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SubjectRe: [RFC PATCH v2] membarrier: expedited private command
On Thu, Jul 27, 2017 at 10:41:25PM +0000, Mathieu Desnoyers wrote:
> ----- On Jul 27, 2017, at 6:13 PM, Paul E. McKenney paulmck@linux.vnet.ibm.com wrote:
>
> > On Thu, Jul 27, 2017 at 05:13:14PM -0400, Mathieu Desnoyers wrote:
> >> Implement MEMBARRIER_CMD_PRIVATE_EXPEDITED with IPIs using cpumask built
> >> from all runqueues for which current thread's mm is the same as the
> >> thread calling sys_membarrier.
> >>
> >> Scheduler-wise, it requires that we add a memory barrier after context
> >> switching between processes (which have different mm). Interestingly,
> >> there is already a memory barrier in mmdrop(), so we only need to add
> >> a barrier when switching from a kernel thread to a userspace thread.
> >> We also don't need to add the barrier when switching to a kernel thread,
> >> because it has no userspace memory mapping, which makes ordering of
> >> user-space memory accesses pretty much useless.
> >>
> >> * Benchmark
> >>
> >> A stress-test benchmark of sched pipe shows that it does not add
> >> significant overhead to the scheduler switching between processes:
> >>
> >> 100 runs of:
> >>
> >> taskset 01 ./perf bench sched pipe
> >>
> >> Running 'sched/pipe' benchmark:
> >> Executed 1000000 pipe operations between two processes
> >>
> >> Hardware: CPU: Intel(R) Xeon(R) CPU E5-2630 v3 @ 2.40GHz
> >>
> >> A) With 4.13.0-rc2+
> >> at commit a97fb594bc7d ("virtio-net: fix module unloading")
> >>
> >> avg.: 2.923 usecs/op
> >> std.dev: 0.057 usecs/op
> >>
> >> B) With this commit:
> >>
> >> avg.: 2.916 usecs/op
> >> std.dev: 0.043 usecs/op
> >>
> >> Changes since v1:
> >> - move membarrier code under kernel/sched/ because it uses the
> >> scheduler runqueue,
> >> - only add the barrier when we switch from a kernel thread. The case
> >> where we switch from a user-space thread is already handled by
> >> the atomic_dec_and_test() in mmdrop().
> >> - add a comment to mmdrop() documenting the requirement on the implicit
> >> memory barrier.
> >>
> >> 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>
> >> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> >
> > Looks much better, thank you!
> >
> > I have queued this in place of my earlier patch for the moment. If there
> > are no objections, I will push this into the upcoming v4.14 merge window.
> > If someone else wants to push it into v4.14, I am of course fine with
> > that, and am happy to give it a reviewed-by along the way. But if there
> > are objections to your patch (suitably modified based on additional
> > review and testing, of course) going into v4.14, I can always fall back
> > to pushing my earlier simpler but less housebroken patch. ;-)
>
> I'm fine about you picking up this patch, even though it's tagged "RFC".
> I'm sure concerns will have plenty of time to be voiced by others until
> it reaches mainline anyway, at which point I'll address them and resubmit
> new versions.

Works for me! My -rcu tree is subject to rebasing, so I can easily
replace the current patch with an updated one.

Thanx, Paul

> Thanks!
