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Subject[RFC PATCH v2] membarrier: expedited private command
Date
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.

Scheduler-wise, it requires that we add a memory barrier after context
switching between processes (which have different mm). Interestingly,
there is already a memory barrier in mmdrop(), so we only need to add
a barrier when switching from a kernel thread to a userspace thread.
We also don't need to add the barrier when switching to a kernel thread,
because it has no userspace memory mapping, which makes ordering of
user-space memory accesses pretty much useless.

* Benchmark

A stress-test benchmark of sched pipe shows that it does not add
significant overhead to the scheduler switching between processes:

100 runs of:

taskset 01 ./perf bench sched pipe

Running 'sched/pipe' benchmark:
Executed 1000000 pipe operations between two processes

Hardware: CPU: Intel(R) Xeon(R) CPU E5-2630 v3 @ 2.40GHz

A) With 4.13.0-rc2+
at commit a97fb594bc7d ("virtio-net: fix module unloading")

avg.: 2.923 usecs/op
std.dev: 0.057 usecs/op

B) With this commit:

avg.: 2.916 usecs/op
std.dev: 0.043 usecs/op

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>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
MAINTAINERS | 2 +-
include/linux/sched/mm.h | 5 +++
include/uapi/linux/membarrier.h | 23 +++++++++++--
kernel/Makefile | 1 -
kernel/sched/Makefile | 1 +
kernel/sched/core.c | 27 ++++++++++++++++
kernel/{ => sched}/membarrier.c | 72 ++++++++++++++++++++++++++++++++++++++++-
7 files changed, 126 insertions(+), 5 deletions(-)
rename kernel/{ => sched}/membarrier.c (59%)

diff --git a/MAINTAINERS b/MAINTAINERS
index f66488dfdbc9..3b035584272f 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -8621,7 +8621,7 @@ M: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
M: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
L: linux-kernel@vger.kernel.org
S: Supported
-F: kernel/membarrier.c
+F: kernel/sched/membarrier.c
F: include/uapi/linux/membarrier.h

MEMORY MANAGEMENT
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 2b24a6974847..5c5384d9ae0f 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -38,6 +38,11 @@ static inline void mmgrab(struct mm_struct *mm)
extern void __mmdrop(struct mm_struct *);
static inline void mmdrop(struct mm_struct *mm)
{
+ /*
+ * Implicit full memory barrier provided by
+ * atomic_dec_and_test() is required by membarrier. See comments
+ * around membarrier_expedited_mb_after_set_current().
+ */
if (unlikely(atomic_dec_and_test(&mm->mm_count)))
__mmdrop(mm);
}
diff --git a/include/uapi/linux/membarrier.h b/include/uapi/linux/membarrier.h
index e0b108bd2624..6d47b3249d8a 100644
--- a/include/uapi/linux/membarrier.h
+++ b/include/uapi/linux/membarrier.h
@@ -40,14 +40,33 @@
* (non-running threads are de facto in such a
* state). This covers threads from all processes
* running on the system. This command returns 0.
+ * @MEMBARRIER_CMD_PRIVATE_EXPEDITED:
+ * Execute a memory barrier on each running
+ * thread belonging to the same process as the current
+ * thread. Upon return from system call, the
+ * caller thread is ensured that all its running
+ * threads siblings have passed through a state
+ * where all memory accesses to user-space
+ * addresses match program order between entry
+ * to and return from the system call
+ * (non-running threads are de facto in such a
+ * state). This only covers threads from the
+ * same processes as the caller thread. This
+ * command returns 0. The "expedited" commands
+ * complete faster than the non-expedited ones,
+ * they never block, but have the downside of
+ * causing extra overhead.
*
* Command to be passed to the membarrier system call. The commands need to
* be a single bit each, except for MEMBARRIER_CMD_QUERY which is assigned to
* the value 0.
*/
enum membarrier_cmd {
- MEMBARRIER_CMD_QUERY = 0,
- MEMBARRIER_CMD_SHARED = (1 << 0),
+ MEMBARRIER_CMD_QUERY = 0,
+ MEMBARRIER_CMD_SHARED = (1 << 0),
+ /* reserved for MEMBARRIER_CMD_SHARED_EXPEDITED (1 << 1) */
+ /* reserved for MEMBARRIER_CMD_PRIVATE (1 << 2) */
+ MEMBARRIER_CMD_PRIVATE_EXPEDITED = (1 << 3),
};

#endif /* _UAPI_LINUX_MEMBARRIER_H */
diff --git a/kernel/Makefile b/kernel/Makefile
index 4cb8e8b23c6e..9c323a6daa46 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -108,7 +108,6 @@ obj-$(CONFIG_CRASH_DUMP) += crash_dump.o
obj-$(CONFIG_JUMP_LABEL) += jump_label.o
obj-$(CONFIG_CONTEXT_TRACKING) += context_tracking.o
obj-$(CONFIG_TORTURE_TEST) += torture.o
-obj-$(CONFIG_MEMBARRIER) += membarrier.o

obj-$(CONFIG_HAS_IOMEM) += memremap.o

diff --git a/kernel/sched/Makefile b/kernel/sched/Makefile
index 53f0164ed362..78f54932ea1d 100644
--- a/kernel/sched/Makefile
+++ b/kernel/sched/Makefile
@@ -25,3 +25,4 @@ obj-$(CONFIG_SCHED_DEBUG) += debug.o
obj-$(CONFIG_CGROUP_CPUACCT) += cpuacct.o
obj-$(CONFIG_CPU_FREQ) += cpufreq.o
obj-$(CONFIG_CPU_FREQ_GOV_SCHEDUTIL) += cpufreq_schedutil.o
+obj-$(CONFIG_MEMBARRIER) += membarrier.o
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 17c667b427b4..01e3b881ab3a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2724,6 +2724,32 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev)
put_user(task_pid_vnr(current), current->set_child_tid);
}

