| Message ID | 20201027180713.7642-3-ionela.voinescu@arm.com (mailing list archive) |
|---|---|
| State | Not Applicable, archived |
| Headers | show |
| Series | condition EAS enablement on FI support | expand |
On Tue, Oct 27, 2020 at 06:07:12PM +0000, Ionela Voinescu wrote: > Task scheduler behavior depends on frequency invariance (FI) support and > the resulting invariant load tracking signals. For example, in order to > make accurate predictions across CPUs for all performance states, Energy > Aware Scheduling (EAS) needs frequency-invariant load tracking signals > and therefore it has a direct dependency on FI. This dependency is known, > but EAS enablement is not yet conditioned on the presence of FI during > the built of the scheduling domain hierarchy. > > Before this is done, the following must be considered: while > arch_scale_freq_invariant() will see changes in FI support and could > be used to condition the use of EAS, it could return different values > during system initialisation. > > For arm64, such a scenario will happen for a system that does not support > cpufreq driven FI, but does support counter-driven FI. For such a system, > arch_scale_freq_invariant() will return false if called before counter > based FI initialisation, but change its status to true after it. > If EAS becomes explicitly dependent on FI this would affect the task > scheduler behavior which builds its scheduling domain hierarchy well > before the late counter-based FI init. During that process, EAS would be > disabled due to its dependency on FI. > > Two points of future early calls to arch_scale_freq_invariant() which > would determine EAS enablement are: > - (1) drivers/base/arch_topology.c:126 <<update_topology_flags_workfn>> > rebuild_sched_domains(); > This will happen after CPU capacity initialisation. > - (2) kernel/sched/cpufreq_schedutil.c:917 <<rebuild_sd_workfn>> > rebuild_sched_domains_energy(); > -->rebuild_sched_domains(); > This will happen during sched_cpufreq_governor_change() for the > schedutil cpufreq governor. > > Therefore, before enforcing the presence of FI support for the use of EAS, > ensure the following: if there is a change in FI support status after > counter init, use the existing rebuild_sched_domains_energy() function to > trigger a rebuild of the scheduling and performance domains that in turn > will determine the enablement of EAS. > > Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com> > Cc: Catalin Marinas <catalin.marinas@arm.com> > Cc: Will Deacon <will@kernel.org> Acked-by: Catalin Marinas <catalin.marinas@arm.com>
diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c index 543c67cae02f..2a9b69fdabc9 100644 --- a/arch/arm64/kernel/topology.c +++ b/arch/arm64/kernel/topology.c @@ -213,6 +213,7 @@ static DEFINE_STATIC_KEY_FALSE(amu_fie_key); static int __init init_amu_fie(void) { + bool invariance_status = topology_scale_freq_invariant(); cpumask_var_t valid_cpus; bool have_policy = false; int ret = 0; @@ -255,6 +256,15 @@ static int __init init_amu_fie(void) if (!topology_scale_freq_invariant()) static_branch_disable(&amu_fie_key); + /* + * Task scheduler behavior depends on frequency invariance support, + * either cpufreq or counter driven. If the support status changes as + * a result of counter initialisation and use, retrigger the build of + * scheduling domains to ensure the information is propagated properly. + */ + if (invariance_status != topology_scale_freq_invariant()) + rebuild_sched_domains_energy(); + free_valid_mask: free_cpumask_var(valid_cpus);
Task scheduler behavior depends on frequency invariance (FI) support and the resulting invariant load tracking signals. For example, in order to make accurate predictions across CPUs for all performance states, Energy Aware Scheduling (EAS) needs frequency-invariant load tracking signals and therefore it has a direct dependency on FI. This dependency is known, but EAS enablement is not yet conditioned on the presence of FI during the built of the scheduling domain hierarchy. Before this is done, the following must be considered: while arch_scale_freq_invariant() will see changes in FI support and could be used to condition the use of EAS, it could return different values during system initialisation. For arm64, such a scenario will happen for a system that does not support cpufreq driven FI, but does support counter-driven FI. For such a system, arch_scale_freq_invariant() will return false if called before counter based FI initialisation, but change its status to true after it. If EAS becomes explicitly dependent on FI this would affect the task scheduler behavior which builds its scheduling domain hierarchy well before the late counter-based FI init. During that process, EAS would be disabled due to its dependency on FI. Two points of future early calls to arch_scale_freq_invariant() which would determine EAS enablement are: - (1) drivers/base/arch_topology.c:126 <<update_topology_flags_workfn>> rebuild_sched_domains(); This will happen after CPU capacity initialisation. - (2) kernel/sched/cpufreq_schedutil.c:917 <<rebuild_sd_workfn>> rebuild_sched_domains_energy(); -->rebuild_sched_domains(); This will happen during sched_cpufreq_governor_change() for the schedutil cpufreq governor. Therefore, before enforcing the presence of FI support for the use of EAS, ensure the following: if there is a change in FI support status after counter init, use the existing rebuild_sched_domains_energy() function to trigger a rebuild of the scheduling and performance domains that in turn will determine the enablement of EAS. Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Will Deacon <will@kernel.org> --- arch/arm64/kernel/topology.c | 10 ++++++++++ 1 file changed, 10 insertions(+)