Re: [PATCH 1/2] mm/migrate: optimize hotplug-time demotion order updates

[David Hildenbrand <david@redhat.com>]

On 24.09.21 18:12, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@linux.intel.com>
> == tl;dr ==
> 
> Automatic demotion opted for a simple, lazy approach to handling
> hotplug events.  This noticeably slows down memory hotplug[1].
> Optimize away updates to the demotion order when memory hotplug
> events should have no effect.
> 
> This has no effect on CPU hotplug.  There is no known problem on
> the CPU side and any work there will be in a separate series.
> 
> == Background ==
> 
> Automatic demotion is a memory migration strategy to ensure that
> new allocations have room in faster memory tiers on tiered memory
> systems.  The kernel maintains an array (node_demotion[]) to
> drive these migrations.
> 
> The node_demotion[] path is calculated by starting at nodes with
> CPUs and then "walking" to nodes with memory.  Only hotplug
> events which online or offline a node with memory (N_ONLINE) or
> CPUs (N_CPU) will actually affect the migration order.
> 
> == Problem ==
> 
> However, the current code is lazy.  It completely regenerates the
> migration order on *any* CPU or memory hotplug event.  The logic
> was that these events are extremely rare and that the overhead
> from indiscriminate order regeneration is minimal.
> 
> Part of the update logic involves a synchronize_rcu(), which is a
> pretty big hammer.  Its overhead was large enough to be detected
> by some 0day tests that watch memory hotplug performance[1].
> 
> == Solution ==
> 
> Add a new helper (node_demotion_topo_changed()) which can
> differentiate between superfluous and impactful hotplug events.
> Skip the expensive update operation for superfluous events.
> 
> == Aside: Locking ==
> 
> It took me a few moments to declare the locking to be safe enough
> for node_demotion_topo_changed() to work.  It all hinges on the
> memory hotplug lock:
> 
> During memory hotplug events, 'mem_hotplug_lock' is held for
> write.  This ensures that two memory hotplug events can not be
> called simultaneously.
> 
> CPU hotplug has a similar lock (cpuhp_state_mutex) which also
> provides mutual exclusion between CPU hotplug events.  In
> addition, the demotion code acquire and hold the mem_hotplug_lock
> for read during its CPU hotplug handlers.  This provides mutual
> exclusion between the demotion memory hotplug callbacks and the
> CPU hotplug callbacks.
> 
> This effectively allows treating the migration target generation
> code to act as if it is single-threaded.
> 
> 1. https://lore.kernel.org/all/20210905135932.GE15026@xsang-OptiPlex-9020/
> 
> Fixes: 884a6e5d1f93 ("mm/migrate: update node demotion order on hotplug events")
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: "Huang, Ying" <ying.huang@intel.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Wei Xu <weixugc@google.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Greg Thelen <gthelen@google.com>
> Cc: Yang Shi <yang.shi@linux.alibaba.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>   b/mm/migrate.c |   12 +++++++++++-
>   1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff -puN mm/migrate.c~faster-node-order mm/migrate.c
> --- a/mm/migrate.c~faster-node-order	2021-09-24 09:12:30.988377798 -0700
> +++ b/mm/migrate.c	2021-09-24 09:12:30.988377798 -0700
> @@ -3239,8 +3239,18 @@ static int migration_offline_cpu(unsigne
>    * set_migration_target_nodes().
>    */
>   static int __meminit migrate_on_reclaim_callback(struct notifier_block *self,
> -						 unsigned long action, void *arg)
> +						 unsigned long action, void *_arg)
>   {
> +	struct memory_notify *arg = _arg;
> +
> +	/*
> +	 * Only update the node migration order when a node is
> +	 * changing status, like online->offline.  This avoids
> +	 * the overhead of synchronize_rcu() in most cases.
> +	 */
> +	if (arg->status_change_nid < 0)
> +		return notifier_from_errno(0);
> +
>   	switch (action) {
>   	case MEM_GOING_OFFLINE:
>   		/*
> _
> 

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb