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Subject[tip:sched/urgent] membarrier: Provide GLOBAL_EXPEDITED command
Commit-ID:  c5f58bd58f432be5d92df33c5458e0bcbee3aadf
Gitweb: https://git.kernel.org/tip/c5f58bd58f432be5d92df33c5458e0bcbee3aadf
Author: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
AuthorDate: Mon, 29 Jan 2018 15:20:13 -0500
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 5 Feb 2018 21:34:31 +0100

membarrier: Provide GLOBAL_EXPEDITED command

Allow expedited membarrier to be used for data shared between processes
through shared memory.

Processes wishing to receive the membarriers register with
MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED. Those which want to issue
membarrier invoke MEMBARRIER_CMD_GLOBAL_EXPEDITED.

This allows extremely simple kernel-level implementation: we have almost
everything we need with the PRIVATE_EXPEDITED barrier code. All we need
to do is to add a flag in the mm_struct that will be used to check
whether we need to send the IPI to the current thread of each CPU.

There is a slight downside to this approach compared to targeting
specific shared memory users: when performing a membarrier operation,
all registered "global" receivers will get the barrier, even if they
don't share a memory mapping with the sender issuing
MEMBARRIER_CMD_GLOBAL_EXPEDITED.

This registration approach seems to fit the requirement of not
disturbing processes that really deeply care about real-time: they
simply should not register with MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED.

In order to align the membarrier command names, the "MEMBARRIER_CMD_SHARED"
command is renamed to "MEMBARRIER_CMD_GLOBAL", keeping an alias of
MEMBARRIER_CMD_SHARED to MEMBARRIER_CMD_GLOBAL for UAPI header backward
compatibility.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrea Parri <parri.andrea@gmail.com>
Cc: Andrew Hunter <ahh@google.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Avi Kivity <avi@scylladb.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Dave Watson <davejwatson@fb.com>
Cc: David Sehr <sehr@google.com>
Cc: Greg Hackmann <ghackmann@google.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Maged Michael <maged.michael@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-api@vger.kernel.org
Link: http://lkml.kernel.org/r/20180129202020.8515-5-mathieu.desnoyers@efficios.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/powerpc/include/asm/membarrier.h | 3 +-
include/linux/sched/mm.h | 6 +-
include/uapi/linux/membarrier.h | 42 ++++++++++--
kernel/sched/membarrier.c | 120 +++++++++++++++++++++++++++++++---
4 files changed, 153 insertions(+), 18 deletions(-)

diff --git a/arch/powerpc/include/asm/membarrier.h b/arch/powerpc/include/asm/membarrier.h
index 98ff4f1..6e20bb5 100644
--- a/arch/powerpc/include/asm/membarrier.h
+++ b/arch/powerpc/include/asm/membarrier.h
@@ -13,7 +13,8 @@ static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
* store to rq->curr.
*/
if (likely(!(atomic_read(&next->membarrier_state) &
- MEMBARRIER_STATE_PRIVATE_EXPEDITED) || !prev))
+ (MEMBARRIER_STATE_PRIVATE_EXPEDITED |
+ MEMBARRIER_STATE_GLOBAL_EXPEDITED)) || !prev))
return;

/*
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index b84e0fd..1c4e40c 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -219,8 +219,10 @@ static inline void memalloc_noreclaim_restore(unsigned int flags)

#ifdef CONFIG_MEMBARRIER
enum {
- MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY = (1U << 0),
- MEMBARRIER_STATE_PRIVATE_EXPEDITED = (1U << 1),
+ MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY = (1U << 0),
+ MEMBARRIER_STATE_PRIVATE_EXPEDITED = (1U << 1),
+ MEMBARRIER_STATE_GLOBAL_EXPEDITED_READY = (1U << 2),
+ MEMBARRIER_STATE_GLOBAL_EXPEDITED = (1U << 3),
};

#ifdef CONFIG_ARCH_HAS_MEMBARRIER_CALLBACKS
diff --git a/include/uapi/linux/membarrier.h b/include/uapi/linux/membarrier.h
index 4e01ad7..d252506 100644
--- a/include/uapi/linux/membarrier.h
+++ b/include/uapi/linux/membarrier.h
@@ -31,7 +31,7 @@
* enum membarrier_cmd - membarrier system call command
* @MEMBARRIER_CMD_QUERY: Query the set of supported commands. It returns
* a bitmask of valid commands.
- * @MEMBARRIER_CMD_SHARED: Execute a memory barrier on all running threads.
+ * @MEMBARRIER_CMD_GLOBAL: Execute a memory barrier on all running threads.
* Upon return from system call, the caller thread
* is ensured that all running threads have passed
* through a state where all memory accesses to
@@ -40,6 +40,28 @@
* (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_GLOBAL_EXPEDITED:
+ * Execute a memory barrier on all running threads
+ * of all processes which previously registered
+ * with MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED.
+ * Upon return from system call, the caller thread
+ * is ensured that all running threads 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 processes
+ * which registered with
+ * MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED.
+ * This command returns 0. Given that
+ * registration is about the intent to receive
+ * the barriers, it is valid to invoke
+ * MEMBARRIER_CMD_GLOBAL_EXPEDITED from a
+ * non-registered process.
+ * @MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED:
+ * Register the process intent to receive
+ * MEMBARRIER_CMD_GLOBAL_EXPEDITED memory
+ * barriers. Always returns 0.
* @MEMBARRIER_CMD_PRIVATE_EXPEDITED:
* Execute a memory barrier on each running
* thread belonging to the same process as the current
@@ -64,18 +86,24 @@
* Register the process intent to use
* MEMBARRIER_CMD_PRIVATE_EXPEDITED. Always
* returns 0.
+ * @MEMBARRIER_CMD_SHARED:
+ * Alias to MEMBARRIER_CMD_GLOBAL. Provided for
+ * header backward compatibility.
*
* 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),
- /* reserved for MEMBARRIER_CMD_SHARED_EXPEDITED (1 << 1) */
- /* reserved for MEMBARRIER_CMD_PRIVATE (1 << 2) */
- MEMBARRIER_CMD_PRIVATE_EXPEDITED = (1 << 3),
- MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED = (1 << 4),
+ MEMBARRIER_CMD_QUERY = 0,
+ MEMBARRIER_CMD_GLOBAL = (1 << 0),
+ MEMBARRIER_CMD_GLOBAL_EXPEDITED = (1 << 1),
+ MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED = (1 << 2),
+ MEMBARRIER_CMD_PRIVATE_EXPEDITED = (1 << 3),
+ MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED = (1 << 4),
+
+ /* Alias for header backward compatibility. */
+ MEMBARRIER_CMD_SHARED = MEMBARRIER_CMD_GLOBAL,
};

