Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH v3] sched/topology: remove sysctl_sched_energy_aware depending on the architecture | Date | Mon, 18 Sep 2023 14:22:17 +0200 |
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On 15/09/23 23:40, Shrikanth Hegde wrote: > On 9/15/23 5:30 PM, Valentin Schneider wrote: >> On 14/09/23 23:26, Shrikanth Hegde wrote: >>> On 9/14/23 9:51 PM, Valentin Schneider wrote: >>>> On 13/09/23 17:18, Shrikanth Hegde wrote: >>>>> sysctl_sched_energy_aware is available for the admin to disable/enable >>>>> energy aware scheduling(EAS). EAS is enabled only if few conditions are >>>>> met by the platform. They are, asymmetric CPU capacity, no SMT, >>>>> valid cpufreq policy, frequency invariant load tracking. It is possible >>>>> platform when booting may not have EAS capability, but can do that after. >>>>> For example, changing/registering the cpufreq policy. >>>>> >>>>> At present, though platform doesn't support EAS, this sysctl is still >>>>> present and it ends up calling rebuild of sched domain on write to 1 and >>>>> NOP when writing to 0. That is confusing and un-necessary. >>>>> >>>> >>> >>> Hi Valentin, Thanks for taking a look at this patch. >>> >>>> But why would you write to it in the first place? Or do you mean to use >>>> this as an indicator for userspace that EAS is supported? >>>> >>> >>> Since this sysctl is present and its value being 1, it gives the >>> impression to the user that EAS is supported when it is not. >>> So its an attempt to correct that part. >>> >> >> Ah, I see. Then how about just making the sysctl return 0 when EAS isn't >> supported? And on top of it, prevent all writes when EAS isn't supported >> (perf domains cannot be built, so there would be no point in forcing a >> rebuild that will do nothing). > > Yes. That's another way. Thats what I had as possible approach in > https://lore.kernel.org/lkml/d2c945d6-c4f0-a096-0623-731b11484f51@linux.vnet.ibm.com/ >
Thanks for the link; and apologies for bringing up topics that have been discussed already.
> > >> >> I can never remember how to properly use the sysctl API, so that's a very >> crude implementation, but something like so? >> >> --- >> >> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c >> index 05a5bc678c089..dadfc5afc4121 100644 >> --- a/kernel/sched/topology.c >> +++ b/kernel/sched/topology.c >> @@ -230,9 +230,28 @@ static int sched_energy_aware_handler(struct ctl_table *table, int write, >> if (write && !capable(CAP_SYS_ADMIN)) >> return -EPERM; >> >> + if (!sched_energy_enabled()) { > > Use of sched_energy_enabled won't work as Pierre has indicated. > > Instead this can be done by adding those checks in a helper function to > do similar checks as done build_perf_domains. > > I can send v4 with this approach if it makes more sense. Please let me know. >
So what I'm thinking is the standard approach seems to be to keep the knobs visible, but change how reads/writes to them are handled.
For instance, SMT support has
/sys/devices/system/cpu/smt /control /active
And a system with CONFIG_HOTPLUG_SMT=y but no actual hardware SMT will have:
/control = notsupported /active = 0
So IMO it would make sense to keep sched_energy_aware around, but make it read 0 and prevent writes for systems that have the software support compiled but don't have the actual hardware support.
In a pinch it also helps to know if CONFIG_ENERGY_MODEL was selected, though that's obvious enough with CONFIG_SCHED_DEBUG=y.
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