From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from Galois.linutronix.de (Galois.linutronix.de [IPv6:2001:470:1f0b:db:abcd:42:0:1]) (using TLSv1.2 with cipher DHE-RSA-AES128-SHA (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 6F0AA1A09F3 for ; Fri, 28 Nov 2014 08:52:20 +1100 (AEDT) Date: Thu, 27 Nov 2014 22:52:08 +0100 (CET) From: Thomas Gleixner To: David Hildenbrand Subject: Re: [RFC 0/2] Reenable might_sleep() checks for might_fault() when atomic In-Reply-To: <20141127161905.7c6220ee@thinkpad-w530> Message-ID: References: <20141126151729.GB9612@redhat.com> <20141126152334.GA9648@redhat.com> <20141126163207.63810fcb@thinkpad-w530> <20141126154717.GB10568@redhat.com> <5475FAB1.1000802@de.ibm.com> <20141126163216.GB10850@redhat.com> <547604FC.4030300@de.ibm.com> <20141126170447.GC11202@redhat.com> <20141127070919.GA4390@osiris> <20141127090301.3ddc3077@thinkpad-w530> <20141127120441.GB4390@osiris> <20141127161905.7c6220ee@thinkpad-w530> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Cc: linux-arch@vger.kernel.org, "Michael S. Tsirkin" , Heiko Carstens , linux-kernel@vger.kernel.org, Christian Borntraeger , paulus@samba.org, schwidefsky@de.ibm.com, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org, mingo@kernel.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, 27 Nov 2014, David Hildenbrand wrote: > > OTOH, there is no reason why we need to disable preemption over that > > page_fault_disabled() region. There are code pathes which really do > > not require to disable preemption for that. > > > > We have that seperated in preempt-rt for obvious reasons and IIRC > > Peter Zijlstra tried to distangle it in mainline some time ago. I > > forgot why that never got merged. > > > > Of course, we can completely separate that in our page fault code by doing > pagefault_disabled() checks instead of in_atomic() checks (even in add on > patches later). > > > We tie way too much stuff on the preemption count already, which is a > > mightmare because we have no clear distinction of protection > > scopes. > > Although it might not be optimal, but keeping a separate counter for > pagefault_disable() as part of the preemption counter seems to be the only > doable thing right now. It needs to be seperate, if it should be useful. Otherwise we just have a extra accounting in preempt_count() which does exactly the same thing as we have now: disabling preemption. Now you might say, that we could mask out that part when checking preempt_count, but that wont work on x86 as x86 has the preempt counter as a per cpu variable and not as a per thread one. But if you want to distangle pagefault disable from preempt disable then you must move it to the thread, because it is a property of the thread. preempt count is very much a per cpu counter as you can only go through schedule when it becomes 0. Btw, I find the x86 representation way more clear, because it documents that preempt count is a per cpu BKL and not a magic thread property. And sadly that is how preempt count is used ... > I am not sure if a completely separated counter is even possible, > increasing the size of thread_info. And adding a ulong to thread_info is going to create exactly which problem? Thanks, tglx