RE: [PATCH] smp: Do not warn if smp_call_function_single() is doing a self call.

From: Dexuan Cui <decui@microsoft.com>
To: Thomas Gleixner <tglx@linutronix.de>, vkuznets <vkuznets@redhat.com>
Cc: "riel@surriel.com" <riel@surriel.com>,
	"jpoimboe@redhat.com" <jpoimboe@redhat.com>,
	"luto@kernel.org" <luto@kernel.org>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Sasha Levin <Alexander.Levin@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	KY Srinivasan <kys@microsoft.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Michael Kelley <mikelley@microsoft.com>,
	"marcelo.cerri@canonical.com" <marcelo.cerri@canonical.com>,
	"apw@canonical.com" <apw@canonical.com>,
	"olaf@aepfle.de" <olaf@aepfle.de>,
	"jasowang@redhat.com" <jasowang@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>
Subject: RE: [PATCH] smp: Do not warn if smp_call_function_single() is doing a self call.
Date: Wed, 17 Apr 2019 23:50:33 +0000	[thread overview]
Message-ID: <PU1P153MB01693370A9E788356ACF989BBF250@PU1P153MB0169.APCP153.PROD.OUTLOOK.COM> (raw)
In-Reply-To: <alpine.DEB.2.21.1904162206590.1780@nanos.tec.linutronix.de>

> From: Thomas Gleixner <tglx@linutronix.de>
> Sent: Tuesday, April 16, 2019 1:13 PM
> > ...
> > True. And before we start digging deeper into this, let's step back: why
> > do we need to do clockevents_unbind_device() on hybernation? Can we just
> > disable the device and re-enable it back on resume?

We do clockevents_unbind_device as part of hv_synic_cleanup(), which is
called as a CPU hotplug callback: see vmbus_bus_init():
    ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "hyperv/vmbus:online",
                        hv_synic_init, hv_synic_cleanup);

Yes, it looks the right thing is to implement the suspend/resume callbacks of
the clock_event_device. Thank you for the suggestion! I'll look into this.

> Yes. That's the right thing to do. Simple solution is to implement the
> suspend/resume callbacks on the clock events device and be done with it.

Agreed.

> > Actually, all usages of clockevents_unbind_device() in kernel are
> > limited to Hyper-V and with Michael's patches moving this out of VMBus
> > driver I think it can go away completely.

Thanks for the heads-up! I'll rebase to Michael's patches.

> Correct. There was a driver which required that, but that's gone by now and
> of course nobody noticed that it was the last user. The reason why this
> exists was to allow switching out an active clocksource similar to the
> sysfs unbind file but without user space interaction.
> 
> 	tglx

Thanks for the background sharing!

- Dexuan