Re: [RFC PATCH 4/7] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jul 21, 2020, at 11:06 AM, Peter Zijlstra peterz@infradead.org wrote:

> On Tue, Jul 21, 2020 at 08:04:27PM +1000, Nicholas Piggin wrote:
> 
>> That being said, the x86 sync core gap that I imagined could be fixed
>> by changing to rq->curr == rq->idle test does not actually exist because
>> the global membarrier does not have a sync core option. So fixing the
>> exit_lazy_tlb points that this series does *should* fix that. So
>> PF_KTHREAD may be less problematic than I thought from implementation
>> point of view, only semantics.
> 
> So I've been trying to figure out where that PF_KTHREAD comes from,
> commit 227a4aadc75b ("sched/membarrier: Fix p->mm->membarrier_state racy
> load") changed 'p->mm' to '!(p->flags & PF_KTHREAD)'.
> 
> So the first version:
> 
>  https://lkml.kernel.org/r/20190906031300.1647-5-mathieu.desnoyers@efficios.com
> 
> appears to unconditionally send the IPI and checks p->mm in the IPI
> context, but then v2:
> 
>  https://lkml.kernel.org/r/20190908134909.12389-1-mathieu.desnoyers@efficios.com
> 
> has the current code. But I've been unable to find the reason the
> 'p->mm' test changed into '!(p->flags & PF_KTHREAD)'.

Looking back at my inbox, it seems like you are the one who proposed to
skip all kthreads: 

https://lkml.kernel.org/r/20190904124333.GQ2332@hirez.programming.kicks-ass.net

> 
> The comment doesn't really help either; sure we have the whole lazy mm
> thing, but that's ->active_mm, not ->mm.
> 
> Possibly it is because {,un}use_mm() do not have sufficient barriers to
> make the remote p->mm test work? Or were we over-eager with the !p->mm
> doesn't imply kthread 'cleanups' at the time?

The nice thing about adding back kthreads to the threads considered for membarrier
IPI is that it has no observable effect on the user-space ABI. No pre-existing kthread
rely on this, and we just provide an additional guarantee for future kthread
implementations.

> Also, I just realized, I still have a fix for use_mm() now
> kthread_use_mm() that seems to have been lost.

I suspect we need to at least document the memory barriers in kthread_use_mm and
kthread_unuse_mm to state that they are required by membarrier if we want to
ipi kthreads as well.

Thanks,

Mathieu

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