linux-kernel - [PATCH V2 2/4] x86/resctrl: Don't move a task to the same resource group

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Date:   Thu, 17 Dec 2020 14:31:19 -0800
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     tglx@...utronix.de, fenghua.yu@...el.com, bp@...en8.de,
        tony.luck@...el.com
Cc:     kuo-lang.tseng@...el.com, shakeelb@...gle.com,
        valentin.schneider@....com, mingo@...hat.com, babu.moger@....com,
        james.morse@....com, hpa@...or.com, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Reinette Chatre <reinette.chatre@...el.com>,
        stable@...r.kernel.org
Subject: [PATCH V2 2/4] x86/resctrl: Don't move a task to the same resource group

From: Fenghua Yu <fenghua.yu@...el.com>

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: e02737d5b826 ("x86/intel_rdt: Add tasks files")
Reported-by: Shakeel Butt <shakeelb@...gle.com>
Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
Signed-off-by: Reinette Chatre <reinette.chatre@...el.com>
Reviewed-by: Tony Luck <tony.luck@...el.com>
Cc: stable@...r.kernel.org
---
V1->V2:
* No changes

 arch/x86/kernel/cpu/resctrl/rdtgroup.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index c5937a12d731..4042e1eb4f5d 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -552,6 +552,13 @@ 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.
-- 
2.26.2