Messages in this thread | | | Date | Tue, 18 Oct 2016 11:07:47 +0200 | From | Peter Zijlstra <> | Subject | Re: [v4.8-rc1 Regression] sched/fair: Apply more PELT fixes |
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On Mon, Oct 17, 2016 at 11:52:39PM +0100, Dietmar Eggemann wrote: > > Something looks weird related to the use of for_each_possible_cpu(i) in > online_fair_sched_group() on my i5-3320M CPU (4 logical cpus). > > In case I print out cpu id and the cpu masks inside the for_each_possible_cpu(i) > I get: > > [ 5.462368] cpu=0 cpu_possible_mask=0-7 cpu_online_mask=0-3 cpu_present_mask=0-3 cpu_active_mask=0-3
OK, you have a buggy BIOS :-) It enumerates too many CPU slots. There is no reason to have 4 empty CPU slots on a machine that cannot do physical hotplug.
This also explains why it doesn't show on many machines, most machines will not have this and possible_mask == present_mask == online_mask for most all cases.
x86 folk, can we detect the lack of physical hotplug capability and FW_WARN on this and lowering possible_mask to present_mask?
> [ 5.462370] cpu=1 cpu_possible_mask=0-7 cpu_online_mask=0-3 cpu_present_mask=0-3 cpu_active_mask=0-3 > [ 5.462370] cpu=2 cpu_possible_mask=0-7 cpu_online_mask=0-3 cpu_present_mask=0-3 cpu_active_mask=0-3 > [ 5.462371] cpu=3 cpu_possible_mask=0-7 cpu_online_mask=0-3 cpu_present_mask=0-3 cpu_active_mask=0-3 > [ 5.462372] *cpu=4* cpu_possible_mask=0-7 cpu_online_mask=0-3 cpu_present_mask=0-3 cpu_active_mask=0-3 > [ 5.462373] *cpu=5* cpu_possible_mask=0-7 cpu_online_mask=0-3 cpu_present_mask=0-3 cpu_active_mask=0-3 > [ 5.462374] *cpu=6* cpu_possible_mask=0-7 cpu_online_mask=0-3 cpu_present_mask=0-3 cpu_active_mask=0-3 > [ 5.462375] *cpu=7* cpu_possible_mask=0-7 cpu_online_mask=0-3 cpu_present_mask=0-3 cpu_active_mask=0-3 > > T430:/sys/fs/cgroup/cpu,cpuacct/system.slice# ls -l | grep '^d' | wc -l > 80 > > /proc/sched_debug: > > cfs_rq[0]:/system.slice > ... > .tg_load_avg : 323584 > ... > > 80 * 1024 * 4 (not existent cpu4-cpu7) = 327680 (with a little bit of decay, > this could be this extra load on the systen.slice tg) > > Using for_each_online_cpu(i) instead of for_each_possible_cpu(i) in > online_fair_sched_group() works on this machine, i.e. the .tg_load_avg > of system.slice tg is 0 after startup.
Right, so the reason for using present_mask is that it avoids having to deal with hotplug, also all the per-cpu memory is allocated and present for !online CPUs anyway, so might as well set it up properly anyway.
(You might want to start booting your laptop with "possible_cpus=4" to save some memory FWIW.)
But yes, we have a bug here too... /me ponders
So aside from funny BIOSes, this should also show up when creating cgroups when you have offlined a few CPUs, which is far more common I'd think.
On IRC you mentioned that adding list_add_leaf_cfs_rq() to online_fair_sched_group() cures this, this would actually match with unregister_fair_sched_group() doing list_del_leaf_cfs_rq() and avoid a few instructions on the enqueue path, so that's all good.
I'm just not immediately seeing how that cures things. The only relevant user of the leaf_cfs_rq list seems to be update_blocked_averages() which is called from the balance code (idle_balance() and rebalance_domains()). But neither should call that for offline (or !present) CPUs.
Humm..
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