linux-kernel - Re: [PATCH -V2] sched,topology: Update sched topology atomically

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Date:   Mon, 25 Apr 2022 16:52:33 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Huang Ying <ying.huang@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Huang Ying <ying.huang@...el.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Ingo Molnar <mingo@...hat.com>, Mel Gorman <mgorman@...e.de>,
        Rik van Riel <riel@...riel.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH -V2] sched,topology: Update sched topology atomically

On 21/04/22 08:31, Huang Ying wrote:
> When Peter Zijlstra reviewed commit 0fb3978b0aac ("sched/numa: Fix
> NUMA topology for systems with CPU-less nodes") [1], he pointed out
> that sched_domains_numa_distance and sched_domains_numa_masks are made
> separate RCU variables.  That could go side-ways if there were a
> function using both, although there isn't for now.
>
> So we update sched_domains_numa_distance and sched_domains_numa_masks
> and some other related sched topology parameters atomically to address
> the potential issues.
>
> [1] https://lkml.kernel.org/r/20220214121553.582248-1-ying.huang@intel.com
>
> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Cc: Valentin Schneider <valentin.schneider@....com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Rik van Riel <riel@...riel.com>
> Cc: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
>
> Changelog:
>
> v2:
>
> - Addressed comments from Valentin Schneider, Thanks!

One small bug and a whitespace nit below, with those fixed:

Reviewed-by: Valentin Schneider <vschneid@...hat.com>

FWIW I briefly tested this vs hotplug on QEMU.

> @@ -1806,8 +1873,7 @@ void sched_init_numa(int offline_node)
>
>                       if (distance < LOCAL_DISTANCE || distance >= NR_DISTANCE_VALUES) {
>                               sched_numa_warn("Invalid distance value range");
> -				bitmap_free(distance_map);
> -				return;
> +                                goto free_bitmap;

The indentation here is wrong (spaces vs tabs).

>                       }
>
>                       bitmap_set(distance_map, distance, 1);

>       /* Compute default topology size */
>       for (i = 0; sched_domain_topology[i].mask; i++);

After the original boot this will now be the default topology with the NUMA
bits on top, so we'll just keep growing the array every time we hotplug a
node. This should use sched_domain_topology_default instead (ditto for the
copy loop further down).