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Tue, 21 Jul 2020 15:19:49 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 76765301AC6; Tue, 21 Jul 2020 17:19:47 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 691E220107F23; Tue, 21 Jul 2020 17:19:47 +0200 (CEST) Date: Tue, 21 Jul 2020 17:19:47 +0200 From: Peter Zijlstra To: Mathieu Desnoyers Cc: Nicholas Piggin , Anton Blanchard , Arnd Bergmann , Jens Axboe , linux-arch , linux-kernel , linux-mm , linuxppc-dev , Andy Lutomirski , Andy Lutomirski , x86 Subject: Re: [RFC PATCH 4/7] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode Message-ID: <20200721151947.GD10769@hirez.programming.kicks-ass.net> 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> <616209816.22376.1595344513051.JavaMail.zimbra@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <616209816.22376.1595344513051.JavaMail.zimbra@efficios.com> X-Rspamd-Queue-Id: A38F41860B X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam03 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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.