>
> Mathieu
>
> >
> > Thanx, Paul
> >
> >> ---
> >> MAINTAINERS | 2 +-
> >> include/linux/sched/mm.h | 5 +++
> >> include/uapi/linux/membarrier.h | 23 +++++++++++--
> >> kernel/Makefile | 1 -
> >> kernel/sched/Makefile | 1 +
> >> kernel/sched/core.c | 27 ++++++++++++++++
> >> kernel/{ => sched}/membarrier.c | 72 ++++++++++++++++++++++++++++++++++++++++-
> >> 7 files changed, 126 insertions(+), 5 deletions(-)
> >> rename kernel/{ => sched}/membarrier.c (59%)
> >>
> >> diff --git a/MAINTAINERS b/MAINTAINERS
> >> index f66488dfdbc9..3b035584272f 100644
> >> --- a/MAINTAINERS
> >> +++ b/MAINTAINERS
> >> @@ -8621,7 +8621,7 @@ M: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> >> M: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> >> L: linux-kernel@vger.kernel.org
> >> S: Supported
> >> -F: kernel/membarrier.c
> >> +F: kernel/sched/membarrier.c
> >> F: include/uapi/linux/membarrier.h
> >>
> >> MEMORY MANAGEMENT
> >> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> >> index 2b24a6974847..5c5384d9ae0f 100644
> >> --- a/include/linux/sched/mm.h
> >> +++ b/include/linux/sched/mm.h
> >> @@ -38,6 +38,11 @@ static inline void mmgrab(struct mm_struct *mm)
> >> extern void __mmdrop(struct mm_struct *);
> >> static inline void mmdrop(struct mm_struct *mm)
> >> {
> >> + /*
> >> + * Implicit full memory barrier provided by
> >> + * atomic_dec_and_test() is required by membarrier. See comments
> >> + * around membarrier_expedited_mb_after_set_current().
> >> + */
> >> if (unlikely(atomic_dec_and_test(&mm->mm_count)))
> >> __mmdrop(mm);
> >> }
> >> diff --git a/include/uapi/linux/membarrier.h b/include/uapi/linux/membarrier.h
> >> index e0b108bd2624..6d47b3249d8a 100644
> >> --- a/include/uapi/linux/membarrier.h
> >> +++ b/include/uapi/linux/membarrier.h
> >> @@ -40,14 +40,33 @@
> >> * (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_PRIVATE_EXPEDITED:
> >> + * Execute a memory barrier on each running
> >> + * thread belonging to the same process as the current
> >> + * thread. Upon return from system call, the
> >> + * caller thread is ensured that all its running
> >> + * threads siblings 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 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 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),
> >> };
> >>
> >> #endif /* _UAPI_LINUX_MEMBARRIER_H */
> >> diff --git a/kernel/Makefile b/kernel/Makefile
> >> index 4cb8e8b23c6e..9c323a6daa46 100644
> >> --- a/kernel/Makefile
> >> +++ b/kernel/Makefile
> >> @@ -108,7 +108,6 @@ obj-$(CONFIG_CRASH_DUMP) += crash_dump.o
> >> obj-$(CONFIG_JUMP_LABEL) += jump_label.o
> >> obj-$(CONFIG_CONTEXT_TRACKING) += context_tracking.o
> >> obj-$(CONFIG_TORTURE_TEST) += torture.o
> >> -obj-$(CONFIG_MEMBARRIER) += membarrier.o
> >>
> >> obj-$(CONFIG_HAS_IOMEM) += memremap.o
> >>
> >> diff --git a/kernel/sched/Makefile b/kernel/sched/Makefile
> >> index 53f0164ed362..78f54932ea1d 100644
> >> --- a/kernel/sched/Makefile
> >> +++ b/kernel/sched/Makefile
> >> @@ -25,3 +25,4 @@ obj-$(CONFIG_SCHED_DEBUG) += debug.o
> >> obj-$(CONFIG_CGROUP_CPUACCT) += cpuacct.o
> >> obj-$(CONFIG_CPU_FREQ) += cpufreq.o
> >> obj-$(CONFIG_CPU_FREQ_GOV_SCHEDUTIL) += cpufreq_schedutil.o
> >> +obj-$(CONFIG_MEMBARRIER) += membarrier.o
> >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> >> index 17c667b427b4..01e3b881ab3a 100644
> >> --- a/kernel/sched/core.c
> >> +++ b/kernel/sched/core.c
> >> @@ -2724,6 +2724,32 @@ asmlinkage __visible void schedule_tail(struct
> >> task_struct *prev)
> >> put_user(task_pid_vnr(current), current->set_child_tid);
> >> }
> >>
> >> +static void membarrier_expedited_mb_after_set_current(struct mm_struct *mm,
> >> + struct mm_struct *oldmm)
> >> +{
> >> + if (!IS_ENABLED(CONFIG_MEMBARRIER))
> >> + return;
> >> + /*
> >> + * __schedule()->
> >> + * finish_task_switch()->
> >> + * if (mm)
> >> + * mmdrop(mm) ->
> >> + * atomic_dec_and_test()
> >> + * takes care of issuing a memory barrier when oldmm is
> >> + * non-NULL. We also don't need the barrier when switching to a
> >> + * kernel thread, nor when we switch between threads belonging
> >> + * to the same process.
