Messages in this thread Patch in this message | | | Date | Thu, 24 Sep 2020 11:26:47 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH 2/3] sched: membarrier: cover kthread_use_mm (v2) |
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----- On Aug 24, 2020, at 10:06 PM, Boqun Feng boqun.feng@gmail.com wrote:
> 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@gmail.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@efficios.com> >> >> Cc: Peter Zijlstra (Intel) <peterz@infradead.org> >> >> Cc: Will Deacon <will@kernel.org> >> >> Cc: Paul E. McKenney <paulmck@kernel.org> >> >> Cc: Nicholas Piggin <npiggin@gmail.com> >> >> Cc: Andy Lutomirski <luto@amacapital.net> >> >> Cc: Andrew Morton <akpm@linux-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.
OK, so as we introduce IPIs to kthreads which are using kthread_use_mm, we need to reconsider how the scheduler deals with runqueues entering lazy TLB state. Currently, membarrier_switch_mm() is not issued when entering lazy TLB state. But as we start considering kthreads as candidates for sending IPIs, we need to update the rq->membarrier_state whenever we enter lazy TLB state as well.
So I plan to do this change to cover this:
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 84758f34cdb0..44521dc5602a 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3736,6 +3736,8 @@ context_switch(struct rq *rq, struct task_struct *prev, */ arch_start_context_switch(prev); + membarrier_switch_mm(rq, prev->mm, next->mm); + /* * kernel -> kernel lazy + transfer active * user -> kernel lazy + mmgrab() active @@ -3752,7 +3754,6 @@ context_switch(struct rq *rq, struct task_struct *prev, else prev->active_mm = NULL; } else { // to user - membarrier_switch_mm(rq, prev->active_mm, next->mm); /* * sys_membarrier() requires an smp_mb() between setting * rq->curr / membarrier_switch_mm() and returning to userspace. diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 3fd283892761..481149066086 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -2592,12 +2592,13 @@ static inline void membarrier_switch_mm(struct rq *rq, struct mm_struct *prev_mm, struct mm_struct *next_mm) { - int membarrier_state; + int membarrier_state = 0; if (prev_mm == next_mm) return; - membarrier_state = atomic_read(&next_mm->membarrier_state); + if (next_mm) + membarrier_state = atomic_read(&next_mm->membarrier_state); if (READ_ONCE(rq->membarrier_state) == membarrier_state) return;
Thoughts ? Thanks,
Mathieu
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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