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SubjectRe: [RFC PATCH] membarrier: expedited private command

----- On Jul 27, 2017, at 4:06 PM, Paul E. McKenney paulmck@linux.vnet.ibm.com wrote:

> On Thu, Jul 27, 2017 at 02:59:43PM -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 our own.
>>
>> Scheduler-wise, it requires that we add a memory barrier after context
>> switching between processes (which have different mm).
>>
>> It would be interesting to benchmark the overhead of this added barrier
>> on the performance of context switching between processes. If the
>> preexisting overhead of switching between mm is high enough, the
>> overhead of adding this extra barrier may be insignificant.
>
> I sent this along to the people asking for faster sys_membarrier(),
> CCing you, thank you!

Thanks! More below,

>
> Please see other feedback inline below.
>
>> [ Compile-tested only! ]
>>
>> CC: Peter Zijlstra <peterz@infradead.org>
>> CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>> CC: Boqun Feng <boqun.feng@gmail.com>
>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> ---
>> include/uapi/linux/membarrier.h | 8 +++--
>> kernel/membarrier.c | 76 ++++++++++++++++++++++++++++++++++++++++-
>> kernel/sched/core.c | 21 ++++++++++++
>> 3 files changed, 102 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/uapi/linux/membarrier.h b/include/uapi/linux/membarrier.h
>> index e0b108bd2624..6a33c5852f6b 100644
>> --- a/include/uapi/linux/membarrier.h
>> +++ b/include/uapi/linux/membarrier.h
>> @@ -40,14 +40,18 @@
>> * (non-running threads are de facto in such a
>> * state). This covers threads from all processes
>> * running on the system. This command returns 0.
>> + * TODO: documentation.
>
> How about something like this?
>
> * @MEMBARRIER_CMD_PRIVATE_EXPEDITED: IPI each CPU running a thread belonging
> * to the same process as the current thread, causing
> * the recipient CPU to execute a full memory-barrier
> * instruction. The same-process determination is
> * made using the task_struct ->mm field: If some
> * other CPU's currently running task has the same
> * value in its ->mm field as the requesting thread,
> * that CPU is IPIed.
>

I would prefer:

* @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.
mainly because this is a uapi/ header, installed into distribution headers. So
I want to remove any implementation reference from that text.


>> *
>> * 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/membarrier.c b/kernel/membarrier.c
>> index 9f9284f37f8d..8c6c0f96f617 100644
>> --- a/kernel/membarrier.c
>> +++ b/kernel/membarrier.c
>> @@ -19,10 +19,81 @@
>> #include <linux/tick.h>
>>
>> /*
>> + * XXX For cpu_rq(). Should we rather move
>> + * membarrier_private_expedited() to sched/core.c or create
>> + * sched/membarrier.c ?
>
> Any reason not to move the entirety of membarrier.c there? Assuming that
> Peter is OK with this, of course.

I'm planning to do exactly this for v2.

>
>> + */
>> +#include "sched/sched.h"
>> +
>> +/*
>> * 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. */
>
> I am good with paranoia! ;-)
>
>> +}
>> +
>> +static void membarrier_private_expedited_ipi_each(void)
>> +{
>> + int cpu;
>> +
>> + for_each_online_cpu(cpu) {
>> + struct task_struct *p;
>> +
>> + rcu_read_lock();
>> + p = task_rcu_dereference(&cpu_rq(cpu)->curr);
>> + if (p && p->mm == current->mm)
>> + smp_call_function_single(cpu, ipi_mb, NULL, 1);
>> + rcu_read_unlock();
>> + }
>> +}
>> +
>> +static void membarrier_private_expedited(void)
>> +{
>> + int cpu, this_cpu;
>> + 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. */
>> + membarrier_private_expedited_ipi_each();
>> + goto end;
>> + }
>> +
>> + this_cpu = raw_smp_processor_id();
>> + 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)
>> + __cpumask_set_cpu(cpu, tmpmask);
>> + rcu_read_unlock();
>> + }
>> + smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
>> + free_cpumask_var(tmpmask);
>> +end:
>> + /*
>> + * 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 +135,9 @@ 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;
>> }
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 17c667b427b4..f171d2aaaf82 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2724,6 +2724,26 @@ asmlinkage __visible void schedule_tail(struct
>> task_struct *prev)
>> put_user(task_pid_vnr(current), current->set_child_tid);
>> }
>>
>> +#ifdef CONFIG_MEMBARRIER
>> +static void membarrier_expedited_mb_after_set_current(struct mm_struct *mm,
>> + struct mm_struct *oldmm)
>> +{
>> + if (likely(mm == oldmm))
>> + return; /* Thread context switch, same mm. */
>> + /*
>> + * When switching between processes, membarrier expedited
>> + * private requires a memory barrier after we set the current
>> + * task.
>> + */
>> + smp_mb();
>> +}
>> +#else /* #ifdef CONFIG_MEMBARRIER */
>> +static void membarrier_expedited_mb_after_set_current(struct mm_struct *mm,
>> + struct mm_struct *oldmm)
>> +{
>> +}
>> +#endif /* #else #ifdef CONFIG_MEMBARRIER */
>
> Why not something like this? Shorter, easier to read, and should generate
> the same code.

Sure, I'll use this in my v2.

Thanks!

Mathieu

>
> static void membarrier_expedited_mb_after_set_current(struct mm_struct *mm,
> struct mm_struct *oldmm)
> {
> if (!IS_ENABLED(CONFIG_MEMBARRIER))
> return;
> if (likely(mm == oldmm))
> return; /* Thread context switch, same mm. */
> /*
> * 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 +2757,7 @@ 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
>> --
>> 2.11.0

--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

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