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SubjectRe: [RFC PATCH v3 1/2] membarrier: Provide register expedited private command
Hi Mathieu,

On Tue, Sep 19, 2017 at 06:13:41PM -0400, Mathieu Desnoyers wrote:
> Provide a new command allowing processes to register their intent to use
> the private expedited command.
>
> This allows 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.
>
> Processes are now required to register before using
> MEMBARRIER_CMD_PRIVATE_EXPEDITED, otherwise that command returns EPERM.
>

Sorry I'm late for the party, but I couldn't stop thinking whether we
could avoid the register thing at all, because the registering makes
sys_membarrier() more complex(both for the interface and the
implementation). So how about we trade-off a little bit by taking
some(not all) the rq->locks?

The idea is in membarrier_private_expedited(), we go through all ->curr
on each CPU and

1) If it's a userspace task and its ->mm is matched, we send an ipi

2) If it's a kernel task, we skip

(Because there will be a smp_mb() implied by mmdrop(), when it
switchs to userspace task).

3) If it's a userspace task and its ->mm is not matched, we take
the corresponding rq->lock and check rq->curr again, if its ->mm
matched, we send an ipi, otherwise we do nothing.

(Because if we observe rq->curr is not matched with rq->lock
held, when a task having matched ->mm schedules in, the rq->lock
pairing along with the smp_mb__after_spinlock() will guarantee
it observes all memory ops before sys_membarrir()).

membarrier_private_expedited() will look like this if we choose this
way:

void membarrier_private_expedited()
{
int cpu;
bool fallback = false;
cpumask_var_t tmpmask;
struct rq_flags rf;


if (num_online_cpus() == 1)
return;

smp_mb();

if (!zalloc_cpumask_var(&tmpmask, GFP_NOWAIT)) {
/* Fallback for OOM. */
fallback = true;
}

cpus_read_lock();
for_each_online_cpu(cpu) {
struct task_struct *p;

if (cpu == raw_smp_processor_id())
continue;

rcu_read_lock();
p = task_rcu_dereference(&cpu_rq(cpu)->curr);

if (!p) {
rcu_read_unlock();
continue;
}

if (p->mm == current->mm) {
if (!fallback)
__cpumask_set_cpu(cpu, tmpmask);
else
smp_call_function_single(cpu, ipi_mb, NULL, 1);
}

if (p->mm == current->mm || !p->mm) {
rcu_read_unlock();
continue;
}

rcu_read_unlock();

/*
* This should be a arch-specific code, as we don't
* need it at else place other than some archs without
* a smp_mb() in switch_mm() (i.e. powerpc)
*/
rq_lock_irq(cpu_rq(cpu), &rf);
if (p->mm == current->mm) {
if (!fallback)
__cpumask_set_cpu(cpu, tmpmask);
else
smp_call_function_single(cpu, ipi_mb, NULL, 1);
}
rq_unlock_irq(cpu_rq(cpu), &rf);
}
if (!fallback) {
smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
free_cpumask_var(tmpmask);
}
cpus_read_unlock();

smp_mb();
}

Thoughts?

