From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from e06smtp12.uk.ibm.com (e06smtp12.uk.ibm.com [195.75.94.108]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 1A0A91A0BC5 for ; Fri, 28 Nov 2014 18:35:04 +1100 (AEDT) Received: from /spool/local by e06smtp12.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Fri, 28 Nov 2014 07:35:00 -0000 Received: from b06cxnps4076.portsmouth.uk.ibm.com (d06relay13.portsmouth.uk.ibm.com [9.149.109.198]) by d06dlp01.portsmouth.uk.ibm.com (Postfix) with ESMTP id D1C3217D8056 for ; Fri, 28 Nov 2014 07:35:15 +0000 (GMT) Received: from d06av04.portsmouth.uk.ibm.com (d06av04.portsmouth.uk.ibm.com [9.149.37.216]) by b06cxnps4076.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id sAS7YxDS17825960 for ; Fri, 28 Nov 2014 07:34:59 GMT Received: from d06av04.portsmouth.uk.ibm.com (localhost [127.0.0.1]) by d06av04.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id sAS7YuVx024003 for ; Fri, 28 Nov 2014 00:34:58 -0700 Date: Fri, 28 Nov 2014 08:34:54 +0100 From: David Hildenbrand To: Thomas Gleixner Subject: Re: [RFC 0/2] Reenable might_sleep() checks for might_fault() when atomic Message-ID: <20141128083454.403d5620@thinkpad-w530> In-Reply-To: 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. Ah right, it's per cpu on x86. So it really belongs to a thread if we want to demangle preemption and pagefault_disable. Would work for now, but for x86 not on the long run. > > 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. Thinking about it, this makes perfect sense! > > 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? If we're allowed to increase the size of thread_info - absolutely fine with me! (I am not sure if some archs have special constraints on the size) Will see what I can come up with. Thanks! > > Thanks, > > tglx >