x86/resctrl: Don't move a task to the same resource group

Shakeel Butt reported in [1] that a user can request a task to be moved
to a resource group even if the task is already in the group. It just
wastes time to do the move operation which could be costly to send IPI
to a different CPU.

Add a sanity check to ensure that the move operation only happens when
the task is not already in the resource group.

[1] https://lore.kernel.org/lkml/CALvZod7E9zzHwenzf7objzGKsdBmVwTgEJ0nPgs0LUFU3SN5Pw@mail.gmail.com/

Fixes: e02737d ("x86/intel_rdt: Add tasks files")
Reported-by: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/962ede65d8e95be793cb61102cca37f7bb018e66.1608243147.git.reinette.chatre@intel.com

Author: Fenghua Yu

arch/x86/kernel/cpu/resctrl/rdtgroup.c

@@ -546,6 +546,13 @@ static void update_task_closid_rmid(struct task_struct *t)
 static int __rdtgroup_move_task(struct task_struct *tsk,
 				struct rdtgroup *rdtgrp)
 {
+	/* If the task is already in rdtgrp, no need to move the task. */
+	if ((rdtgrp->type == RDTCTRL_GROUP && tsk->closid == rdtgrp->closid &&
+	     tsk->rmid == rdtgrp->mon.rmid) ||
+	    (rdtgrp->type == RDTMON_GROUP && tsk->rmid == rdtgrp->mon.rmid &&
+	     tsk->closid == rdtgrp->mon.parent->closid))
+		return 0;
+
 	/*
 	 * Set the task's closid/rmid before the PQR_ASSOC MSR can be
 	 * updated by them.