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x86/resctrl: Use task_curr() instead of task_struct->on_cpu to prevent unnecessary IPI

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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>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/e9e68ce1441a73401e08b641cc3b9a3cf13fe6d4.1608243147.git.reinette.chatre@intel.com
This commit is contained in:
Reinette Chatre 2020-12-17 14:31:20 -08:00 committed by Borislav Petkov
parent 3ff4ec0e28
commit e0ad6dc896
1 changed files with 5 additions and 9 deletions
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14
arch/x86/kernel/cpu/resctrl/rdtgroup.c
View file

@ -2313,19 +2313,15 @@ 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);