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Subject[PATCH V2 3/4] x86/resctrl: Use task_curr() instead of task_struct->on_cpu to prevent unnecessary IPI
Date
James reported in [1] that there could be two tasks running on the same CPU
with task_struct->on_cpu set. Using task_struct->on_cpu as a test if a task
is running on a CPU may thus match the old task for a CPU while the
scheduler is running and IPI it unnecessarily.

task_curr() is the correct helper to use. While doing so move the #ifdef
check of the CONFIG_SMP symbol to be a C conditional used to determine
if this helper should be used to ensure the code is always checked for
correctness by the compiler.

[1] https://lore.kernel.org/lkml/a782d2f3-d2f6-795f-f4b1-9462205fd581@arm.com

Reported-by: James Morse <james.morse@arm.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
---
V1->V2:
* New patch in series

arch/x86/kernel/cpu/resctrl/rdtgroup.c | 14 +++++---------
1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index 4042e1eb4f5d..9bd36210d220 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -2319,19 +2319,15 @@ static void rdt_move_group_tasks(struct rdtgroup *from, struct rdtgroup *to,
t->closid = to->closid;
t->rmid = to->mon.rmid;

-#ifdef CONFIG_SMP
/*
- * This is safe on x86 w/o barriers as the ordering
- * of writing to task_cpu() and t->on_cpu is
- * reverse to the reading here. The detection is
- * inaccurate as tasks might move or schedule
- * before the smp function call takes place. In
- * such a case the function call is pointless, but
+ * If the task is on a CPU, set the CPU in the mask.
+ * The detection is inaccurate as tasks might move or
+ * schedule before the smp function call takes place.
+ * In such a case the function call is pointless, but
* there is no other side effect.
*/
- if (mask && t->on_cpu)
+ if (IS_ENABLED(CONFIG_SMP) && mask && task_curr(t))
cpumask_set_cpu(task_cpu(t), mask);
-#endif
}
}
read_unlock(&tasklist_lock);
--
2.26.2
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