> >> + */
> >> + if (likely(oldmm || !mm || mm == oldmm))
> >> + return;
> >> + /*
> >> + * When switching between processes, membarrier expedited
> >> + * private requires a memory barrier after we set the current
> >> + * task.
> >> + */
> >> + smp_mb();
> >> +}
> >> +
> >> /*
> >> * context_switch - switch to the new MM and the new thread's register state.
> >> */
> >> @@ -2737,6 +2763,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
> >>
> >> mm = next->mm;
> >> oldmm = prev->active_mm;
> >> + membarrier_expedited_mb_after_set_current(mm, oldmm);
> >> /*
> >> * For paravirt, this is coupled with an exit in switch_to to
> >> * combine the page table reload and the switch backend into
> >> diff --git a/kernel/membarrier.c b/kernel/sched/membarrier.c
> >> similarity index 59%
> >> rename from kernel/membarrier.c
> >> rename to kernel/sched/membarrier.c
> >> index 9f9284f37f8d..f80828b0b607 100644
> >> --- a/kernel/membarrier.c
> >> +++ b/kernel/sched/membarrier.c
> >> @@ -17,12 +17,79 @@
> >> #include <linux/syscalls.h>
> >> #include <linux/membarrier.h>
> >> #include <linux/tick.h>
> >> +#include <linux/cpumask.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)
> >> +#define MEMBARRIER_CMD_BITMASK \
> >> + (MEMBARRIER_CMD_SHARED | MEMBARRIER_CMD_PRIVATE_EXPEDITED)
> >> +
> >> +static void ipi_mb(void *info)
> >> +{
> >> + smp_mb(); /* IPIs should be serializing but paranoid. */
> >> +}
> >> +
> >> +static void membarrier_private_expedited(void)
> >> +{
> >> + int cpu, this_cpu;
> >> + bool fallback = false;
> >> + cpumask_var_t tmpmask;
> >> +
> >> + if (num_online_cpus() == 1)
> >> + return;
> >> +
> >> + /*
> >> + * Matches memory barriers around rq->curr modification in
> >> + * scheduler.
> >> + */
> >> + smp_mb(); /* system call entry is not a mb. */
> >> +
> >> + if (!alloc_cpumask_var(&tmpmask, GFP_NOWAIT)) {
> >> + /* Fallback for OOM. */
> >> + fallback = true;
> >> + }
> >> +
> >> + /*
> >> + * 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.
> >> + */
> >> + this_cpu = raw_smp_processor_id();
> >> + cpus_read_lock();
> >> + for_each_online_cpu(cpu) {
> >> + struct task_struct *p;
> >> +
> >> + if (cpu == this_cpu)
> >> + 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();
> >> + }
> >> + cpus_read_unlock();
> >> + if (!fallback) {
> >> + smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
> >> + free_cpumask_var(tmpmask);
> >> + }
> >> +
> >> + /*
> >> + * 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 */
> >> +}
> >>
> >> /**
> >> * sys_membarrier - issue memory barriers on a set of threads
> >> @@ -64,6 +131,9 @@ SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
> >> if (num_online_cpus() > 1)
> >> synchronize_sched();
> >> return 0;
> >> + case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
> >> + membarrier_private_expedited();
> >> + return 0;
> >> default:
> >> return -EINVAL;
> >> }
> >> --
> >> 2.11.0
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
>

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