[PATCH 19/19] context_tracking: Exempt CONFIG_HAVE_CONTEXT_TRACKING_USER_OFFSTACK from non-active tracking

[Frederic Weisbecker]

Since a CPU may save the state of the context tracking using
exception_enter() before calling into schedule(), we need all CPUs in
the system to track user <-> kernel transitions and not just those that
really need it (nohz_full CPUs).

The following illustrates the issue that could otherwise happen:

     CPU 0 (not tracking)                       CPU 1 (tracking)
     -------------------                       --------------------
     // we are past user_enter()
     // but this CPU is always in
     // CONTEXT_KERNEL
     // because it doesn't track user <-> kernel

     ctx = exception_enter(); //ctx == CONTEXT_KERNEL
     schedule();
     ===========================================>
                                               return from schedule();
                                               exception_exit(ctx);
                                               //go to user in CONTEXT_KERNEL

However CONFIG_HAVE_CONTEXT_TRACKING_USER_OFFSTACK doesn't play those
games because schedule() can't be called between user_enter() and
user_exit() under such config. In this situation we can spare context
tracking on the CPUs that don't need it.

Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
Cc: Uladzislau Rezki <uladzislau.rezki@xxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Nicolas Saenz Julienne <nsaenz@xxxxxxxxxx>
Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
Cc: Xiongfeng Wang <wangxiongfeng2@xxxxxxxxxx>
Cc: Yu Liao<liaoyu15@xxxxxxxxxx>
Cc: Phil Auld <pauld@xxxxxxxxxx>
Cc: Paul Gortmaker<paul.gortmaker@xxxxxxxxxxxxx>
Cc: Alex Belits <abelits@xxxxxxxxxxx>
---
 kernel/context_tracking.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index 87e7b748791c..b1934264f77f 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -374,7 +374,7 @@ void noinstr __ct_user_enter(enum ctx_state state)
 			 * when the CPU runs in userspace.
 			 */
 			ct_kernel_exit(true, RCU_DYNTICKS_IDX + state);
-		} else {
+		} else if (!IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_USER_OFFSTACK)) {
 			/*
 			 * Even if context tracking is disabled on this CPU, because it's outside
 			 * the full dynticks mask for example, we still have to keep track of the
@@ -384,7 +384,8 @@ void noinstr __ct_user_enter(enum ctx_state state)
 			 * handler and then migrate to another CPU, that new CPU must know where
 			 * the exception returns by the time we call exception_exit().
 			 * This information can only be provided by the previous CPU when it called
-			 * exception_enter().
+			 * exception_enter(). CONFIG_HAVE_CONTEXT_TRACKING_USER_OFFSTACK is
+			 * excused though because it doesn't use exception_enter().
 			 * OTOH we can spare the calls to vtime and RCU when context_tracking.active
 			 * is false because we know that CPU is not tickless.
 			 */
@@ -460,7 +461,7 @@ void noinstr __ct_user_exit(enum ctx_state state)
 				trace_user_exit(0);
 				instrumentation_end();
 			}
-		} else {
+		} else if (!IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_USER_OFFSTACK)) {
 			atomic_sub(state, &ct->state);
 		}
 	}
-- 
2.25.1