linux-kernel - Re: [RFC PATCH 4/7] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before

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Date:   Tue, 21 Jul 2020 17:19:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        Anton Blanchard <anton@...abs.org>,
        Arnd Bergmann <arnd@...db.de>, Jens Axboe <axboe@...nel.dk>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Andy Lutomirski <luto@...capital.net>,
        Andy Lutomirski <luto@...nel.org>, x86 <x86@...nel.org>
Subject: Re: [RFC PATCH 4/7] x86: use exit_lazy_tlb rather than
 membarrier_mm_sync_core_before_usermode

On Tue, Jul 21, 2020 at 11:15:13AM -0400, Mathieu Desnoyers wrote:
> ----- On Jul 21, 2020, at 11:06 AM, Peter Zijlstra peterz@...radead.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

I had a feeling it might've been me ;-) I just couldn't find the email.

> > 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.

Right, so going by that email you found it was mostly a case of being
lazy, but yes, if we audit the kthread_{,un}use_mm() barriers and add
any other bits that might be needed, covering kthreads should be
possible.

No objections from me for making it so.