linux-kernel - SMP lockup in virtualized environment

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Message-ID: <462E4223.1050109@trango-systems.com>
Date:	Tue, 24 Apr 2007 19:45:07 +0200
From:	LAPLACE Cyprien <cyprien.laplace@...ngo-systems.com>
To:	virtualization@...ts.linux-foundation.org
CC:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: SMP lockup in virtualized environment

In a previous mail, Jeremy Fitzhardinge wrote:
> The softlockup watchdog is currently a nuisance in a virtual machine,
> since the whole system could have the CPU stolen from it for a long
> period of time.  While it would be unlikely for a guest domain to be
> denied timer interrupts for over 10s, it could happen and any
> softlockup message would be completely spurious.

I wonder how the guest domain can be denied timer interrupts for such a
long time ? The only reason I see is that the guest domain is not
scheduled at all (host domain or another higher priority guest running).

Now in SMP host and guest, what happens if a guest CPU is not scheduled
for a while ?

An example: in kernel/pid.c:alloc_pid(), if one of the guest CPUs is
descheduled when holding the pidmap_lock, what happens to the other
guest CPUs who want to alloc/free pids ? Are they blocked too ?

-- 
Cyprien Laplace

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