From mboxrd@z Thu Jan 1 00:00:00 1970 From: Glauber Costa Subject: Re: [PATCH v2 3/5] change number_of_cpusets to an atomic Date: Tue, 24 Apr 2012 13:30:05 -0300 Message-ID: <4F96D50D.4020804@parallels.com> References: <1335209867-1831-1-git-send-email-glommer@parallels.com> <1335209867-1831-4-git-send-email-glommer@parallels.com> <4F96D1A8.6040604@parallels.com> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1"; format=flowed Content-Transfer-Encoding: 7bit Cc: Tejun Heo , , , Li Zefan , , David Miller , To: Christoph Lameter Return-path: In-Reply-To: Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: netdev.vger.kernel.org On 04/24/2012 01:24 PM, Christoph Lameter wrote: > On Tue, 24 Apr 2012, Glauber Costa wrote: > >>> Would this not also be a good case to introduce static branching? >>> >>> number_of_cpusets is used to avoid going through unnecessary processing >>> should there be no cpusets in use. >> >> static branches comes with a set of problems themselves, so I usually prefer >> to use them only in places where we don't want to pay even a cache miss if we >> can avoid, or a function call, or anything like that - like the slub cache >> alloc as you may have seen in my kmem memcg series. >> >> It doesn't seem to be the case here. > > How did you figure that? number_of_cpusets was introduced exactly because > the functions are used in places where we do not pay the cost of calling > __cpuset_node_allowed_soft/hardwall. Have a look at these. They may take > locks etc etc in critical allocation paths I am not arguing that. You want to avoid the cost of processing a function, that's fair. (Note that by "function call cost" I don't mean the cost of processing a function, but the cost of a (potentially empty) function call.) The real question is: Are you okay with the cost of a branch + a global variable (which is almost read only) fetch? The test of a global variable can - and do as of right now - avoid all the expensive operations like locking, sleeping, etc, and if you don't need to squeeze every nanosecond you can, they are often simpler - and therefore better - than static branching. Just to mention one point I am coming across these days - that initiated all this: static patching holds the cpu_hotplug.lock. So it can't be called if you hold any lock that has been already held under the cpu_hotplug.lock. This will probably mean any lock the cpuset cgroup needs to take, because it is called - and to do a lot of things - from the cpu hotplug handler, that holds the cpu_hotplug.lock. So if if were a case of simple static branch usage, I am not opposed to it. But I foresee it getting so complicated, that a global variable seems to do the job we need just fine.