On Mon, Apr 13, 2015 at 05:46:37PM +0530, Preeti U Murthy wrote: > On 04/13/2015 12:31 PM, Peter Zijlstra wrote: > > Would it not make sense to make that a mount option and limit the amount > > of semantic variants of cpusets? > > I spent some time analyzing if this would be a better option than the > sysfs knob and I think not for the following reasons: > > 1. Mount options tend to be generic across the controllers of a cgroup. > But use case addressed by this patch is specific to the cpuset controller. Surely we can get around that somehow. > 2. The behavior that this patch is trying to bring about is not a > drastic one to call for a mount option equivalent to the __SANE_BEHAVIOR > one that existed earlier. This option was used to switch the legacy > design to the default one. > > However this patch is not *wholly* mimicking the default hierarchy > behavior. The behavior when cpusets become empty is left untouched for > instance. The patch borrows one of the behaviors from the default > hierarchy only and hence just not justify the use of a mount flag. So the 'problem' I have is that you introduce a 3rd semantic for the cpuset thing. You also do not answer if you can live with the default hierarchy behaviour, only that your patch mimicks a subset of it. Why not all of it? > 3. cpuset controller already has flags that allow a runtime change in > the semantics of scheduling, memory allocation. > Eg: CS_SCHED_LOAD_BALANCE, CS_SPREAD_PAGE. This page intends to bring > about a runtime change in the semantics of maintaining the allowed cpus > and mem masks in the face of hotplug. So adding a flag to take care of > this looks better and the design falls in place. s/page/patch/ I disagree, you change global semantics, the ONLY_ON_ROOT is a hint here. If such a file really is the only option, then at least take the entire semantics from default hierarchy, not only part of it. > 4. The worry was around possible race conditions when the user tries to > change this sysfs knob during runtime after mounting a cgroup hierarchy. > But the sites are protected by the cpuset_lock. Moreover all of the > cpuset behavior enforced by its flags can be changed after mounting the > cgroup. So cpuset controller already expects twiddling of these knobs at > will by the user. That just doesn't parse. cpuset_lock doesn't protect anything userspace. You cannot atomically read this flag and do something else. It being ONLY_ON_ROOT might mean you don't even have access to the flag. -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
