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Date:   Wed, 29 May 2019 11:20:40 +0200 (CEST)
From:   Jiri Kosina <jikos@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
cc:     Dongli Zhang <dongli.zhang@oracle.com>,
        Keith Busch <kbusch@kernel.org>, Jens Axboe <axboe@kernel.dk>,
        Sagi Grimberg <sagi@grimberg.me>, linux-kernel@vger.kernel.org,
        linux-nvme@lists.infradead.org,
        Keith Busch <keith.busch@intel.com>,
        Hannes Reinecke <hare@suse.de>, Christoph Hellwig <hch@lst.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>, linux-pm@vger.kernel.org,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: "nosmt" breaks resuming from hibernation (was Re: [5.2-rc1
 regression]: nvme vs. hibernation)
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On Wed, 29 May 2019, Peter Zijlstra wrote:

> > > I verified that it succesfully makes it to the point where restore_image()
> > > is called from swsusp_arch_resume() (and verified that only BSP is alive
> > > at that time), but the old kernel never comes back and triplefault-like
> > > reboot happens.
> 
> which means that even without nosmt all 'other' CPUs are offline. And
> when I look at resume_target_kernel() I see it call
> hibernate_resume_nonboot_cpu_disable().
> 
> So how is the SMT offline different from that offline? afaict they all 
> get into play_dead()->native_play_dead()->mwait_play_dead().

There is no way those other CPUs have been offlined before to the 
native_play_dead() state, as this is way before any userspace was alive to 
initiate any kind of hotplug.

So they are guaranteed to have been all online, and then offlined properly 
to resume_play_dead(). 'nosmt' is the only exception there, as it's the 
only kind of offlining that has already happened at this point.

Let's continue in the other thread.

Thanks,

-- 
Jiri Kosina
SUSE Labs