#endif /* _UAPI_LINUX_MEMBARRIER_H */
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index 6785772..d2087d5 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -27,7 +27,9 @@
* except MEMBARRIER_CMD_QUERY.
*/
#define MEMBARRIER_CMD_BITMASK \
- (MEMBARRIER_CMD_SHARED | MEMBARRIER_CMD_PRIVATE_EXPEDITED \
+ (MEMBARRIER_CMD_GLOBAL | MEMBARRIER_CMD_GLOBAL_EXPEDITED \
+ | MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED \
+ | MEMBARRIER_CMD_PRIVATE_EXPEDITED \
| MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED)

static void ipi_mb(void *info)
@@ -35,6 +37,73 @@ static void ipi_mb(void *info)
smp_mb(); /* IPIs should be serializing but paranoid. */
}

+static int membarrier_global_expedited(void)
+{
+ int cpu;
+ bool fallback = false;
+ cpumask_var_t tmpmask;
+
+ if (num_online_cpus() == 1)
+ return 0;
+
+ /*
+ * 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 && (atomic_read(&p->mm->membarrier_state) &
+ MEMBARRIER_STATE_GLOBAL_EXPEDITED)) {
+ if (!fallback)
+ __cpumask_set_cpu(cpu, tmpmask);
+ else
+ smp_call_function_single(cpu, ipi_mb, NULL, 1);
+ }
+ rcu_read_unlock();
+ }
+ if (!fallback) {
+ preempt_disable();
+ smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
+ preempt_enable();
+ 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 */
+ return 0;
+}
+
static int membarrier_private_expedited(void)
{
int cpu;
@@ -105,7 +174,38 @@ static int membarrier_private_expedited(void)
return 0;
}

-static void membarrier_register_private_expedited(void)
+static int membarrier_register_global_expedited(void)
+{
+ struct task_struct *p = current;
+ struct mm_struct *mm = p->mm;
+
+ if (atomic_read(&mm->membarrier_state) &
+ MEMBARRIER_STATE_GLOBAL_EXPEDITED_READY)
+ return 0;
+ atomic_or(MEMBARRIER_STATE_GLOBAL_EXPEDITED, &mm->membarrier_state);
+ if (atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1) {
+ /*
+ * For single mm user, single threaded process, we can
+ * simply issue a memory barrier after setting
+ * MEMBARRIER_STATE_GLOBAL_EXPEDITED to guarantee that
+ * no memory access following registration is reordered
+ * before registration.
+ */
+ smp_mb();
+ } else {
+ /*
+ * For multi-mm user threads, we need to ensure all
+ * future scheduler executions will observe the new
+ * thread flag state for this mm.
+ */
+ synchronize_sched();
+ }
+ atomic_or(MEMBARRIER_STATE_GLOBAL_EXPEDITED_READY,
+ &mm->membarrier_state);
+ return 0;
+}
+
+static int membarrier_register_private_expedited(void)
{
struct task_struct *p = current;
struct mm_struct *mm = p->mm;
@@ -117,7 +217,7 @@ static void membarrier_register_private_expedited(void)
*/
if (atomic_read(&mm->membarrier_state)
& MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY)
- return;
+ return 0;
atomic_or(MEMBARRIER_STATE_PRIVATE_EXPEDITED, &mm->membarrier_state);
if (!(atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1)) {
/*
@@ -128,6 +228,7 @@ static void membarrier_register_private_expedited(void)
}
atomic_or(MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY,
&mm->membarrier_state);
+ return 0;
}

/**
@@ -167,21 +268,24 @@ SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
int cmd_mask = MEMBARRIER_CMD_BITMASK;

if (tick_nohz_full_enabled())
- cmd_mask &= ~MEMBARRIER_CMD_SHARED;
+ cmd_mask &= ~MEMBARRIER_CMD_GLOBAL;
return cmd_mask;
}
- case MEMBARRIER_CMD_SHARED:
- /* MEMBARRIER_CMD_SHARED is not compatible with nohz_full. */
+ case MEMBARRIER_CMD_GLOBAL:
+ /* MEMBARRIER_CMD_GLOBAL 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_GLOBAL_EXPEDITED:
+ return membarrier_global_expedited();
+ case MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED:
+ return membarrier_register_global_expedited();
case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
return membarrier_private_expedited();
case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED:
- membarrier_register_private_expedited();
- return 0;
+ return membarrier_register_private_expedited();
default:
return -EINVAL;
}
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