[RFC PATCH v3] membarrier: expedited private command
From: Mathieu Desnoyers
Date: Fri Jul 28 2017 - 12:55:14 EST
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 a memory barrier before and after context
switching between processes (which have different mm). The memory
barrier before context switch is already present. After context switch,
finish_lock_switch() acts as a RELEASE barrier, which is a full memory
barrier on all architectures except PowerPC. Add a
smp_mb__after_unlock_lock() to promote this barrier to a full memory
barrier. This is a no-op on all architectures but PowerPC.
* 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 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@xxxxxxxxxxxxx>
CC: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CC: Boqun Feng <boqun.feng@xxxxxxxxx>
CC: Andrew Hunter <ahh@xxxxxxxxxx>
CC: Maged Michael <maged.michael@xxxxxxxxx>
CC: gromer@xxxxxxxxxx
CC: Avi Kivity <avi@xxxxxxxxxxxx>
CC: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
CC: Paul Mackerras <paulus@xxxxxxxxx>
CC: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
---
MAINTAINERS | 2 +-
include/uapi/linux/membarrier.h | 23 +++++-
kernel/Makefile | 1 -
kernel/membarrier.c | 70 ------------------
kernel/sched/Makefile | 1 +
kernel/sched/core.c | 5 ++
kernel/sched/membarrier.c | 152 ++++++++++++++++++++++++++++++++++++++++
7 files changed, 180 insertions(+), 74 deletions(-)
delete mode 100644 kernel/membarrier.c
create mode 100644 kernel/sched/membarrier.c
diff --git a/MAINTAINERS b/MAINTAINERS
index f66488dfdbc9..3b035584272f 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -8621,7 +8621,7 @@ M: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
M: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
L: linux-kernel@xxxxxxxxxxxxxxx
S: Supported
-F: kernel/membarrier.c
+F: kernel/sched/membarrier.c
F: include/uapi/linux/membarrier.h
MEMORY MANAGEMENT
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/membarrier.c b/kernel/membarrier.c
deleted file mode 100644
index 9f9284f37f8d..000000000000
--- a/kernel/membarrier.c
+++ /dev/null
@@ -1,70 +0,0 @@
-/*
- * Copyright (C) 2010, 2015 Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
- *
- * membarrier system call
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation; either version 2 of the License, or
- * (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
- * GNU General Public License for more details.
- */
-
-#include <linux/syscalls.h>
-#include <linux/membarrier.h>
-#include <linux/tick.h>
-
-/*
- * Bitmask made from a "or" of all commands within enum membarrier_cmd,
- * except MEMBARRIER_CMD_QUERY.
- */
-#define MEMBARRIER_CMD_BITMASK (MEMBARRIER_CMD_SHARED)
-
-/**
- * sys_membarrier - issue memory barriers on a set of threads
- * @cmd: Takes command values defined in enum membarrier_cmd.
- * @flags: Currently needs to be 0. For future extensions.
- *
- * If this system call is not implemented, -ENOSYS is returned. If the
- * command specified does not exist, or if the command argument is invalid,
- * this system call returns -EINVAL. For a given command, with flags argument
- * set to 0, this system call is guaranteed to always return the same value
- * until reboot.
