From: Peter Newman peternewman@google.com
commit fe1f0714385fbcf76b0cbceb02b7277d842014fc upstream.
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: 0efc89be9471 ("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: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/kernel/cpu/resctrl/rdtgroup.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-)
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c @@ -580,8 +580,10 @@ static int __rdtgroup_move_task(struct t /* * 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 @@ -2364,6 +2366,14 @@ static void rdt_move_group_tasks(struct WRITE_ONCE(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 * schedule before the smp function call takes place.