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Date:   Sat, 14 Mar 2020 10:04:16 +0800
From:   Ming Lei <>
To:     Thomas Gleixner <>
Cc:     Peter Xu <>,,
        Ingo Molnar <>,
        Peter Zijlstra <>,
        Ming Lei <>,
        Juri Lelli <>
Subject: Re: [PATCH] x86/vector: Allow to free vector for managed IRQ

On Fri, Mar 13, 2020 at 03:24:08PM +0100, Thomas Gleixner wrote:
> Peter Xu <> writes:
> > After we introduced the "managed_irq" sub-parameter for isolcpus, it's
> > possible to free a kernel managed irq vector now.
> >
> > It can be triggered easily by booting a VM with a few vcpus, with one
> > virtio-blk device and then mark some cores as HK_FLAG_MANAGED_IRQ (in
> > below case, there're 4 vcpus, with vcpu 3 isolated with managed_irq):
> >
> > [    2.889911] ------------[ cut here ]------------
> > [    2.889964] WARNING: CPU: 3 PID: 0 at arch/x86/kernel/apic/vector.c:853 free_moved_vector+0x126/0x160
> <SNIP>
> > [    2.890026] softirqs last disabled at (8757): [<ffffffffbb0ecccd>] irq_enter+0x4d/0x70
> > [    2.890027] ---[ end trace deb5d563d2acb13f ]---
> What is this backtrace for? It's completly useless as it merily shows
> that the warning triggers. Also even if it'd be useful then it wants to
> be trimmed properly.
> > I believe the same thing will happen to bare metals.
> Believe is not really relevant in engineering.
> The problem has nothing to do with virt or bare metal. It's a genuine
> issue.
> > When allocating the IRQ for the device, activate_managed() will try to
> > allocate a vector based on what we've calculated for kernel managed
> > IRQs (which does not take HK_FLAG_MANAGED_IRQ into account).  However
> > when we bind the IRQ to the IRQ handler, we'll do irq_startup() and
> > irq_do_set_affinity(), in which we will start to consider the whole
> > HK_FLAG_MANAGED_IRQ logic.  This means the chosen core can be
> > different from when we do the allocation.  When that happens, we'll
> > need to be able to properly free the old vector on the old core.
> There's lots of 'we' in that text. We do nothing really. Please describe
> things in neutral and factual language.
> Also there is another way to trigger this: Offline all non-isolated CPUs
> in the mask and then bring one online again.
> Ming, I really have to ask why these two situations were not tested
> before the final submission of that isolation patch. Both issues have
> been discussed during review of the different versions. So the warning
> should have triggered back then already....

Hi Thomas,

I run CPU hotplug & unplug stress test with isolcpus:managed_irq, however
the test just checks if the irq's effective vector is setup correctly and
IO can be run as expected.

Looks dmesg warning is missed to check, sorry for that.


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