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Subject[RFC PATCH v7 for 4.15 16/24] membarrier: powerpc: Skip memory barrier in switch_mm()
Date
Allow PowerPC to skip the full memory barrier in switch_mm(), and
only issue the barrier when scheduling into a task belonging to a
process that has registered to use expedited private.

Threads targeting the same VM but which belong to different thread
groups is a tricky case. It has a few consequences:

It turns out that we cannot rely on get_nr_threads(p) to count the
number of threads using a VM. We can use
(atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1)
instead to skip the synchronize_sched() for cases where the VM only has
a single user, and that user only has a single thread.

It also turns out that we cannot use for_each_thread() to set
thread flags in all threads using a VM, as it only iterates on the
thread group.

Therefore, test the membarrier state variable directly rather than
relying on thread flags. This means
membarrier_register_private_expedited() needs to set the
MEMBARRIER_STATE_PRIVATE_EXPEDITED flag, issue synchronize_sched(), and
only then set MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY which allows
private expedited membarrier commands to succeed.
membarrier_arch_switch_mm() now tests for the
MEMBARRIER_STATE_PRIVATE_EXPEDITED flag.

Changes since v1:
- Use test_ti_thread_flag(next, ...) instead of test_thread_flag() in
powerpc membarrier_arch_sched_in(), given that we want to specifically
check the next thread state.
- Add missing ARCH_HAS_MEMBARRIER_HOOKS in Kconfig.
- Use task_thread_info() to pass thread_info from task to
*_ti_thread_flag().

Changes since v2:
- Move membarrier_arch_sched_in() call to finish_task_switch().
- Check for NULL t->mm in membarrier_arch_fork().
- Use membarrier_sched_in() in generic code, which invokes the
arch-specific membarrier_arch_sched_in(). This fixes allnoconfig
build on PowerPC.
- Move asm/membarrier.h include under CONFIG_MEMBARRIER, fixing
allnoconfig build on PowerPC.
- Build and runtime tested on PowerPC.

Changes since v3:
- Simply rely on copy_mm() to copy the membarrier_private_expedited mm
field on fork.
- powerpc: test thread flag instead of reading
membarrier_private_expedited in membarrier_arch_fork().
- powerpc: skip memory barrier in membarrier_arch_sched_in() if coming
from kernel thread, since mmdrop() implies a full barrier.
- Set membarrier_private_expedited to 1 only after arch registration
code, thus eliminating a race where concurrent commands could succeed
when they should fail if issued concurrently with process
registration.
- Use READ_ONCE() for membarrier_private_expedited field access in
membarrier_private_expedited. Matches WRITE_ONCE() performed in
process registration.

Changes since v4:
- Move powerpc hook from sched_in() to switch_mm(), based on feedback
from Nicholas Piggin.

Changes since v5:
- Rebase on v4.14-rc6.
- Fold "Fix: membarrier: Handle CLONE_VM + !CLONE_THREAD correctly on
powerpc (v2)"

Changes since v6:
- Rename MEMBARRIER_STATE_SWITCH_MM to MEMBARRIER_STATE_PRIVATE_EXPEDITED.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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: 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>
CC: Dave Watson <davejwatson@fb.com>
CC: Alan Stern <stern@rowland.harvard.edu>
CC: Will Deacon <will.deacon@arm.com>
CC: Andy Lutomirski <luto@kernel.org>
CC: Ingo Molnar <mingo@redhat.com>
CC: Alexander Viro <viro@zeniv.linux.org.uk>
CC: Nicholas Piggin <npiggin@gmail.com>
CC: linuxppc-dev@lists.ozlabs.org
CC: linux-arch@vger.kernel.org
---
MAINTAINERS | 1 +
arch/powerpc/Kconfig | 1 +
arch/powerpc/include/asm/membarrier.h | 25 +++++++++++++++++++++++++
arch/powerpc/mm/mmu_context.c | 7 +++++++
include/linux/sched/mm.h | 12 +++++++++++-
init/Kconfig | 3 +++
kernel/sched/core.c | 10 ----------
kernel/sched/membarrier.c | 9 +++++++++
8 files changed, 57 insertions(+), 11 deletions(-)
create mode 100644 arch/powerpc/include/asm/membarrier.h

diff --git a/MAINTAINERS b/MAINTAINERS
index d0bf5c5b4267..cf41984201d2 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -8842,6 +8842,7 @@ L: linux-kernel@vger.kernel.org
S: Supported
F: kernel/sched/membarrier.c
F: include/uapi/linux/membarrier.h
+F: arch/powerpc/include/asm/membarrier.h