Regards,
Boqun

> 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.
>
> 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: gromer@google.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: linux-arch@vger.kernel.org
> ---
> MAINTAINERS | 2 ++
> arch/powerpc/Kconfig | 1 +
> arch/powerpc/include/asm/membarrier.h | 40 +++++++++++++++++++++++++
> arch/powerpc/include/asm/thread_info.h | 3 ++
> arch/powerpc/kernel/Makefile | 2 ++
> arch/powerpc/kernel/membarrier.c | 45 ++++++++++++++++++++++++++++
> fs/exec.c | 1 +
> include/linux/mm_types.h | 3 ++
> include/linux/sched/mm.h | 55 ++++++++++++++++++++++++++++++++++
> include/uapi/linux/membarrier.h | 23 +++++++++-----
> init/Kconfig | 3 ++
> kernel/fork.c | 2 ++
> kernel/sched/core.c | 16 ++--------
> kernel/sched/membarrier.c | 25 +++++++++++++---
> 14 files changed, 197 insertions(+), 24 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 2281af4b41b6..fe6d34d8a2e0 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -8805,6 +8805,8 @@ L: linux-kernel@vger.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@kvack.org
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 809c468edab1..6f44c5f74f71 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -138,6 +138,7 @@ 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..ee208455674d
> --- /dev/null
> +++ b/arch/powerpc/include/asm/membarrier.h
> @@ -0,0 +1,40 @@
> +#ifndef _ASM_POWERPC_MEMBARRIER_H
> +#define _ASM_POWERPC_MEMBARRIER_H
> +
> +static inline void membarrier_arch_sched_in(struct task_struct *prev,
> + struct task_struct *next)
> +{
> + /*
> + * Only need the full barrier when switching between processes.
> + */
> + if (likely(!test_ti_thread_flag(task_thread_info(next),
> + TIF_MEMBARRIER_PRIVATE_EXPEDITED)
> + || prev->mm == next->mm))
> + return;
> +
> + /*
> + * The membarrier system call requires a full memory barrier
> + * after storing to rq->curr, before going back to user-space.
> + */
> + smp_mb();
> +}
> +static inline void membarrier_arch_fork(struct task_struct *t,
> + unsigned long clone_flags)
> +{
> + /*
> + * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
> + * fork is protected by siglock. membarrier_arch_fork is called
> + * with siglock held.
> + */
> + if (t->mm && t->mm->membarrier_private_expedited)
> + set_ti_thread_flag(task_thread_info(t),
> + TIF_MEMBARRIER_PRIVATE_EXPEDITED);
> +}
> +static inline void membarrier_arch_execve(struct task_struct *t)
> +{
> + clear_ti_thread_flag(task_thread_info(t),
> + TIF_MEMBARRIER_PRIVATE_EXPEDITED);
> +}
> +void membarrier_arch_register_private_expedited(struct task_struct *t);
> +
> +#endif /* _ASM_POWERPC_MEMBARRIER_H */
> diff --git a/arch/powerpc/include/asm/thread_info.h b/arch/powerpc/include/asm/thread_info.h
> index a941cc6fc3e9..2a208487724b 100644
> --- a/arch/powerpc/include/asm/thread_info.h
> +++ b/arch/powerpc/include/asm/thread_info.h
> @@ -100,6 +100,7 @@ static inline struct thread_info *current_thread_info(void)
> #if defined(CONFIG_PPC64)
> #define TIF_ELF2ABI 18 /* function descriptors must die! */
> #endif
> +#define TIF_MEMBARRIER_PRIVATE_EXPEDITED 19 /* membarrier */
>
> /* as above, but as bit values */
> #define _TIF_SYSCALL_TRACE (1<<TIF_SYSCALL_TRACE)
> @@ -119,6 +120,8 @@ static inline struct thread_info *current_thread_info(void)
> #define _TIF_SYSCALL_TRACEPOINT (1<<TIF_SYSCALL_TRACEPOINT)
> #define _TIF_EMULATE_STACK_STORE (1<<TIF_EMULATE_STACK_STORE)
> #define _TIF_NOHZ (1<<TIF_NOHZ)
> +#define _TIF_MEMBARRIER_PRIVATE_EXPEDITED \
> + (1<<TIF_MEMBARRIER_PRIVATE_EXPEDITED)
> #define _TIF_SYSCALL_DOTRACE (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | \
> _TIF_SECCOMP | _TIF_SYSCALL_TRACEPOINT | \
> _TIF_NOHZ)
> diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
> index 91960f83039c..2dd4b9e3313a 100644
> --- a/arch/powerpc/kernel/Makefile
> +++ b/arch/powerpc/kernel/Makefile
