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 11:22:28 -0400 (EDT) Message-ID: <2141750915.22379.1595344948206.JavaMail.zimbra@efficios.com> References: <1594868476.6k5kvx8684.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> <616209816.22376.1595344513051.JavaMail.zimbra@efficios.com> <20200721151947.GD10769@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]:48708 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726436AbgGUPW3 (ORCPT ); Tue, 21 Jul 2020 11:22:29 -0400 In-Reply-To: <20200721151947.GD10769@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 11:19 AM, Peter Zijlstra peterz@infradead.org wrote: > On Tue, Jul 21, 2020 at 11:15:13AM -0400, Mathieu Desnoyers wrote: >> ----- 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 > > 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. I'm OK on making membarrier cover kthreads using mm as well, provided we audit kthread_{,un}use_mm() to make sure the proper barriers are in place after setting task->mm and before clearing it. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com