linux-kernel - Re: [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug

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Date:   Tue, 24 Sep 2019 16:48:46 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug_lock

On Tue 24-09-19 16:36:15, David Hildenbrand wrote:
> Since commit 3f906ba23689 ("mm/memory-hotplug: switch locking to a percpu
> rwsem") we do a cpus_read_lock() in mem_hotplug_begin(). This was
> introduced to fix a potential deadlock between get_online_mems() and
> get_online_cpus() - the memory and cpu hotplug lock. The root issue was
> that build_all_zonelists() -> stop_machine() required the cpu hotplug lock:
>     The reason is that memory hotplug takes the memory hotplug lock and
>     then calls stop_machine() which calls get_online_cpus().  That's the
>     reverse lock order to get_online_cpus(); get_online_mems(); in
>     mm/slub_common.c
> 
> So memory hotplug never really required any cpu lock itself, only
> stop_machine() and lru_add_drain_all() required it. Back then,
> stop_machine_cpuslocked() and lru_add_drain_all_cpuslocked() were used
> as the cpu hotplug lock was now obtained in the caller.
> 
> Since commit 11cd8638c37f ("mm, page_alloc: remove stop_machine from build
> all_zonelists"), the stop_machine_cpuslocked() call is gone.
> build_all_zonelists() does no longer require the cpu lock and does no
> longer make use of stop_machine().
> 
> Since commit 9852a7212324 ("mm: drop hotplug lock from
> lru_add_drain_all()"), lru_add_drain_all() "Doesn't need any cpu hotplug
> locking because we do rely on per-cpu kworkers being shut down before our
> page_alloc_cpu_dead callback is executed on the offlined cpu.". The
> lru_add_drain_all_cpuslocked() variant was removed.
> 
> So there is nothing left that requires the cpu hotplug lock. The memory
> hotplug lock and the device hotplug lock are sufficient.

I would love to see this happen. The biggest offenders should be gone. 
I really hated how those two locks have been conflated which likely
resulted in some undocumented/unintended dependencies. So for now, I
cannot really tell you whether the patch is correct. It would really
require a lot of testing. I am not sure this is reasonably reviewable.

So please add some testing results (ideally cpu hotplug racing a lot
with the memory hotplug). Then I would be willing to give this a try
and see. First by keeping it in linux-next for a release or two and then
eyes closed, fingers crossed and go to the wild. Do we have a tag for
that Dared-by maybe?

> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Pavel Tatashin <pasha.tatashin@...een.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
> 
> RFC -> v1:
> - Reword and add more details why the cpu hotplug lock was needed here
>   in the first place, and why we no longer require it.
> 
> ---
>  mm/memory_hotplug.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index c3e9aed6023f..5fa30f3010e1 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -88,14 +88,12 @@ __setup("memhp_default_state=", setup_memhp_default_state);
>  
>  void mem_hotplug_begin(void)
>  {
> -	cpus_read_lock();
>  	percpu_down_write(&mem_hotplug_lock);
>  }
>  
>  void mem_hotplug_done(void)
>  {
>  	percpu_up_write(&mem_hotplug_lock);
> -	cpus_read_unlock();
>  }
>  
>  u64 max_mem_size = U64_MAX;
> -- 
> 2.21.0

-- 
Michal Hocko
SUSE Labs