linux/kernel/sched/membarrier.c
Mathieu Desnoyers a961e40917 membarrier: Provide register expedited private command
This introduces a "register private expedited" membarrier command which
allows eventual removal of important memory barrier constraints on the
scheduler fast-paths. It changes how the "private expedited" membarrier
command (new to 4.14) is used from user-space.

This new command allows processes to register their intent to use the
private expedited command.  This affects how the expedited private
command introduced in 4.14-rc is meant to be used, and should be merged
before 4.14 final.

Processes are now required to register before using
MEMBARRIER_CMD_PRIVATE_EXPEDITED, otherwise that command returns EPERM.

This fixes a problem that arose when designing requested extensions to
sys_membarrier() to allow JITs to efficiently flush old code from
instruction caches.  Several potential algorithms are much less painful
if the user register intent to use this functionality early on, for
example, before the process spawns the second thread.  Registering at
this time removes the need to interrupt each and every thread in that
process at the first expedited sys_membarrier() system call.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2017-10-19 22:13:40 -04:00

179 lines
5.2 KiB
C

/*
* Copyright (C) 2010-2017 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
*
* 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 <linux/atomic.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 \
| MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED)
static void ipi_mb(void *info)
{
smp_mb(); /* IPIs should be serializing but paranoid. */
}
static int membarrier_private_expedited(void)
{
int cpu;
bool fallback = false;
cpumask_var_t tmpmask;
if (!(atomic_read(&current->mm->membarrier_state)
& MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY))
return -EPERM;
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 == 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 */
return 0;
}
static void membarrier_register_private_expedited(void)
{
struct task_struct *p = current;
struct mm_struct *mm = p->mm;
/*
* We need to consider threads belonging to different thread
* groups, which use the same mm. (CLONE_VM but not
* CLONE_THREAD).
*/
if (atomic_read(&mm->membarrier_state)
& MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY)
return;
atomic_or(MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY,
&mm->membarrier_state);
}
/**
* 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:
return membarrier_private_expedited();
case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED:
membarrier_register_private_expedited();
return 0;
default:
return -EINVAL;
}
}