[Xen-devel] [RFC/RFT][PATCH 2 of 3] Remove VCPU pausing while adjusting domain scheduling parameters.

[Dario Faggioli <raistlin@xxxxxxxx>]

Pausing all the non-current VCPUs of a domain while changing its
scheduling parameters is dangerous (what if two VCPUs try to pause
all the other VCPUs but themselves at the same time?). Moreover, it
does not appear to be needed, since:
 - races are (or should be) avoided within each pluggable scheduler
   by means of proper locking;
 - changes in values using during actual scheduling are (or should be)
   prevented by runq locking.

Just remove it.

Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>

diff -r 6b5e2eb81706 xen/common/schedule.c
--- a/xen/common/schedule.c     Wed Nov 23 15:09:14 2011 +0100
+++ b/xen/common/schedule.c     Wed Nov 23 15:20:28 2011 +0100
@@ -1005,7 +1005,6 @@ int sched_id(void)
 /* Adjust scheduling parameter for a given domain. */
 long sched_adjust(struct domain *d, struct xen_domctl_scheduler_op *op)
 {
-    struct vcpu *v;
     long ret;
     
     if ( (op->sched_id != DOM2OP(d)->sched_id) ||
@@ -1013,27 +1012,11 @@ long sched_adjust(struct domain *d, stru
           (op->cmd != XEN_DOMCTL_SCHEDOP_getinfo)) )
         return -EINVAL;
 
-    /*
-     * Most VCPUs we can simply pause. If we are adjusting this VCPU then
-     * concurrent updates shall be prevented within the actual pluggable
-     * scheduler.
-     */
-
-    for_each_vcpu ( d, v )
-    {
-        if ( v != current )
-            vcpu_pause(v);
-    }
-
+    /* NB: the pluggable scheduler code needs to take care
+     * of locking by itself. */
     if ( (ret = SCHED_OP(DOM2OP(d), adjust, d, op)) == 0 )
         TRACE_1D(TRC_SCHED_ADJDOM, d->domain_id);
 
-    for_each_vcpu ( d, v )
-    {
-        if ( v != current )
-            vcpu_unpause(v);
-    }
-
     return ret;
 }
 

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-------------------------------------------------------------------
Dario Faggioli, http://retis.sssup.it/people/faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)
PhD Candidate, ReTiS Lab, Scuola Superiore Sant'Anna, Pisa (Italy)

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