> @@ -135,6 +135,8 @@ 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..b0d79a5f5981
> --- /dev/null
> +++ b/arch/powerpc/kernel/membarrier.c
> @@ -0,0 +1,45 @@
> +/*
> + * Copyright (C) 2010-2017 Mathieu Desnoyers <mathieu.desnoyers@efficios.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>
> +
> +void membarrier_arch_register_private_expedited(struct task_struct *p)
> +{
> + struct task_struct *t;
> +
> + if (get_nr_threads(p) == 1) {
> + set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED);
> + return;
> + }
> + /*
> + * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
> + * fork is protected by siglock.
> + */
> + spin_lock(&p->sighand->siglock);
> + for_each_thread(p, t)
> + set_ti_thread_flag(task_thread_info(t),
> + TIF_MEMBARRIER_PRIVATE_EXPEDITED);
> + spin_unlock(&p->sighand->siglock);
> + /*
> + * Ensure all future scheduler executions will observe the new
> + * thread flag state for this process.
> + */
> + synchronize_sched();
> +}
> diff --git a/fs/exec.c b/fs/exec.c
> index ac34d9724684..b2448f2731b3 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1802,6 +1802,7 @@ static int do_execveat_common(int fd, struct filename *filename,
> /* execve succeeded */
> current->fs->in_exec = 0;
> current->in_execve = 0;
> + membarrier_execve(current);
> acct_update_integrals(current);
> task_numa_free(current);
> free_bprm(bprm);
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 46f4ecf5479a..5e0fe8ce053b 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -445,6 +445,9 @@ struct mm_struct {
> unsigned long flags; /* Must use atomic bitops to access the bits */
>
> struct core_state *core_state; /* coredumping support */
> +#ifdef CONFIG_MEMBARRIER
> + int membarrier_private_expedited;
> +#endif
> #ifdef CONFIG_AIO
> spinlock_t ioctx_lock;
> struct kioctx_table __rcu *ioctx_table;
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 766cc47c4d7c..b88f85c8474e 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -210,4 +210,59 @@ static inline void memalloc_noreclaim_restore(unsigned int flags)
> current->flags = (current->flags & ~PF_MEMALLOC) | flags;
> }
>
> +#ifdef CONFIG_MEMBARRIER
> +
> +#ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS
> +#include <asm/membarrier.h>
> +#else
> +static inline void membarrier_arch_sched_in(struct task_struct *prev,
> + struct task_struct *next)
> +{
> +}
> +static inline void membarrier_arch_fork(struct task_struct *t,
> + unsigned long clone_flags)
> +{
> +}
> +static inline void membarrier_arch_execve(struct task_struct *t)
> +{
> +}
> +static inline void membarrier_arch_register_private_expedited(
> + struct task_struct *p)
> +{
> +}
> +#endif
> +
> +static inline void membarrier_sched_in(struct task_struct *prev,
> + struct task_struct *next)
> +{
> + membarrier_arch_sched_in(prev, next);
> +}
> +static inline void membarrier_fork(struct task_struct *t,
> + unsigned long clone_flags)
> +{
> + if (!current->mm || !t->mm)
> + return;
> + t->mm->membarrier_private_expedited =
> + current->mm->membarrier_private_expedited;
> + membarrier_arch_fork(t, clone_flags);
> +}
> +static inline void membarrier_execve(struct task_struct *t)
> +{
> + t->mm->membarrier_private_expedited = 0;
> + membarrier_arch_execve(t);
> +}
> +#else
> +static inline void membarrier_sched_in(struct task_struct *prev,
> + struct task_struct *next)
> +{
> +}
> +static inline void membarrier_fork(struct task_struct *t,
> + unsigned long clone_flags)
> +{
> +}
> +static inline void membarrier_execve(struct task_struct *t)
> +{
> +}
> +#endif
> +
> #endif /* _LINUX_SCHED_MM_H */
> diff --git a/include/uapi/linux/membarrier.h b/include/uapi/linux/membarrier.h
> index 6d47b3249d8a..4e01ad7ffe98 100644
> --- a/include/uapi/linux/membarrier.h
> +++ b/include/uapi/linux/membarrier.h
> @@ -52,21 +52,30 @@
> * (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 returns 0 on success. The
> + * "expedited" commands complete faster than
