Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH v4 03/10] sched/topology: Propagate SD_ASYM_CPUCAPACITY upwards | Date | Thu, 06 Aug 2020 17:19:26 +0100 |
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On 06/08/20 15:20, Ingo Molnar wrote: > * Valentin Schneider <valentin.schneider@arm.com> wrote: > >> We currently set this flag *only* on domains whose topology level exactly >> match the level where we detect asymmetry (as returned by >> asym_cpu_capacity_level()). This is rather problematic. >> >> Say there are two clusters in the system, one with a lone big CPU and the >> other with a mix of big and LITTLE CPUs (as is allowed by DynamIQ): >> >> DIE [ ] >> MC [ ][ ] >> 0 1 2 3 4 >> L L B B B >> >> asym_cpu_capacity_level() will figure out that the MC level is the one >> where all CPUs can see a CPU of max capacity, and we will thus set >> SD_ASYM_CPUCAPACITY at MC level for all CPUs. >> >> That lone big CPU will degenerate its MC domain, since it would be alone in >> there, and will end up with just a DIE domain. Since the flag was only set >> at MC, this CPU ends up not seeing any SD with the flag set, which is >> broken. >> >> Rather than clearing dflags at every topology level, clear it before >> entering the topology level loop. This will properly propagate upwards >> flags that are set starting from a certain level. >> >> Reviewed-by: Quentin Perret <qperret@google.com> >> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com> >> --- >> kernel/sched/topology.c | 3 +-- >> 1 file changed, 1 insertion(+), 2 deletions(-) >> >> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c >> index 865fff3ef20a..42b89668e1e4 100644 >> --- a/kernel/sched/topology.c >> +++ b/kernel/sched/topology.c >> @@ -1985,11 +1985,10 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att >> /* Set up domains for CPUs specified by the cpu_map: */ >> for_each_cpu(i, cpu_map) { >> struct sched_domain_topology_level *tl; >> + int dflags = 0; >> >> sd = NULL; >> for_each_sd_topology(tl) { >> - int dflags = 0; >> - >> if (tl == tl_asym) { >> dflags |= SD_ASYM_CPUCAPACITY; >> has_asym = true; > > I'd suggest ordering all patches with potential side effects at the > end, to make them easier to bisect. > > I.e. I'd reorder this series to do: > > - Obviously correct renamings & cleanups > > - Convert the code over to the new instrumented sd-flags method. This > will presumably spew a few warnings for problems the new debugging > checks catch in existing topologies. > > - Do all the behavioral changes and fixes like this patch, even if we > think that they have no serious side effects. > > In that sense it might make sense to order the two ARM patches to the > later stage as well - but I suppose it's OK to do those two first as > well. >
This does sound sensible; I can shuffle this around for v5.
FWIW the reason I had this very patch before the instrumentation is that IMO it really wants to be propagated and could thus directly be tagged with SDF_SHARED_PARENT when the instrumentation hits. It's a minor thing, but having it after the instrumentation means that I'll first have to tag it without any hierarchical metaflag, and then tag it with SDF_SHARED_PARENT in the propagation fix.
If that sounds fine by you, I'll do just that.
> Nice series otherwise, these new checks look really useful and already > caught bugs. >
Thanks!
> Thanks, > > Ingo
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