[Xen-devel] [PATCH] xen: credit1: fix tickling when it happens from a remote pCPU

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

especially if that is also from a different cpupool than the
processor of the vCPU that triggered the tickling.

In fact, it is possible that we get as far as calling vcpu_unblock()-->
vcpu_wake()-->csched_vcpu_wake()-->__runq_tickle() for the vCPU 'vc',
but all while running on a pCPU that is different from 'vc->processor'.

For instance, this can happen when an HVM domain runs in a cpupool,
with a different scheduler than the default one, and issues IOREQs
to Dom0, running in Pool-0 with the default scheduler.
In fact, right in this case, the following crash can be observed:

(XEN) ----[ Xen-4.7-unstable  x86_64  debug=y  Tainted:    C ]----
(XEN) CPU:    7
(XEN) RIP:    e008:[<ffff82d0801230de>] __runq_tickle+0x18f/0x430
(XEN) RFLAGS: 0000000000010086   CONTEXT: hypervisor (d1v0)
(XEN) rax: 0000000000000001   rbx: ffff8303184fee00   rcx: 0000000000000000
(XEN) ... ... ...
(XEN) Xen stack trace from rsp=ffff83031fa57a08:
(XEN)    ffff82d0801fe664 ffff82d08033c820 0000000100000002 0000000a00000001
(XEN)    0000000000006831 0000000000000000 0000000000000000 0000000000000000
(XEN) ... ... ...
(XEN) Xen call trace:
(XEN)    [<ffff82d0801230de>] __runq_tickle+0x18f/0x430
(XEN)    [<ffff82d08012348a>] csched_vcpu_wake+0x10b/0x110
(XEN)    [<ffff82d08012b421>] vcpu_wake+0x20a/0x3ce
(XEN)    [<ffff82d08012b91c>] vcpu_unblock+0x4b/0x4e
(XEN)    [<ffff82d080167bd0>] vcpu_kick+0x17/0x61
(XEN)    [<ffff82d080167c46>] vcpu_mark_events_pending+0x2c/0x2f
(XEN)    [<ffff82d08010ac35>] evtchn_fifo_set_pending+0x381/0x3f6
(XEN)    [<ffff82d08010a0f6>] notify_via_xen_event_channel+0xc9/0xd6
(XEN)    [<ffff82d0801c29ed>] hvm_send_ioreq+0x3e9/0x441
(XEN)    [<ffff82d0801bba7d>] hvmemul_do_io+0x23f/0x2d2
(XEN)    [<ffff82d0801bbb43>] hvmemul_do_io_buffer+0x33/0x64
(XEN)    [<ffff82d0801bc92b>] hvmemul_do_pio_buffer+0x35/0x37
(XEN)    [<ffff82d0801cc49f>] handle_pio+0x58/0x14c
(XEN)    [<ffff82d0801eabcb>] vmx_vmexit_handler+0x16b3/0x1bea
(XEN)    [<ffff82d0801efd21>] vmx_asm_vmexit_handler+0x41/0xc0

In this case, pCPU 7 is not in Pool-0, while the (Dom0's) vCPU being
woken is. pCPU's 7 pool has a different scheduler than credit, but it
is, however, right from pCPU 7 that we are waking the Dom0's vCPUs.
Therefore, the current code tries to access csched_balance_mask for
pCPU 7, but that is not defined, and hence the Oops.

(Note that, in case the two pools run the same scheduler we see no
Oops, but things are still conceptually wrong.)

Cure things by providing a second macro allowing to fetch the scratch
mask of a specific pCPU (instead than always using smp_processor_id()),
and use that one in __runq_tickle(), with such pCPU equal to the
processor of the vCPU being woken.

Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
---
Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
---
 xen/common/sched_credit.c |   15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index a1945ac..f58c3d9 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -168,14 +168,16 @@ struct csched_pcpu {
 };
 
 /*
- * Convenience macro for accessing the per-PCPU cpumask we need for
+ * Convenience macros for accessing the per-PCPU cpumask we need for
  * implementing the two steps (soft and hard affinity) balancing logic.
  * It is stored in csched_pcpu so that serialization is not an issue,
- * as there is a csched_pcpu for each PCPU and we always hold the
- * runqueue spin-lock when using this.
+ * as there is a csched_pcpu for each PCPU, and we always hold the
+ * runqueue lock for the proper PCPU when using these.
  */
 #define csched_balance_mask (CSCHED_PCPU(smp_processor_id())->balance_mask)
 
+#define csched_balance_mask_cpu(c) (CSCHED_PCPU(c)->balance_mask)
+
 /*
  * Virtual CPU
  */
@@ -412,9 +414,10 @@ __runq_tickle(unsigned int cpu, struct csched_vcpu *new)
 
             /* Are there idlers suitable for new (for this balance step)? */
             csched_balance_cpumask(new->vcpu, balance_step,
-                                   csched_balance_mask);
-            cpumask_and(csched_balance_mask, csched_balance_mask, &idle_mask);
-            new_idlers_empty = cpumask_empty(csched_balance_mask);
+                                   csched_balance_mask_cpu(cpu));
+            cpumask_and(csched_balance_mask_cpu(cpu),
+                        csched_balance_mask_cpu(cpu), &idle_mask);
+            new_idlers_empty = cpumask_empty(csched_balance_mask_cpu(cpu));
 
             /*
              * Let's not be too harsh! If there aren't idlers suitable


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