+static void membarrier_expedited_mb_after_set_current(struct mm_struct *mm,
+ struct mm_struct *oldmm)
+{
+ if (!IS_ENABLED(CONFIG_MEMBARRIER))
+ return;
+ /*
+ * __schedule()->
+ * finish_task_switch()->
+ * if (mm)
+ * mmdrop(mm) ->
+ * atomic_dec_and_test()
+ * takes care of issuing a memory barrier when oldmm is
+ * non-NULL. We also don't need the barrier when switching to a
+ * kernel thread, nor when we switch between threads belonging
+ * to the same process.
+ */
+ if (likely(oldmm || !mm || mm == oldmm))
+ return;
+ /*
+ * When switching between processes, membarrier expedited
+ * private requires a memory barrier after we set the current
+ * task.
+ */
+ smp_mb();
+}
+
/*
* context_switch - switch to the new MM and the new thread's register state.
*/
@@ -2737,6 +2763,7 @@ context_switch(struct rq *rq, struct task_struct *prev,

mm = next->mm;
oldmm = prev->active_mm;
+ membarrier_expedited_mb_after_set_current(mm, oldmm);
/*
* For paravirt, this is coupled with an exit in switch_to to
* combine the page table reload and the switch backend into
diff --git a/kernel/membarrier.c b/kernel/sched/membarrier.c
similarity index 59%
rename from kernel/membarrier.c
rename to kernel/sched/membarrier.c
index 9f9284f37f8d..f80828b0b607 100644
--- a/kernel/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -17,12 +17,79 @@
#include <linux/syscalls.h>
#include <linux/membarrier.h>
#include <linux/tick.h>
+#include <linux/cpumask.h>
+
+#include "sched.h" /* for cpu_rq(). */

/*
* Bitmask made from a "or" of all commands within enum membarrier_cmd,
* except MEMBARRIER_CMD_QUERY.
*/
-#define MEMBARRIER_CMD_BITMASK (MEMBARRIER_CMD_SHARED)
+#define MEMBARRIER_CMD_BITMASK \
+ (MEMBARRIER_CMD_SHARED | MEMBARRIER_CMD_PRIVATE_EXPEDITED)
+
+static void ipi_mb(void *info)
+{
+ smp_mb(); /* IPIs should be serializing but paranoid. */
+}
+
+static void membarrier_private_expedited(void)
+{
+ int cpu, this_cpu;
+ bool fallback = false;
+ cpumask_var_t tmpmask;
+
+ if (num_online_cpus() == 1)
+ return;
+
+ /*
+ * Matches memory barriers around rq->curr modification in
+ * scheduler.
+ */
+ smp_mb(); /* system call entry is not a mb. */
+
+ if (!alloc_cpumask_var(&tmpmask, GFP_NOWAIT)) {
+ /* Fallback for OOM. */
+ fallback = true;
+ }
+
+ /*
+ * Skipping the current CPU is OK even through we can be
+ * migrated at any point. The current CPU, at the point where we
+ * read raw_smp_processor_id(), is ensured to be in program
+ * order with respect to the caller thread. Therefore, we can
+ * skip this CPU from the iteration.
+ */
+ this_cpu = raw_smp_processor_id();
+ cpus_read_lock();
+ for_each_online_cpu(cpu) {
+ struct task_struct *p;
+
+ if (cpu == this_cpu)
+ continue;
+ rcu_read_lock();
+ p = task_rcu_dereference(&cpu_rq(cpu)->curr);
+ if (p && p->mm == current->mm) {
+ if (!fallback)
+ __cpumask_set_cpu(cpu, tmpmask);
+ else
+ smp_call_function_single(cpu, ipi_mb, NULL, 1);
+ }
+ rcu_read_unlock();
+ }
+ cpus_read_unlock();
+ if (!fallback) {
+ smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
+ free_cpumask_var(tmpmask);
+ }
+
+ /*
+ * Memory barrier on the caller thread _after_ we finished
+ * waiting for the last IPI. Matches memory barriers around
+ * rq->curr modification in scheduler.
+ */
+ smp_mb(); /* exit from system call is not a mb */
+}

/**
* sys_membarrier - issue memory barriers on a set of threads
@@ -64,6 +131,9 @@ SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
if (num_online_cpus() > 1)
synchronize_sched();
return 0;
+ case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
+ membarrier_private_expedited();
+ return 0;
default:
return -EINVAL;
}
--
2.11.0
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