MEMORY MANAGEMENT
L: linux-mm@kvack.org
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 41d1dae3b1b5..e54a822e5fb9 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -139,6 +139,7 @@ config PPC
select ARCH_HAS_ELF_RANDOMIZE
select ARCH_HAS_FORTIFY_SOURCE
select ARCH_HAS_GCOV_PROFILE_ALL
+ select ARCH_HAS_MEMBARRIER_HOOKS
select ARCH_HAS_SCALED_CPUTIME if VIRT_CPU_ACCOUNTING_NATIVE
select ARCH_HAS_SG_CHAIN
select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
diff --git a/arch/powerpc/include/asm/membarrier.h b/arch/powerpc/include/asm/membarrier.h
new file mode 100644
index 000000000000..046f96768ab5
--- /dev/null
+++ b/arch/powerpc/include/asm/membarrier.h
@@ -0,0 +1,25 @@
+#ifndef _ASM_POWERPC_MEMBARRIER_H
+#define _ASM_POWERPC_MEMBARRIER_H
+
+static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
+ struct mm_struct *next, struct task_struct *tsk)
+{
+ /*
+ * Only need the full barrier when switching between processes.
+ * Barrier when switching from kernel to userspace is not
+ * required here, given that it is implied by mmdrop(). Barrier
+ * when switching from userspace to kernel is not needed after
+ * store to rq->curr.
+ */
+ if (likely(!(atomic_read(&next->membarrier_state)
+ & MEMBARRIER_STATE_PRIVATE_EXPEDITED) || !prev))
+ return;
+
+ /*
+ * The membarrier system call requires a full memory barrier
+ * after storing to rq->curr, before going back to user-space.
+ */
+ smp_mb();
+}
+
+#endif /* _ASM_POWERPC_MEMBARRIER_H */
diff --git a/arch/powerpc/mm/mmu_context.c b/arch/powerpc/mm/mmu_context.c
index 0f613bc63c50..22f5c91cdc38 100644
--- a/arch/powerpc/mm/mmu_context.c
+++ b/arch/powerpc/mm/mmu_context.c
@@ -12,6 +12,7 @@

#include <linux/mm.h>
#include <linux/cpu.h>
+#include <linux/sched/mm.h>

#include <asm/mmu_context.h>

@@ -67,6 +68,10 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
*
* On the read side the barrier is in pte_xchg(), which orders
* the store to the PTE vs the load of mm_cpumask.
+ *
+ * This full barrier is needed by membarrier when switching
+ * between processes after store to rq->curr, before user-space
+ * memory accesses.
*/
smp_mb();

@@ -89,6 +94,8 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,

if (new_on_cpu)
radix_kvm_prefetch_workaround(next);
+ else
+ membarrier_arch_switch_mm(prev, next, tsk);

/*
* The actual HW switching method differs between the various
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 3d49b91b674d..7077253d0df4 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -215,14 +215,24 @@ static inline void memalloc_noreclaim_restore(unsigned int flags)
#ifdef CONFIG_MEMBARRIER
enum {
MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY = (1U << 0),
- MEMBARRIER_STATE_SWITCH_MM = (1U << 1),
+ MEMBARRIER_STATE_PRIVATE_EXPEDITED = (1U << 1),
};

+#ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS
+#include <asm/membarrier.h>
+#endif
+
static inline void membarrier_execve(struct task_struct *t)
{
atomic_set(&t->mm->membarrier_state, 0);
}
#else
+#ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS
+static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
+ struct mm_struct *next, struct task_struct *tsk)
+{
+}
+#endif
static inline void membarrier_execve(struct task_struct *t)
{
}
diff --git a/init/Kconfig b/init/Kconfig
index e4fbb5dd6a24..609296e764d6 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1400,6 +1400,9 @@ config MEMBARRIER

If unsure, say Y.

+config ARCH_HAS_MEMBARRIER_HOOKS
+ bool
+
config RSEQ
bool "Enable rseq() system call" if EXPERT
default y
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e547f93a46c2..0ac96e8329d5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2684,16 +2684,6 @@ 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();

diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index dd7908743dab..b045974346d0 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -116,6 +116,15 @@ static void membarrier_register_private_expedited(void)
if (atomic_read(&mm->membarrier_state)
& MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY)
return;
+ atomic_or(MEMBARRIER_STATE_PRIVATE_EXPEDITED,
+ &mm->membarrier_state);
+ if (!(atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1)) {
+ /*
+ * Ensure all future scheduler executions will observe the
+ * new thread flag state for this process.
+ */
+ synchronize_sched();
+ }
atomic_or(MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY,
&mm->membarrier_state);
}
--
2.11.0
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