- *
- * All memory accesses performed in program order from each targeted thread
- * is guaranteed to be ordered with respect to sys_membarrier(). If we use
- * the semantic "barrier()" to represent a compiler barrier forcing memory
- * accesses to be performed in program order across the barrier, and
- * smp_mb() to represent explicit memory barriers forcing full memory
- * ordering across the barrier, we have the following ordering table for
- * each pair of barrier(), sys_membarrier() and smp_mb():
- *
- * The pair ordering is detailed as (O: ordered, X: not ordered):
- *
- * barrier() smp_mb() sys_membarrier()
- * barrier() X X O
- * smp_mb() X O O
- * sys_membarrier() O O O
- */
-SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
-{
- /* MEMBARRIER_CMD_SHARED is not compatible with nohz_full. */
- if (tick_nohz_full_enabled())
- return -ENOSYS;
- if (unlikely(flags))
- return -EINVAL;
- switch (cmd) {
- case MEMBARRIER_CMD_QUERY:
- return MEMBARRIER_CMD_BITMASK;
- case MEMBARRIER_CMD_SHARED:
- if (num_online_cpus() > 1)
- synchronize_sched();
- return 0;
- default:
- return -EINVAL;
- }
-}
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..dd677fb2ee92 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2636,6 +2636,11 @@ static struct rq *finish_task_switch(struct task_struct *prev)
vtime_task_switch(prev);
perf_event_task_sched_in(prev, current);
finish_lock_switch(rq, prev);
+ /*
+ * The membarrier system call requires a full memory barrier
+ * after storing to rq->curr, before going back to user-space.
+ */
+ smp_mb__after_unlock_lock();
finish_arch_post_lock_switch();
fire_sched_in_preempt_notifiers(current);
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
new file mode 100644
index 000000000000..287a3f88f9b2
--- /dev/null
+++ b/kernel/sched/membarrier.c
@@ -0,0 +1,152 @@
+/*
+ * Copyright (C) 2010, 2015 Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
+ *
+ * membarrier system call
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ */
+
+#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 | 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;
+ 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. */
+
+ /*
+ * Expedited membarrier commands guarantee that they won't
+ * block, hence the GFP_NOWAIT allocation flag and fallback
+ * implementation.
+ */
+ if (!zalloc_cpumask_var(&tmpmask, GFP_NOWAIT)) {
+ /* Fallback for OOM. */
+ fallback = true;
+ }
+
+ cpus_read_lock();
+ for_each_online_cpu(cpu) {
+ struct task_struct *p;
+
+ /*
+ * 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.
+ */
+ if (cpu == raw_smp_processor_id())
+ 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();
+ }
+ if (!fallback) {
+ smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
+ free_cpumask_var(tmpmask);
+ }
+ cpus_read_unlock();
+
+ /*
+ * 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
+ * @cmd: Takes command values defined in enum membarrier_cmd.
+ * @flags: Currently needs to be 0. For future extensions.
+ *
+ * If this system call is not implemented, -ENOSYS is returned. If the
+ * command specified does not exist, not available on the running
+ * kernel, or if the command argument is invalid, this system call
+ * returns -EINVAL. For a given command, with flags argument set to 0,
+ * this system call is guaranteed to always return the same value until
+ * reboot.
+ *
+ * All memory accesses performed in program order from each targeted thread
+ * is guaranteed to be ordered with respect to sys_membarrier(). If we use
+ * the semantic "barrier()" to represent a compiler barrier forcing memory
+ * accesses to be performed in program order across the barrier, and
+ * smp_mb() to represent explicit memory barriers forcing full memory
+ * ordering across the barrier, we have the following ordering table for
+ * each pair of barrier(), sys_membarrier() and smp_mb():
+ *
+ * The pair ordering is detailed as (O: ordered, X: not ordered):
+ *
+ * barrier() smp_mb() sys_membarrier()
+ * barrier() X X O
+ * smp_mb() X O O
+ * sys_membarrier() O O O
+ */
+SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
+{
+ if (unlikely(flags))
+ return -EINVAL;
+ switch (cmd) {
+ case MEMBARRIER_CMD_QUERY:
+ {
+ int cmd_mask = MEMBARRIER_CMD_BITMASK;
+
+ if (tick_nohz_full_enabled())
+ cmd_mask &= ~MEMBARRIER_CMD_SHARED;
+ return cmd_mask;
+ }
+ case MEMBARRIER_CMD_SHARED:
+ /* MEMBARRIER_CMD_SHARED is not compatible with nohz_full. */
+ if (tick_nohz_full_enabled())
+ return -EINVAL;
+ 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