[PATCH] perf/x86/amd/uncore: Avoid context migration from non-owners

[Sandipan Das]

Each uncore context is owned by a single CPU that hosts all events for
the corresponding uncore domain and is advertised through the sysfs
cpumask attribute. When a CPU goes offline, the context is migrated to
an online sibling if the departing CPU is the owner.

Before commit d6389d3ccc13 ("perf/x86/amd/uncore: Refactor uncore
management"), uncore_down_prepare() checked whether the CPU going
offline owned the context. The check was lost when the function was
replaced by amd_uncore_ctx_move(). Hence, when a CPU that is not the
owner goes offline, ownership is always handed over to the first online
sibling in the uncore domain irrespective of whether another sibling
already owns it. This goes unnoticed as long as the owner is the
lowest-numbered online CPU in the domain, since the first online
sibling is then the owner itself. Once ownership has moved, that no
longer holds.

For example, consider an L3 domain spanning CPUs 8-15,136-143 and owned
by CPU 8.

  $ cat /sys/bus/event_source/devices/amd_l3/cpumask
  0,8,16,24,32,40,48,56,64,72,80,88,96,104,112,120

  $ echo 0 | sudo tee /sys/devices/system/cpu/cpu8/online
  $ cat /sys/bus/event_source/devices/amd_l3/cpumask
  0,9,16,24,32,40,48,56,64,72,80,88,96,104,112,120

  $ echo 1 | sudo tee /sys/devices/system/cpu/cpu8/online
  $ cat /sys/bus/event_source/devices/amd_l3/cpumask
  0,9,16,24,32,40,48,56,64,72,80,88,96,104,112,120

  $ echo 0 | sudo tee /sys/devices/system/cpu/cpu10/online
  $ cat /sys/bus/event_source/devices/amd_l3/cpumask
  0,8-9,16,24,32,40,48,56,64,72,80,88,96,104,112,120

Offlining CPU 8 correctly moves ownership to CPU 9 and onlining it
again leaves CPU 9 as the owner. However, offlining CPU 10 then makes
CPU 8 the owner while existing events remain on CPU 9. Both CPUs are
now advertised for the same domain, so tools that open an event on
every CPU in the cpumask count the domain twice, and the context gets
modified from two CPUs without any synchronization.

Restore the ownership check that was lost during the refactor.

Fixes: d6389d3ccc13 ("perf/x86/amd/uncore: Refactor uncore management")
Signed-off-by: Sandipan Das <sandipan.das@xxxxxxx>
---
 arch/x86/events/amd/uncore.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/events/amd/uncore.c b/arch/x86/events/amd/uncore.c
index 7aa5a5b652a2..c307323f6390 100644
--- a/arch/x86/events/amd/uncore.c
+++ b/arch/x86/events/amd/uncore.c
@@ -595,7 +595,7 @@ static void amd_uncore_ctx_move(struct amd_uncore *uncore, unsigned int cpu)
 	for (i = 0; i < uncore->num_pmus; i++) {
 		pmu = &uncore->pmus[i];
 		curr = *per_cpu_ptr(pmu->ctx, cpu);
-		if (!curr)
+		if (!curr || curr->cpu != cpu)
 			continue;
 
 		/* Migrate to a shared sibling if possible */
-- 
2.53.0