summaryrefslogtreecommitdiffstats
path: root/kernel/sched/core.c
diff options
context:
space:
mode:
authorMathieu Desnoyers <mathieu.desnoyers@efficios.com>2017-07-28 16:40:40 -0400
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2017-08-17 07:28:05 -0700
commit22e4ebb975822833b083533035233d128b30e98f (patch)
tree973b3dda0e0b79af883482f66b49da2dd74225f1 /kernel/sched/core.c
parent955dbdf4ce87fd9be4bc8378e26b8c2eb8b3d184 (diff)
downloadlinux-22e4ebb975822833b083533035233d128b30e98f.tar.bz2
membarrier: Provide expedited private command
Implement MEMBARRIER_CMD_PRIVATE_EXPEDITED with IPIs using cpumask built from all runqueues for which current thread's mm is the same as the thread calling sys_membarrier. It executes faster than the non-expedited variant (no blocking). It also works on NOHZ_FULL configurations. Scheduler-wise, it requires a memory barrier before and after context switching between processes (which have different mm). The memory barrier before context switch is already present. For the barrier after context switch: * Our TSO archs can do RELEASE without being a full barrier. Look at x86 spin_unlock() being a regular STORE for example. But for those archs, all atomics imply smp_mb and all of them have atomic ops in switch_mm() for mm_cpumask(), and on x86 the CR3 load acts as a full barrier. * From all weakly ordered machines, only ARM64 and PPC can do RELEASE, the rest does indeed do smp_mb(), so there the spin_unlock() is a full barrier and we're good. * ARM64 has a very heavy barrier in switch_to(), which suffices. * PPC just removed its barrier from switch_to(), but appears to be talking about adding something to switch_mm(). So add a smp_mb__after_unlock_lock() for now, until this is settled on the PPC side. Changes since v3: - Properly document the memory barriers provided by each architecture. Changes since v2: - Address comments from Peter Zijlstra, - Add smp_mb__after_unlock_lock() after finish_lock_switch() in finish_task_switch() to add the memory barrier we need after storing to rq->curr. This is much simpler than the previous approach relying on atomic_dec_and_test() in mmdrop(), which actually added a memory barrier in the common case of switching between userspace processes. - Return -EINVAL when MEMBARRIER_CMD_SHARED is used on a nohz_full kernel, rather than having the whole membarrier system call returning -ENOSYS. Indeed, CMD_PRIVATE_EXPEDITED is compatible with nohz_full. Adapt the CMD_QUERY mask accordingly. Changes since v1: - move membarrier code under kernel/sched/ because it uses the scheduler runqueue, - only add the barrier when we switch from a kernel thread. The case where we switch from a user-space thread is already handled by the atomic_dec_and_test() in mmdrop(). - add a comment to mmdrop() documenting the requirement on the implicit memory barrier. CC: Peter Zijlstra <peterz@infradead.org> CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com> CC: Boqun Feng <boqun.feng@gmail.com> CC: Andrew Hunter <ahh@google.com> CC: Maged Michael <maged.michael@gmail.com> CC: gromer@google.com CC: Avi Kivity <avi@scylladb.com> CC: Benjamin Herrenschmidt <benh@kernel.crashing.org> CC: Paul Mackerras <paulus@samba.org> CC: Michael Ellerman <mpe@ellerman.id.au> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Dave Watson <davejwatson@fb.com>
Diffstat (limited to 'kernel/sched/core.c')
-rw-r--r--kernel/sched/core.c25
1 files changed, 25 insertions, 0 deletions
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bfee6ea7db49..f77269c6c2f8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2640,6 +2640,16 @@ static struct rq *finish_task_switch(struct task_struct *prev)
prev_state = prev->state;
vtime_task_switch(prev);
perf_event_task_sched_in(prev, current);
+ /*
+ * The membarrier system call requires a full memory barrier
+ * after storing to rq->curr, before going back to user-space.
+ *
+ * TODO: This smp_mb__after_unlock_lock can go away if PPC end
+ * up adding a full barrier to switch_mm(), or we should figure
+ * out if a smp_mb__after_unlock_lock is really the proper API
+ * to use.
+ */
+ smp_mb__after_unlock_lock();
finish_lock_switch(rq, prev);
finish_arch_post_lock_switch();
@@ -3329,6 +3339,21 @@ static void __sched notrace __schedule(bool preempt)
if (likely(prev != next)) {
rq->nr_switches++;
rq->curr = next;
+ /*
+ * The membarrier system call requires each architecture
+ * to have a full memory barrier after updating
+ * rq->curr, before returning to user-space. For TSO
+ * (e.g. x86), the architecture must provide its own
+ * barrier in switch_mm(). For weakly ordered machines
+ * for which spin_unlock() acts as a full memory
+ * barrier, finish_lock_switch() in common code takes
+ * care of this barrier. For weakly ordered machines for
+ * which spin_unlock() acts as a RELEASE barrier (only
+ * arm64 and PowerPC), arm64 has a full barrier in
+ * switch_to(), and PowerPC has
+ * smp_mb__after_unlock_lock() before
+ * finish_lock_switch().
+ */
++*switch_count;
trace_sched_switch(preempt, prev, next);