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SubjectRe: [RFC PATCH] membarrier: expedited private command
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!

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.

> *
> * 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.

> + */
> +#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.

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
>

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