Hi James,
Thank you very much for your review.
On 12/9/2020 8:47 AM, James Morse wrote:
Hi Reinette, Fenghua,
On 03/12/2020 23:25, Reinette Chatre wrote:
From: Fenghua Yu fenghua.yu@intel.com
The code of setting the CPU on which a task is running in a CPU mask is moved into a couple of helpers. The new helper task_on_cpu() will be reused shortly.
diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c index 6f4ca4bea625..68db7d2dec8f 100644 --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c @@ -525,6 +525,38 @@ static void rdtgroup_remove(struct rdtgroup *rdtgrp)
+#ifdef CONFIG_SMP
(using IS_ENABLED(CONFIG_SMP) lets the compiler check all the code in one go, then dead-code-remove the stuff that will never happen... its also easier on the eye!)
Thank you for catching this. New fix (see below) uses this.
+/* Get the CPU if the task is on it. */ +static bool task_on_cpu(struct task_struct *t, int *cpu) +{
- /*
* 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 there is no other side effect.*/
- if (t->on_cpu) {
kernel/sched/core.c calls out that there can be two tasks on one CPU with this set. (grep astute) I think that means this series will falsely match the old task for a CPU while the scheduler is running, and IPI it unnecessarily.
task_curr() is the helper that knows not to do this.
Thank you very much for catching this. I did not know this. This exposes an issue with the current implementation of moving tasks as part of directory removal. I now plan to replace this patch with a new fix to address this new issue you exposed: the fix will replace the current usage of t->on_cpu with task_curr(). Since I also follow your suggestion for patch #2 this original patch is no longer needed, which is something Borislav also suggested but I could not see a way to do it at the time.
This new fix does seem to fall into the "This could be a problem.." category of issues referred to in stable-kernel-rules.rst so while I plan on adding a Fixes tag I plan to not cc the stable team on this one. I am unsure about the right thing to do here so if you have an opinion I would appreciate it.
What do you think of this replacement patch (that will be moved to end of series)?
Reinette
----8<------ x86/resctrl: Replace t->on_cpu with task_curr() to prevent unnecessary IPI
James reported in [1] that there could be two tasks running on the same CPU with t->on_cpu set. Using t->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 so ensure the code is always checked for correctness by the compiler.
[1] https://lore.kernel.org/lkml/a782d2f3-d2f6-795f-f4b1-9462205fd581@arm.com
Fixes: 0efc89be9471 ("x86/intel_rdt: Update task closid immediately on CPU in rmdir and unmount") Reported-by: James Morse james.morse@arm.com Signed-off-by: Reinette Chatre reinette.chatre@intel.com --- arch/x86/kernel/cpu/resctrl/rdtgroup.c | 14 ++------------ 1 file changed, 2 insertions(+), 12 deletions(-)
diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c index 5e5a49f38fa1..c64fb37f0aec 100644 --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c @@ -2317,19 +2317,9 @@ 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 - * there is no other side effect. - */ - if (mask && t->on_cpu) + /* If the task is on a CPU, set the CPU in the mask. */ + if (IS_ENABLED(CONFIG_SMP) && mask && task_curr(t)) cpumask_set_cpu(task_cpu(t), mask); -#endif } } read_unlock(&tasklist_lock);