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Re: [Xen-devel] Live-Patch application failure in core-scheduling mode

On 07.02.20 09:23, Jan Beulich wrote:
On 07.02.2020 09:04, Jürgen Groß wrote:
On 06.02.20 15:02, Sergey Dyasli wrote:
On 06/02/2020 11:05, Sergey Dyasli wrote:
On 06/02/2020 09:57, Jürgen Groß wrote:
On 05.02.20 17:03, Sergey Dyasli wrote:

I'm currently investigating a Live-Patch application failure in core-
scheduling mode and this is an example of what I usually get:
(it's easily reproducible)

       (XEN) [  342.528305] livepatch: lp: CPU8 - IPIing the other 15 CPUs
       (XEN) [  342.558340] livepatch: lp: Timed out on semaphore in CPU 
quiesce phase 13/15
       (XEN) [  342.558343] bad cpus: 6 9

       (XEN) [  342.559293] CPU:    6
       (XEN) [  342.559562] Xen call trace:
       (XEN) [  342.559565]    [<ffff82d08023f304>] R 
       (XEN) [  342.559568]    [<ffff82d08023f8aa>] F 
       (XEN) [  342.559571]    [<ffff82d080240d5a>] F 
       (XEN) [  342.559574]    [<ffff82d080278ec5>] F 

       (XEN) [  342.559761] CPU:    9
       (XEN) [  342.560026] Xen call trace:
       (XEN) [  342.560029]    [<ffff82d080241661>] R _spin_lock_irq+0x11/0x40
       (XEN) [  342.560032]    [<ffff82d08023f323>] F 
       (XEN) [  342.560036]    [<ffff82d08023f8aa>] F 
       (XEN) [  342.560039]    [<ffff82d080240d5a>] F 
       (XEN) [  342.560042]    [<ffff82d080279db5>] F 

The first HT sibling is waiting for the second in the LP-application
context while the second waits for the first in the scheduler context.

Any suggestions on how to improve this situation are welcome.

Can you test the attached patch, please? It is only tested to boot, so
I did no livepatch tests with it.

Thank you for the patch! It seems to fix the issue in my manual testing.
I'm going to submit automatic LP testing for both thread/core modes.

Andrew suggested to test late ucode loading as well and so I did.
It uses stop_machine() to rendezvous cpus and it failed with a similar
backtrace for a problematic CPU. But in this case the system crashed
since there is no timeout involved:

      (XEN) [  155.025168] Xen call trace:
      (XEN) [  155.040095]    [<ffff82d0802417f2>] R _spin_unlock_irq+0x22/0x30
      (XEN) [  155.069549]    [<ffff82d08023f3c2>] S 
      (XEN) [  155.109696]    [<ffff82d08023f728>] F 
      (XEN) [  155.145521]    [<ffff82d080240e1a>] F 
      (XEN) [  155.180223]    [<ffff82d0803716f6>] F 

It looks like your patch provides a workaround for LP case, but other
cases like stop_machine() remain broken since the underlying issue with
the scheduler is still there.

And here is the fix for ucode loading (that was in fact the only case
where stop_machine_run() wasn't already called in a tasklet).

This is a rather odd restriction, and hence will need explaining.

stop_machine_run() is using a tasklet on each online cpu (excluding the
one it was called one) for doing a rendezvous of all cpus. With tasklets
always being executed on idle vcpus it is mandatory for
stop_machine_run() to be called on an idle vcpu as well when core
scheduling is active, as otherwise a deadlock will occur. This is being
accomplished by the use of continue_hypercall_on_cpu().

Without it being entirely clear that there's no alternative to
it, I don't think I'd be fine with re-introduction of
continue_hypercall_on_cpu(0, ...) into ucode loading.

I don't see a viable alternative. As the hypercall needs to wait until
the loading has been performed for being able to report the result I
can't see how this can be done else.

Also two remarks on the patch itself: struct ucode_buf's len
field can be unsigned int, seeing the very first check done in
microcode_update(). And instead of xmalloc_bytes() please see
whether you can make use of xmalloc_flex_struct() there.

Both are fine with me.


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