Re: [Xen-devel] [PATCH 2/4] xen: x86 / cpupool: clear the proper cpu_valid bit on pCPU teardown

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Thu, 2015-06-25 at 15:20 +0100, Andrew Cooper wrote:
> On 25/06/15 13:15, Dario Faggioli wrote:
> > In fact, if a pCPU belonging to some other pool than
> > cpupool0 goes down, we want to clear the relevant bit
> > from its actual pool, rather than always from cpupool0.
> 
> This sentence is a little hard to parse.
> 
> I presume you mean "use the correct cpupools valid mask, rather than
> cpupool0's".
> 
Yes, that's a better way to say what I meant.

> > # xl cpupool-cpu-remove Pool-0 8-15
> > # xl cpupool-create name=\"Pool-1\"
> > # xl cpupool-cpu-add Pool-1 8-15
> > --> suspend
> > --> resume
> > (XEN) ----[ Xen-4.6-unstable  x86_64  debug=y  Tainted:    C ]----
> > (XEN) CPU:    8
> > (XEN) RIP:    e008:[<ffff82d080123078>] csched_schedule+0x4be/0xb97
> > (XEN) RFLAGS: 0000000000010087   CONTEXT: hypervisor
> > (XEN) rax: 80007d2f7fccb780   rbx: 0000000000000009   rcx: 0000000000000000
> > (XEN) rdx: ffff82d08031ed40   rsi: ffff82d080334980   rdi: 0000000000000000
> > (XEN) rbp: ffff83010000fe20   rsp: ffff83010000fd40   r8:  0000000000000004
> > (XEN) r9:  0000ffff0000ffff   r10: 00ff00ff00ff00ff   r11: 0f0f0f0f0f0f0f0f
> > (XEN) r12: ffff8303191ea870   r13: ffff8303226aadf0   r14: 0000000000000009
> > (XEN) r15: 0000000000000008   cr0: 000000008005003b   cr4: 00000000000026f0
> > (XEN) cr3: 00000000dba9d000   cr2: 0000000000000000
> > (XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: 0000   cs: e008
> > (XEN) ... ... ...
> > (XEN) Xen call trace:
> > (XEN)    [<ffff82d080123078>] csched_schedule+0x4be/0xb97
> > (XEN)    [<ffff82d08012c732>] schedule+0x12a/0x63c
> > (XEN)    [<ffff82d08012f8c8>] __do_softirq+0x82/0x8d
> > (XEN)    [<ffff82d08012f920>] do_softirq+0x13/0x15
> > (XEN)    [<ffff82d080164791>] idle_loop+0x5b/0x6b
> > (XEN)
> > (XEN) ****************************************
> > (XEN) Panic on CPU 8:
> > (XEN) GENERAL PROTECTION FAULT
> > (XEN) [error_code=0000]
> > (XEN) ****************************************
> 
> What is the actual cause of the #GP fault?  There are no obviously
> poised registers.  
>
IIRC, CPU 8 has been just brought up and is scheduling. Not any other
CPU from Pool-1 is online yet. We are on CPU 8, in
csched_load_balance(), more specifically here:

    ...
    BUG_ON( cpu != snext->vcpu->processor );
    online = cpupool_scheduler_cpumask(per_cpu(cpupool, cpu));
    ...
    for_each_csched_balance_step( bstep )
    {
        /*
         * We peek at the non-idling CPUs in a node-wise fashion. In fact,
         * it is more likely that we find some affine work on our same
         * node, not to mention that migrating vcpus within the same node
         * could well expected to be cheaper than across-nodes (memory
         * stays local, there might be some node-wide cache[s], etc.).
         */
        peer_node = node;
        do
        {
            /* Find out what the !idle are in this node */
            cpumask_andnot(&workers, online, prv->idlers);
            cpumask_and(&workers, &workers, &node_to_cpumask(peer_node));
            __cpumask_clear_cpu(cpu, &workers);

            peer_cpu = cpumask_first(&workers);
            if ( peer_cpu >= nr_cpu_ids )
                goto next_node;
            do
            {
                /*
                 * Get ahold of the scheduler lock for this peer CPU.
                 *
                 * Note: We don't spin on this lock but simply try it. Spinning
                 * could cause a deadlock if the peer CPU is also load
                 * balancing and trying to lock this CPU.
                 */
                spinlock_t *lock = pcpu_schedule_trylock(peer_cpu);

Because of the fact that we did not clear Pool-1->cpu_valid online is
8-15. Also, since we _did_ clear bits 8-15 in prv->idlers when tearing
them down, during suspend, they're all (or all but 8) workers, as far as
the code above can tell.

We therefore enter the inner do{}while with, for instance (that's what
I've seen in my debugging), peer_cpu=9, but we've not yet done
cpu_schedule_up()-->alloc_pdata()-->etc. for that CPU, so we die at (or
shortly after) the end of the code snippet shown above.

> Is it something we should modify to be a BUG or ASSERT?
> 
Not sure how/where. Note that some more fixing of similar situations
happen in other patches in the series, and that includes also adding
ASSERT-s (although, no, they probably won't cover this case).

I can try to think at it and to come up with something if you think it's
important...

Thanks and Regards,
Dario

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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