Re: [PATCH] sched: support dynamiQ cluster

[Morten Rasmussen]

On Wed, Mar 28, 2018 at 09:46:55AM +0200, Vincent Guittot wrote:
> Arm DynamiQ system can integrate cores with different micro architecture
> or max OPP under the same DSU so we can have cores with different compute
> capacity at the LLC (which was not the case with legacy big/LITTLE
> architecture). Such configuration is similar in some way to ITMT on intel
> platform which allows some cores to be boosted to higher turbo frequency
> than others and which uses SD_ASYM_PACKING feature to ensures that CPUs with
> highest capacity, will always be used in priortiy in order to provide
> maximum throughput.
> 
> Add arch_asym_cpu_priority() for arm64 as this function is used to
> differentiate CPUs in the scheduler. The CPU's capacity is used to order
> CPUs in the same DSU.
> 
> Create sched domain topolgy level for arm64 so we can set SD_ASYM_PACKING
> at MC level.
> 
> Some tests have been done on a hikey960 platform (quad cortex-A53,
> quad cortex-A73). For the test purpose, the CPUs topology of the hikey960
> has been modified so the 8 heterogeneous cores are described as being part
> of the same cluster and sharing resources (MC level) like with a DynamiQ DSU.
> 
> Results below show the time in seconds to run sysbench --test=cpu with an
> increasing number of threads. The sysbench test run 32 times
> 
>              without patch     with patch    diff
> 1 threads    11.04(+/- 30%)    8.86(+/- 0%)  -19%
> 2 threads     5.59(+/- 14%)    4.43(+/- 0%)  -20%
> 3 threads     3.80(+/- 13%)    2.95(+/- 0%)  -22%
> 4 threads     3.10(+/- 12%)    2.22(+/- 0%)  -28%
> 5 threads     2.47(+/-  5%)    1.95(+/- 0%)  -21%
> 6 threads     2.09(+/-  0%)    1.73(+/- 0%)  -17%
> 7 threads     1.64(+/-  0%)    1.56(+/- 0%)  - 7%
> 8 threads     1.42(+/-  0%)    1.42(+/- 0%)    0%
> 
> Results show a better and stable results across iteration with the patch
> compared to mainline because we are always using big cores in priority whereas
> with mainline, the scheduler randomly choose a big or a little cores when
> there are more cores than number of threads.
> With 1 thread, the test duration varies in the range [8.85 .. 15.86] for
> mainline whereas it stays in the range [8.85..8.87] with the patch

Using ASYM_PACKING is essentially an easier but somewhat less accurate
way to achieve the same behaviour for big.LITTLE system as with the
"misfit task" series that been under review here for the last couple of
months.

As I see it, the main differences is that ASYM_PACKING attempts to pack
all tasks regardless of task utilization on the higher capacity cpus
whereas the "misfit task" series carefully picks cpus with tasks they
can't handle so we don't risk migrating tasks which are perfectly
suitable to for a little cpu to a big cpu unnecessarily. Also it is
based directly on utilization and cpu capacity like the capacity
awareness we already have to deal with big.LITTLE in the wake-up path.
Furthermore, it should work for all big.LITTLE systems regardless of the
topology, where I think ASYM_PACKING might not work well for systems
with separate big and little sched_domains.

Have to tried taking the misfit patches for a spin on your setup? I
expect them give you the same behaviour as you report above.

Morten