> + * the non-expedited ones, they never block,
> + * but have the downside of causing extra
> + * overhead. A process needs to register its
> + * intent to use the private expedited command
> + * prior to using it, otherwise this command
> + * returns -EPERM.
> + * @MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED:
> + * Register the process intent to use
> + * MEMBARRIER_CMD_PRIVATE_EXPEDITED. Always
> + * returns 0.
> *
> * 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),
> + MEMBARRIER_CMD_PRIVATE_EXPEDITED = (1 << 3),
> + MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED = (1 << 4),
> };
>
> #endif /* _UAPI_LINUX_MEMBARRIER_H */
> diff --git a/init/Kconfig b/init/Kconfig
> index 78cb2461012e..a3dc6a66f0d1 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/fork.c b/kernel/fork.c
> index 10646182440f..bd4a93915e08 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1840,6 +1840,8 @@ static __latent_entropy struct task_struct *copy_process(
> */
> copy_seccomp(p);
>
> + membarrier_fork(p, clone_flags);
> +
> /*
> * Process group and session signals need to be delivered to just the
> * parent before the fork or both the parent and the child after the
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7977b25acf54..08095bb1cfe6 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2643,17 +2643,8 @@ static struct rq *finish_task_switch(struct task_struct *prev)
> */
> prev_state = prev->state;
> vtime_task_switch(prev);
> + membarrier_sched_in(prev, current);
> 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();
>
> @@ -3363,9 +3354,8 @@ static void __sched notrace __schedule(bool preempt)
> * care of this barrier. For weakly ordered machines for
> * which spin_unlock() acts as a RELEASE barrier (only
> * arm64 and PowerPC), arm64 has a full barrier in
> - * switch_to(), and PowerPC has
> - * smp_mb__after_unlock_lock() before
> - * finish_lock_switch().
> + * switch_to(), and PowerPC has a full barrier in
> + * membarrier_arch_sched_in().
> */
> ++*switch_count;
>
> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
> index a92fddc22747..00a2618a36ba 100644
> --- a/kernel/sched/membarrier.c
> +++ b/kernel/sched/membarrier.c
> @@ -26,21 +26,25 @@
> * except MEMBARRIER_CMD_QUERY.
> */
> #define MEMBARRIER_CMD_BITMASK \
> - (MEMBARRIER_CMD_SHARED | MEMBARRIER_CMD_PRIVATE_EXPEDITED)
> + (MEMBARRIER_CMD_SHARED | MEMBARRIER_CMD_PRIVATE_EXPEDITED \
> + | MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED)
>
> static void ipi_mb(void *info)
> {
> smp_mb(); /* IPIs should be serializing but paranoid. */
> }
>
> -static void membarrier_private_expedited(void)
> +static int membarrier_private_expedited(void)
> {
> int cpu;
> bool fallback = false;
> cpumask_var_t tmpmask;
>
> + if (!current->mm->membarrier_private_expedited)
> + return -EPERM;
> +
> if (num_online_cpus() == 1)
> - return;
> + return 0;
>
> /*
> * Matches memory barriers around rq->curr modification in
> @@ -94,6 +98,17 @@ static void membarrier_private_expedited(void)
> * rq->curr modification in scheduler.
> */
> smp_mb(); /* exit from system call is not a mb */
> + return 0;
> +}
> +
> +static void membarrier_register_private_expedited(void)
> +{
> + struct task_struct *p = current;
> +
> + if (READ_ONCE(p->mm->membarrier_private_expedited))
> + return;
> + WRITE_ONCE(p->mm->membarrier_private_expedited, 1);
> + membarrier_arch_register_private_expedited(p);
> }
>
> /**
> @@ -144,7 +159,9 @@ SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
> synchronize_sched();
> return 0;
> case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
> - membarrier_private_expedited();
> + return membarrier_private_expedited();
> + case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED:
> + membarrier_register_private_expedited();
> return 0;
> default:
> return -EINVAL;
> --
> 2.11.0
>
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