From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mathieu Desnoyers Subject: Re: [RFC PATCH 4/7] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode Date: Tue, 21 Jul 2020 11:15:13 -0400 (EDT) Message-ID: <616209816.22376.1595344513051.JavaMail.zimbra@efficios.com> References: <1594868476.6k5kvx8684.astroid@bobo.none> <20200716110038.GA119549@hirez.programming.kicks-ass.net> <1594906688.ikv6r4gznx.astroid@bobo.none> <1314561373.18530.1594993363050.JavaMail.zimbra@efficios.com> <1595213677.kxru89dqy2.astroid@bobo.none> <2055788870.20749.1595263590675.JavaMail.zimbra@efficios.com> <1595324577.x3bf55tpgu.astroid@bobo.none> <20200721150656.GN119549@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: Received: from mail.efficios.com ([167.114.26.124]:45222 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728089AbgGUPPP (ORCPT ); Tue, 21 Jul 2020 11:15:15 -0400 In-Reply-To: <20200721150656.GN119549@hirez.programming.kicks-ass.net> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Peter Zijlstra Cc: Nicholas Piggin , Anton Blanchard , Arnd Bergmann , Jens Axboe , linux-arch , linux-kernel , linux-mm , linuxppc-dev , Andy Lutomirski , Andy Lutomirski , x86 ----- 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