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Message-ID: <20200825020640.GA72178@debian-boqun.qqnc3lrjykvubdpftowmye0fmh.lx.internal.cloudapp.net>
Date: Tue, 25 Aug 2020 10:06:40 +0800
From: Boqun Feng <boqun.feng@...il.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Will Deacon <will@...nel.org>, paulmck <paulmck@...nel.org>,
Andy Lutomirski <luto@...capital.net>,
Andrew Morton <akpm@...ux-foundation.org>,
Alan Stern <stern@...land.harvard.edu>,
Nicholas Piggin <npiggin@...il.com>
Subject: Re: [RFC PATCH 2/3] sched: membarrier: cover kthread_use_mm (v2)
On Mon, Aug 24, 2020 at 11:27:49AM -0400, Mathieu Desnoyers wrote:
> ----- On Aug 16, 2020, at 11:29 AM, Boqun Feng boqun.feng@...il.com wrote:
>
> > On Fri, Aug 14, 2020 at 12:43:57PM -0400, Mathieu Desnoyers wrote:
> >> Add comments and memory barrier to kthread_use_mm and kthread_unuse_mm
> >> to allow the effect of membarrier(2) to apply to kthreads accessing
> >> user-space memory as well.
> >>
> >> Given that no prior kthread use this guarantee and that it only affects
> >> kthreads, adding this guarantee does not affect user-space ABI.
> >>
> >> Refine the check in membarrier_global_expedited to exclude runqueues
> >> running the idle thread rather than all kthreads from the IPI cpumask.
> >>
> >> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> >> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> >> Cc: Will Deacon <will@...nel.org>
> >> Cc: Paul E. McKenney <paulmck@...nel.org>
> >> Cc: Nicholas Piggin <npiggin@...il.com>
> >> Cc: Andy Lutomirski <luto@...capital.net>
> >> Cc: Andrew Morton <akpm@...ux-foundation.org>
> >> ---
> >> Changes since v1:
> >> - Add WARN_ON_ONCE(current->mm) in play_idle_precise (PeterZ),
> >> - Use smp_mb__after_spinlock rather than smp_mb after task_lock.
> >> ---
> >> kernel/kthread.c | 19 +++++++++++++++++++
> >> kernel/sched/idle.c | 1 +
> >> kernel/sched/membarrier.c | 8 ++------
> >> 3 files changed, 22 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/kernel/kthread.c b/kernel/kthread.c
> >> index 3edaa380dc7b..77aaaa7bc8d9 100644
> >> --- a/kernel/kthread.c
> >> +++ b/kernel/kthread.c
> >> @@ -1255,8 +1255,19 @@ void kthread_use_mm(struct mm_struct *mm)
> >> finish_arch_post_lock_switch();
> >> #endif
> >>
> >> + /*
> >> + * When a kthread starts operating on an address space, the loop
> >> + * in membarrier_{private,global}_expedited() may not observe
> >> + * that tsk->mm, and not issue an IPI. Membarrier requires a
> >> + * memory barrier after storing to tsk->mm, before accessing
> >> + * user-space memory. A full memory barrier for membarrier
> >> + * {PRIVATE,GLOBAL}_EXPEDITED is implicitly provided by
> >> + * mmdrop(), or explicitly with smp_mb().
> >> + */
> >> if (active_mm != mm)
> >> mmdrop(active_mm);
> >> + else
> >> + smp_mb();
> >
> > Similar question here: could smp_mb() guarantee the correctness of
> > GLOBAL_EXPEDITED? Don't you need membarrier_switch_mm() here and in
> > kthread_unuse_mm(), too?
> >
> > Am I miss something here?
>
> I think you have a good point there. Which brings me to wonder why we
> don't have membarrier_switch_mm() when entering/leaving lazy TLB mode.
> This means an IPI can be sent to a kthread even if it does not use an
> mm, just because the membarrier state in the runqueue is that of the
> active_mm.
>
> Thoughts ?
>
Right, I think we should also handle the percpu membarrier_state. The
basic rule is whenever we change current->mm or current (i.e. rq->curr)
itself, we need to update the percpu membarrier_state accordingly.
Regards,
Boqun
> Thanks,
>
> Mathieu
>
> >
> > Regards,
> > Boqun
> >
> >>
> >> to_kthread(tsk)->oldfs = force_uaccess_begin();
> >> }
> >> @@ -1276,6 +1287,14 @@ void kthread_unuse_mm(struct mm_struct *mm)
> >> force_uaccess_end(to_kthread(tsk)->oldfs);
> >>
> >> task_lock(tsk);
> >> + /*
> >> + * When a kthread stops operating on an address space, the loop
> >> + * in membarrier_{private,global}_expedited() may not observe
> >> + * that tsk->mm, and not issue an IPI. Membarrier requires a
> >> + * memory barrier after accessing user-space memory, before
> >> + * clearing tsk->mm.
> >> + */
> >> + smp_mb__after_spinlock();
> >> sync_mm_rss(mm);
> >> local_irq_disable();
> >> tsk->mm = NULL;
> >> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> >> index 6bf34986f45c..3443ee8335d0 100644
> >> --- a/kernel/sched/idle.c
> >> +++ b/kernel/sched/idle.c
> >> @@ -341,6 +341,7 @@ void play_idle_precise(u64 duration_ns, u64 latency_ns)
> >> WARN_ON_ONCE(!(current->flags & PF_KTHREAD));
> >> WARN_ON_ONCE(!(current->flags & PF_NO_SETAFFINITY));
> >> WARN_ON_ONCE(!duration_ns);
> >> + WARN_ON_ONCE(current->mm);
> >>
> >> rcu_sleep_check();
> >> preempt_disable();
> >> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
> >> index 168479a7d61b..8a294483074d 100644
> >> --- a/kernel/sched/membarrier.c
> >> +++ b/kernel/sched/membarrier.c
> >> @@ -100,13 +100,9 @@ static int membarrier_global_expedited(void)
> >> MEMBARRIER_STATE_GLOBAL_EXPEDITED))
> >> continue;
> >>
> >> - /*
> >> - * Skip the CPU if it runs a kernel thread. The scheduler
> >> - * leaves the prior task mm in place as an optimization when
> >> - * scheduling a kthread.
> >> - */
> >> + /* Skip the CPU if it runs the idle thread. */
> >> p = rcu_dereference(cpu_rq(cpu)->curr);
> >> - if (p->flags & PF_KTHREAD)
> >> + if (is_idle_task(p))
> >> continue;
> >>
> >> __cpumask_set_cpu(cpu, tmpmask);
> >> --
> >> 2.11.0
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
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