From mboxrd@z Thu Jan 1 00:00:00 1970 From: nicolas.pitre@linaro.org (Nicolas Pitre) Date: Thu, 7 Mar 2013 23:42:34 +0800 (HKT) Subject: [PATCH 00/16] big.LITTLE low-level CPU and cluster power management In-Reply-To: <20130307145106.GA7451@amd.pavel.ucw.cz> References: <1357777251-13541-1-git-send-email-nicolas.pitre@linaro.org> <20130307082721.GB28982@xo-6d-61-c0.localdomain> <20130307094017.GA6337@amd.pavel.ucw.cz> <20130307145106.GA7451@amd.pavel.ucw.cz> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Thu, 7 Mar 2013, Pavel Machek wrote: > Hi! > > > > > > I'd say: > > > > > > > > > > * expose all 8 cores. > > > > > > > > You may do that now. However the resulting power efficiency is far from > > > > optimal. > > > > > > Really? Assuming the lwn article is accurate, it will be exactly the > > > same. > > > > > > I propose exposing all 8 cores, then keeping 4 of them offline in > > > normal operation. This should have characteristics of your solution, > > > except that cpu does not change unexpectedly under the pinned tasks. > > > > > > > > * as long as scheduler is not ready, you can offline "the other" set of cores... > > > > > /sys/.../cpu/..../online > > > > > > > > You may do that now also. But system performance is far from optimal. > > > > > > Really? Do the offlining from the cpufreq code, and solution will be > > > equivalent to yours, except the hacks. > > > > No it is not equivalent. Offlining a CPU is a fairly heavy operation in > > Linux and you don't want to do that multiple times per second for > > example. Whereas the switcher "hack" is similar to a context switch in > > terms of software complexity. > > Well, why not. Logically, you _are_ saving state, offlining, onlining, > and restoring state, multiple times per second, already. Maybe "logically", but in practice there is no actual offlining/onlining but rather a migration. > Fast offline/online benefits everyone... Absolutely! You could ask tglx and cie where they are with their attempt at making hotplug less costly? > As for the hacks... what does /proc/cpuinfo show on your system? What > happens when process requests binding to specific cpu? I fear that > switching hack is easy now, but will have unnice impact on unexpected > places. > > BTW I guess this should be discussed on linux-kernel, not on the arm > list. There is nothing to discuss until I'm ready to show you the code. Nicolas