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Message-Id: <20171108183514.3306-3-mathieu.desnoyers@efficios.com>
Date: Wed, 8 Nov 2017 13:35:10 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
Peter Zijlstra <peterz@...radead.org>,
Andy Lutomirski <luto@...nel.org>,
Boqun Feng <boqun.feng@...il.com>,
Andrew Hunter <ahh@...gle.com>,
Maged Michael <maged.michael@...il.com>, gromer@...gle.com,
Avi Kivity <avi@...lladb.com>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Paul Mackerras <paulus@...ba.org>,
Michael Ellerman <mpe@...erman.id.au>,
Dave Watson <davejwatson@...com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
"H . Peter Anvin" <hpa@...or.com>,
Andrea Parri <parri.andrea@...il.com>, x86@...nel.org,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Alan Stern <stern@...land.harvard.edu>,
Will Deacon <will.deacon@....com>,
Alexander Viro <viro@...iv.linux.org.uk>,
Nicholas Piggin <npiggin@...il.com>,
linuxppc-dev@...ts.ozlabs.org, linux-arch@...r.kernel.org
Subject: [RFC PATCH for 4.15 2/6] membarrier: powerpc: Skip memory barrier in switch_mm() (v6)
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_SWITCH_MM 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_SWITCH_MM 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)"
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC: Peter Zijlstra <peterz@...radead.org>
CC: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
CC: Boqun Feng <boqun.feng@...il.com>
CC: Andrew Hunter <ahh@...gle.com>
CC: Maged Michael <maged.michael@...il.com>
CC: gromer@...gle.com
CC: Avi Kivity <avi@...lladb.com>
CC: Benjamin Herrenschmidt <benh@...nel.crashing.org>
CC: Paul Mackerras <paulus@...ba.org>
CC: Michael Ellerman <mpe@...erman.id.au>
CC: Dave Watson <davejwatson@...com>
CC: Alan Stern <stern@...land.harvard.edu>
CC: Will Deacon <will.deacon@....com>
CC: Andy Lutomirski <luto@...nel.org>
CC: Ingo Molnar <mingo@...hat.com>
CC: Alexander Viro <viro@...iv.linux.org.uk>
CC: Nicholas Piggin <npiggin@...il.com>
CC: linuxppc-dev@...ts.ozlabs.org
CC: linux-arch@...r.kernel.org
---
MAINTAINERS | 2 ++
arch/powerpc/Kconfig | 1 +
arch/powerpc/include/asm/membarrier.h | 26 +++++++++++++++++++++++++
arch/powerpc/kernel/Makefile | 2 ++
arch/powerpc/kernel/membarrier.c | 36 +++++++++++++++++++++++++++++++++++
arch/powerpc/mm/mmu_context.c | 7 +++++++
include/linux/sched/mm.h | 15 +++++++++++++++
init/Kconfig | 3 +++
kernel/sched/core.c | 10 ----------
kernel/sched/membarrier.c | 1 +
10 files changed, 93 insertions(+), 10 deletions(-)
create mode 100644 arch/powerpc/include/asm/membarrier.h
create mode 100644 arch/powerpc/kernel/membarrier.c
diff --git a/MAINTAINERS b/MAINTAINERS
index 2f4e462aa4a2..34687a0ec28c 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -8829,6 +8829,8 @@ L: linux-kernel@...r.kernel.org
S: Supported
F: kernel/sched/membarrier.c
F: include/uapi/linux/membarrier.h
+F: arch/powerpc/kernel/membarrier.c
+F: arch/powerpc/include/asm/membarrier.h
MEMORY MANAGEMENT
L: linux-mm@...ck.org
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index cb782ac1c35d..86077c4cd0a4 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..0951646253d9
--- /dev/null
+++ b/arch/powerpc/include/asm/membarrier.h
@@ -0,0 +1,26 @@
+#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_SWITCH_MM) || !prev))
+ return;
+
+ /*
+ * The membarrier system call requires a full memory barrier
+ * after storing to rq->curr, before going back to user-space.
+ */
+ smp_mb();
+}
+void membarrier_arch_register_private_expedited(struct task_struct *t);
+
+#endif /* _ASM_POWERPC_MEMBARRIER_H */
diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index 6c6cce937dd8..2ad3b55fa2b6 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -136,6 +136,8 @@ endif
obj-$(CONFIG_EPAPR_PARAVIRT) += epapr_paravirt.o epapr_hcalls.o
obj-$(CONFIG_KVM_GUEST) += kvm.o kvm_emul.o
+obj-$(CONFIG_MEMBARRIER) += membarrier.o
+
# Disable GCOV & sanitizers in odd or sensitive code
GCOV_PROFILE_prom_init.o := n
UBSAN_SANITIZE_prom_init.o := n
diff --git a/arch/powerpc/kernel/membarrier.c b/arch/powerpc/kernel/membarrier.c
new file mode 100644
index 000000000000..4795ad59b833
--- /dev/null
+++ b/arch/powerpc/kernel/membarrier.c
@@ -0,0 +1,36 @@
+/*
+ * Copyright (C) 2010-2017 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
+ *
+ * membarrier system call - PowerPC architecture code
+ *
+ * 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/sched/mm.h>
+#include <linux/sched/signal.h>
+#include <linux/thread_info.h>
+#include <linux/spinlock.h>
+#include <linux/rcupdate.h>
+#include <linux/atomic.h>
+
+void membarrier_arch_register_private_expedited(struct task_struct *p)
+{
+ struct mm_struct *mm = p->mm;
+
+ atomic_or(MEMBARRIER_STATE_SWITCH_MM, &mm->membarrier_state);
+ if (atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1)
+ return;
+ /*
+ * Ensure all future scheduler executions will observe the new
+ * thread flag state for this process.
+ */
+ synchronize_sched();
+}
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..bbc3861d14c8 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -218,11 +218,26 @@ enum {
MEMBARRIER_STATE_SWITCH_MM = (1U << 1),
};
+#ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS
+#include <asm/membarrier.h>
+#else
+static inline void membarrier_arch_register_private_expedited(
+ struct task_struct *p)
+{
+}
+#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 3c1faaa2af4a..ba7f17971554 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1400,6 +1400,9 @@ config MEMBARRIER
If unsure, say Y.
+config ARCH_HAS_MEMBARRIER_HOOKS
+ bool
+
config EMBEDDED
bool "Embedded system"
option allnoconfig_y
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d17c5da523a0..f29525e453c4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2644,16 +2644,6 @@ 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..23bd256f8458 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -116,6 +116,7 @@ static void membarrier_register_private_expedited(void)
if (atomic_read(&mm->membarrier_state)
& MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY)
return;
+ membarrier_arch_register_private_expedited(p);
atomic_or(MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY,
&mm->membarrier_state);
}
--
2.11.0
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