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Tue, 21 Jul 2020 15:07:00 +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 40027300446; Tue, 21 Jul 2020 17:06:56 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 2C5FB203B8783; Tue, 21 Jul 2020 17:06:56 +0200 (CEST) Date: Tue, 21 Jul 2020 17:06:56 +0200 From: peterz@infradead.org To: Nicholas Piggin Cc: Mathieu Desnoyers , 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: <20200721150656.GN119549@hirez.programming.kicks-ass.net> References: <1594868476.6k5kvx8684.astroid@bobo.none> <20200716085032.GO10769@hirez.programming.kicks-ass.net> <1594892300.mxnq3b9a77.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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1595324577.x3bf55tpgu.astroid@bobo.none> X-Rspamd-Queue-Id: 0E558182D5EED X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam04 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 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)'. 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? Also, I just realized, I still have a fix for use_mm() now kthread_use_mm() that seems to have been lost.