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x86/resctrl: Fix task CLOSID/RMID update race

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[ Upstream commit fe1f0714385fbcf76b0cbceb02b7277d842014fc ]

When the user moves a running task to a new rdtgroup using the task's
file interface or by deleting its rdtgroup, the resulting change in
CLOSID/RMID must be immediately propagated to the PQR_ASSOC MSR on the
task(s) CPUs.

x86 allows reordering loads with prior stores, so if the task starts
running between a task_curr() check that the CPU hoisted before the
stores in the CLOSID/RMID update then it can start running with the old
CLOSID/RMID until it is switched again because __rdtgroup_move_task()
failed to determine that it needs to be interrupted to obtain the new
CLOSID/RMID.

Refer to the diagram below:

CPU 0                                   CPU 1
-----                                   -----
__rdtgroup_move_task():
  curr <- t1->cpu->rq->curr
                                        __schedule():
                                          rq->curr <- t1
                                        resctrl_sched_in():
                                          t1->{closid,rmid} -> {1,1}
  t1->{closid,rmid} <- {2,2}
  if (curr == t1) // false
   IPI(t1->cpu)

A similar race impacts rdt_move_group_tasks(), which updates tasks in a
deleted rdtgroup.

In both cases, use smp_mb() to order the task_struct::{closid,rmid}
stores before the loads in task_curr().  In particular, in the
rdt_move_group_tasks() case, simply execute an smp_mb() on every
iteration with a matching task.

It is possible to use a single smp_mb() in rdt_move_group_tasks(), but
this would require two passes and a means of remembering which
task_structs were updated in the first loop. However, benchmarking
results below showed too little performance impact in the simple
approach to justify implementing the two-pass approach.

Times below were collected using `perf stat` to measure the time to
remove a group containing a 1600-task, parallel workload.

CPU: Intel(R) Xeon(R) Platinum P-8136 CPU @ 2.00GHz (112 threads)

  # mkdir /sys/fs/resctrl/test
  # echo $$ > /sys/fs/resctrl/test/tasks
  # perf bench sched messaging -g 40 -l 100000

task-clock time ranges collected using:

  # perf stat rmdir /sys/fs/resctrl/test

Baseline:                     1.54 - 1.60 ms
smp_mb() every matching task: 1.57 - 1.67 ms

  [ bp: Massage commit message. ]

Fixes: ae28d1aae48a ("x86/resctrl: Use an IPI instead of task_work_add() to update PQR_ASSOC MSR")
Fixes: 0efc89be94 ("x86/intel_rdt: Update task closid immediately on CPU in rmdir and unmount")
Signed-off-by: Peter Newman <peternewman@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Babu Moger <babu.moger@amd.com>
Cc: <stable@kernel.org>
Link: https://lore.kernel.org/r/20221220161123.432120-1-peternewman@google.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Peter Newman 2022-12-20 17:11:23 +01:00 committed by Greg Kroah-Hartman
parent 22c4eeafc3
commit 3030f11f27
1 changed files with 11 additions and 1 deletions
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12
arch/x86/kernel/cpu/resctrl/rdtgroup.c
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@ -577,8 +577,10 @@ static int __rdtgroup_move_task(struct task_struct *tsk,
/*
* Ensure the task's closid and rmid are written before determining if
* the task is current that will decide if it will be interrupted.
* This pairs with the full barrier between the rq->curr update and
* resctrl_sched_in() during context switch.
*/
barrier();
smp_mb();
/*
* By now, the task's closid and rmid are set. If the task is current
@ -2178,6 +2180,14 @@ static void rdt_move_group_tasks(struct rdtgroup *from, struct rdtgroup *to,
t->closid = to->closid;
t->rmid = to->mon.rmid;
/*
* Order the closid/rmid stores above before the loads
* in task_curr(). This pairs with the full barrier
* between the rq->curr update and resctrl_sched_in()
* during context switch.
*/
smp_mb();
/*
* If the task is on a CPU, set the CPU in the mask.
* The detection is inaccurate as tasks might move or