Why does Linux's scheduler put two threads onto the same physical core on processors with HyperThreading?

Question

I've read in multiple places that Linux's default scheduler is hyperthreading aware on multi-core machines, meaning that if you have a machine with 2 real cores (4 HT), it won't schedule two busy threads onto logical cores in a way that they both run on the same physical cores (which would lead to 2x performance cost in many cases).

But when I run stress -c 2 (spawns two threads to run on 100% CPU) on my Intel i5-2520M, it often schedules (and keeps) the two threads onto HT cores 1 and 2, which map to the same physical core. Even if the system is idle otherwise.

This also happens with real programs (I'm using stress here because it makes it easy to reproduce), and when that happens, my program understandably takes twice as long to run. Setting affinity manually with taskset fixes that for my program, but I'd expect the a HT aware scheduler to do that correctly by itself.

You can find the HT->physical core assgnment with egrep "processor|physical id|core id" /proc/cpuinfo | sed 's/^processor/\nprocessor/g'.

So my question is: Why does the scheduler put my threads onto the same physical core here?

Notes:

This question is very similar to this other question, the answers to which say that Linux has quite a sophisticated thread scheduler which is HT aware. As described above, I cannot observe this fact (check for yourself with stress -c), and would like to know why.
I know that I can set processors affinity manually for my programs, e.g. with the taskset tool or with the sched_setaffinity function. This is not what I'm looking for, I would expect the scheduler to know by itself that mapping two busy threads to a physical core and leaving one physical core completely empty is not a good idea.
I'm aware that there are some situations in which you would prefer threads to be scheduled onto the same physical core and leave the other core free, but it seems nonsensical that the scheduler would do that roughly 1/4 of the cases. It seems to me that the HT cores that it picks are completely random, or maybe those HT cores that had least activity at the time of scheduling, but that wouldn't be very hyperthreading aware, given how clearly programs with the characteristics of stress benefit from running on separate physical cores.

Try running stress in two processes with one thread each. I haven't looked into the specifics of the Linux scheduler (which may have even changed since the last time I was researching it). It's possible that the kernel prefers to schedule threads in the same process on the same physical processor for reasons like cache-locality. — joshperry
@joshperry "It's possible that the kernel prefers to schedule threads in the same process on the same physical processor" <- it does both, sometimes it schedules them as I expect sometimes, the other way, it seems random and not be biased to either. — nh2
Linux tracks HT threads (and last-level caches and NUMA nodes) via scheduler domains. Please show what awk '/^domain/ { print $1, $2; } /^cpu/ { print $1; }' /proc/schedstat prints, it will show CPU masks for scheduler domains. — myaut

myaut myaut · Accepted Answer · 2015-04-12T08:43:54

I think it's time to summarize some knowledge from comments.

Linux scheduler is aware of HyperThreading -- information about it should be read from ACPI SRAT/SLIT tables, which are provided by BIOS/UEFI -- than Linux builds scheduler domains from that.

Domains have hierarchy -- i.e. on 2-CPU servers you will get three layers of domains: all-cpus, per-cpu-package, and per-cpu-core domain. You may check it from /proc/schedstat:

$ awk '/^domain/ { print $1, $2; } /^cpu/ { print $1; }' /proc/schedstat
cpu0
domain0 0000,00001001     <-- all cpus from core 0
domain1 0000,00555555     <-- all cpus from package 0
domain2 0000,00ffffff     <-- all cpus in the system

Part of CFS scheduler is load balancer -- the beast that should steal tasks from your busy core to another core. Here are its description from the Kernel documentation:

While doing that, it checks to see if the current domain has exhausted its rebalance interval. If so, it runs load_balance() on that domain. It then checks the parent sched_domain (if it exists), and the parent of the parent and so forth.

Initially, load_balance() finds the busiest group in the current sched domain. If it succeeds, it looks for the busiest runqueue of all the CPUs' runqueues in that group. If it manages to find such a runqueue, it locks both our initial CPU's runqueue and the newly found busiest one and starts moving tasks from it to our runqueue. The exact number of tasks amounts to an imbalance previously computed while iterating over this sched domain's groups.

From: https://www.kernel.org/doc/Documentation/scheduler/sched-domains.txt

You can monitor for activities of load balancer by comparing numbers in /proc/schedstat. I wrote a script for doing that: schedstat.py

Counter alb_pushed shows that load balancer was successfully moved out task:

Sun Apr 12 14:15:52 2015              cpu0    cpu1    ...    cpu6    cpu7    cpu8    cpu9    cpu10   ...
.domain1.alb_count                                    ...      1       1                       1  
.domain1.alb_pushed                                   ...      1       1                       1  
.domain2.alb_count                              1     ...                                         
.domain2.alb_pushed                             1     ...

However, logic of load balancer is complex, so it is hard to determine what reasons can stop it from doing its work well and how they are related with schedstat counters. Neither me nor @thatotherguy can reproduce your issue.

I see two possibilities for that behavior:

You have some aggressive power saving policy that tries to save one core to reduce power consumption of CPU.
You really encountered a bug with scheduling subsystem, than you should go to LKML and carefully share your findings (including mpstat and schedstat data)

Why does Linux's scheduler put two threads onto the same physical core on processors with HyperThreading?

3 Answers