Messages in this thread | | | Date | Wed, 19 Nov 2008 14:11:57 -0800 | From | "Nish Aravamudan" <> | Subject | Re: Using cpusets for configuration/isolation [Was Re: RT sched: cpupri_vec lock contention with def_root_domain and no load balance] |
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Max,
On Tue, Nov 18, 2008 at 9:14 PM, Max Krasnyansky <maxk@qualcomm.com> wrote: > Nish Aravamudan wrote: >> On Tue, Nov 18, 2008 at 5:59 PM, Max Krasnyansky <maxk@qualcomm.com> wrote: >>> I do not see how 'partfs' that you described would be different from >>> 'cpusets' that we have now. Just ignore 'tasks' files in the cpusets and you >>> already have your 'partfs'. You do _not_ have to use cpuset for assigning >>> tasks if you do not want to. Just use them to define sets of cpus and keep >>> all the tasks in the 'root' set. You can then explicitly pin your threads >>> down with pthread_set_affinity(). >> >> I guess you're right. It still feels a bit kludgy, but that is probably just me. >> >> I have wondered, though, if it makes sense to provide an "isolated" >> file in /sys/devices/system/cpu/cpuX/ to do most of the offline >> sequence, break sched_domains and remove a CPU from the load balancer >> (rather than turning the load balancer off), rather than requiring a >> user to explicitly do an offline/online. > I do not see any benefits in exposing a special 'isolated' bit and have it do > the same thing that the cpu hotplug already does. As I explained in other > threads cpu hotplug is a _perfect_ fit for the isolation purposes. In order to > isolate a CPU dynamically (ie at runtime) we need to flush pending work, flush > chaches, move tasks and timers, etc. Which is _exactly_ what cpu hotplug code > does when it brings CPU down. There is no point in reimplementing it.
Ok, I guess I was just referring to the intent of the administrator and making it a bit clearer. But using syspart or even a simple script, it's easy enough to alias the offline/online sequence.
> btw It sounds like you misunderstood the meaning of the > cpuset.sched_load_balance flag. It's does not turn really turn load balancer > off, it simply causes cpus in different cpusets to be put into separate sched > domains. In other words it already does exactly what you're asking for.
Ok, I'm re-reading the cpusets.txt section. Sorry for my confusion and thanks for the clarification.
>> I guess it can all be rather >> transparently masked via a userspace tool, but we don't have a common >> one yet. > I do :). It's called 'syspart' > http://git.kernel.org/?p=linux/kernel/git/maxk/syspart.git;a=summary > I'll push an updated version in a couple of days.
Has there been any effort to start driving this into distributions?
>> I do have a question, though: is your recommendation to just turn the >> load balancer off in the cpuset you create that has the isolated CPUs? >> I guess the conceptual issue I was having was that the root cpuset (I >> think) always contains all CPUs and all memory nodes. So even if you >> put some CPUs in a cpuset under the root one, and isolate them using >> hotplug + disabling the load balancer in that cpuset, those CPUs are >> still available to tasks in the root cpuset? Maybe I'm just missing a >> step in the configuration, but it seems like as long as the global >> (root cpuset) load balancer is on, a CPU can't be guaranteed to stay >> isolated? > Take a look at what 'syspart' does. In short yes, of course we need to set > sched_load_balance flag in root cpuset to 0.
Will do, thanks, Nish
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