However that would not be true.
> > > >
> > > > When Nodes are onlined and CPUs from those nodes are hotplugged,
> > > > the max node distance would be 40.
> > > >
> > > > To handle such scenarios, let scheduler allow architectures to populate
> > > > the distance map. Architectures that like to populate the distance map
> > > > can overload arch_populate_distance_map().
> > >
> > > Why? Why can't your node_distance() DTRT? The arch interface is
> > > nr_node_ids and node_distance(), I don't see why we need something new
> > > and then replace one special use of it.
> > >
> > > By virtue of you being able to actually implement this new hook, you
> > > supposedly can actually do node_distance() right too.
> >
> > Since for an offline node, arch interface code doesn't have the info.
> > As far as I know/understand, in POWER, unless there is an active memory or
> > CPU that's getting onlined, arch can't fetch the correct node distance.
> >
> > Taking the above example: node 3 is offline, then node_distance of (3,X)
> > where X is anything other than 3, is not reliable. The moment node 3 is
> > onlined, the node distance is reliable.
> >
> > This problem will not happen even on POWER if all the nodes have either
> > memory or CPUs active at the time of boot.
>
> But then how can you implement this new hook? Going by the fact that
> both nr_node_ids and distance_ref_points_depth are fixed, how many
> possible __node_distance() configurations are there left?
>
distance_ref_point_depth is provided as a different property and is readily
available at boot. The new api will use just use that. So based on the
distance_ref_point_depth, we know all possible node distances for that
platform.
For an offline node, we don't have that specific nodes distance_lookup_table
array entries. Each array would be of distance_ref_point_depth entries.
Without the distance_lookup_table for an array populated, we will not be
able to tell how far the node is with respect to other nodes.
We can lookup the correct distance_lookup_table for a node based on memory
or the CPUs attached to that node. Since in an offline node, both of them
would not be around, the distance_lookup_table will have stale values.
> The example provided above does not suggest there's much room for
> alternatives, and hence for actual need of this new interface.
>
--
Thanks and Regards
Srikar Dronamraju
next prev parent reply other threads:[~2021-05-21 10:06 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-20 15:44 [PATCH 0/3] Skip numa distance for offline nodes Srikar Dronamraju
2021-05-20 15:44 ` [PATCH 1/3] sched/topology: Allow archs to populate distance map Srikar Dronamraju
2021-05-20 18:56 ` Peter Zijlstra
2021-05-21 2:38 ` Srikar Dronamraju
2021-05-21 8:14 ` Peter Zijlstra
2021-05-21 9:28 ` Srikar Dronamraju [this message]
2021-05-24 14:16 ` Valentin Schneider
2021-05-24 16:18 ` Srikar Dronamraju
2021-05-25 10:21 ` Valentin Schneider
2021-05-25 11:32 ` Srikar Dronamraju
2021-05-28 5:21 ` Srikar Dronamraju
2021-05-28 8:43 ` Peter Zijlstra
2021-05-28 10:24 ` Srikar Dronamraju
2021-05-20 15:44 ` [PATCH 2/3] powerpc/numa: Populate distance map correctly Srikar Dronamraju
2021-05-24 14:16 ` Valentin Schneider
2021-05-24 14:50 ` Srikar Dronamraju
2021-05-20 15:44 ` [PATCH 3/3] sched/topology: Skip updating masks for non-online nodes Srikar Dronamraju
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