From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pb0-f44.google.com (mail-pb0-f44.google.com [209.85.160.44]) by kanga.kvack.org (Postfix) with ESMTP id CC05F6B006E for ; Wed, 2 Oct 2013 11:17:54 -0400 (EDT) Received: by mail-pb0-f44.google.com with SMTP id xa7so1024301pbc.3 for ; Wed, 02 Oct 2013 08:17:54 -0700 (PDT) Date: Wed, 2 Oct 2013 17:17:34 +0200 From: Peter Zijlstra Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus() Message-ID: <20131002151734.GT3081@twins.programming.kicks-ass.net> References: <7632387.20FXkuCITr@vostro.rjw.lan> <524B0233.8070203@linux.vnet.ibm.com> <20131001173615.GW3657@laptop.programming.kicks-ass.net> <20131001174508.GA17411@redhat.com> <20131001175640.GQ15690@laptop.programming.kicks-ass.net> <20131001180750.GA18261@redhat.com> <20131002090859.GE12926@twins.programming.kicks-ass.net> <20131002121356.GA21581@redhat.com> <20131002133137.GG28601@twins.programming.kicks-ass.net> <20131002140020.GA25256@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131002140020.GA25256@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: Oleg Nesterov Cc: "Srivatsa S. Bhat" , "Rafael J. Wysocki" , "Paul E. McKenney" , Mel Gorman , Rik van Riel , Srikar Dronamraju , Ingo Molnar , Andrea Arcangeli , Johannes Weiner , Linux-MM , LKML , Thomas Gleixner , Steven Rostedt , Viresh Kumar On Wed, Oct 02, 2013 at 04:00:20PM +0200, Oleg Nesterov wrote: > And again, even > > for (;;) { > percpu_down_write(); > percpu_up_write(); > } > > should not completely block the readers. Sure there's a tiny window, but don't forget that a reader will have to wait for the gp_state cacheline to transfer to shared state and the per-cpu refcount cachelines to be brought back into exclusive mode and the above can be aggressive enough that by that time we'll observe state == blocked again. So I don't think that in practise a reader will get in. Also, since the write side is exposed to userspace; you've got an effective DoS. So I'll stick to waitcount -- as you can see in the patches I've just posted. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org