On Wed, Sep 22, 2021 at 01:05:12PM +0200, Peter Zijlstra wrote:
Use rcu_user_{enter,exit}() calls to provide SMP ordering on context
tracking state stores:
__context_tracking_exit()
__this_cpu_write(context_tracking.state, CONTEXT_KERNEL)
rcu_user_exit()
rcu_eqs_exit()
rcu_dynticks_eqs_eit()
rcu_dynticks_inc()
atomic_add_return() /* smp_mb */
__context_tracking_enter()
rcu_user_enter()
rcu_eqs_enter()
rcu_dynticks_eqs_enter()
rcu_dynticks_inc()
atomic_add_return() /* smp_mb */
__this_cpu_write(context_tracking.state, state)
This separates USER/KERNEL state with an smp_mb() on each side,
therefore, a user of context_tracking_state_cpu() can say the CPU must
pass through an smp_mb() before changing.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
For the transformation to negative errno return value and name change
from an RCU perspective:
Acked-by: Paul E. McKenney <paulmck@kernel.org>
For the sampling of nohz_full userspace state:
Another approach is for the rcu_data structure's ->dynticks variable to
use the lower two bits to differentiate between idle, nohz_full userspace
and kernel. In theory, inlining should make this zero cost for idle
transition, and should allow you to safely sample nohz_full userspace
state with a load and a couple of memory barriers instead of an IPI.
To make this work nicely, the low-order bits have to be 00 for kernel,
and (say) 01 for idle and 10 for nohz_full userspace. 11 would be an
error.
The trick would be for rcu_user_enter() and rcu_user_exit() to atomically
increment ->dynticks by 2, for rcu_nmi_exit() to increment by 1 and
rcu_nmi_enter() to increment by 3. The state sampling would need to
change accordingly.
Does this make sense, or am I missing something?
Thanx, Paul
quoted hunk ↗ jump to hunk
---
include/linux/context_tracking_state.h | 12 ++++++++++++
kernel/context_tracking.c | 7 ++++---
2 files changed, 16 insertions(+), 3 deletions(-)
--- a/include/linux/context_tracking_state.h
+++ b/include/linux/context_tracking_state.h
@@ -45,11 +45,23 @@ static __always_inline bool context_trac
{
return __this_cpu_read(context_tracking.state) == CONTEXT_USER;
}
+
+static __always_inline bool context_tracking_state_cpu(int cpu)
+{
+ struct context_tracking *ct = per_cpu_ptr(&context_tracking);
+
+ if (!context_tracking_enabled() || !ct->active)
+ return CONTEXT_DISABLED;
+
+ return ct->state;
+}
+
#else
static inline bool context_tracking_in_user(void) { return false; }
static inline bool context_tracking_enabled(void) { return false; }
static inline bool context_tracking_enabled_cpu(int cpu) { return false; }
static inline bool context_tracking_enabled_this_cpu(void) { return false; }
+static inline bool context_tracking_state_cpu(int cpu) { return CONTEXT_DISABLED; }
#endif /* CONFIG_CONTEXT_TRACKING */
#endif
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c@@ -82,7 +82,7 @@ void noinstr __context_tracking_enter(en
vtime_user_enter(current);
instrumentation_end();
}
- rcu_user_enter();
+ rcu_user_enter(); /* smp_mb */
}
/*
* Even if context tracking is disabled on this CPU, because it's outside
@@ -149,12 +149,14 @@ void noinstr __context_tracking_exit(enu
return;
if (__this_cpu_read(context_tracking.state) == state) {
+ __this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
+
if (__this_cpu_read(context_tracking.active)) {
/*
* We are going to run code that may use RCU. Inform
* RCU core about that (ie: we may need the tick again).
*/
- rcu_user_exit();
+ rcu_user_exit(); /* smp_mb */
if (state == CONTEXT_USER) {
instrumentation_begin();
vtime_user_exit(current);@@ -162,7 +164,6 @@ void noinstr __context_tracking_exit(enu
instrumentation_end();
}
}
- __this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
}
context_tracking_recursion_exit();
}