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SubjectRe: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value
On Thu, May 28, 2020 at 05:51:31PM +0100, Qais Yousef wrote:
> > Indeed, that one. The fact that regular distros cannot enable this
> > feature due to performance overhead is unfortunate. It means there is a
> > lot less potential for this stuff.
>
> I had a humble try to catch the overhead but wasn't successful. The observation
> wasn't missed by us too then.
>

As with all things, it's perfectly possible I was looking at a workload
where the cost is more obvious but given that the functions are inlined,
it's not trivial to spot. I just happened to spot it because I was paying
close attention to try_to_wake_up() at the time.

> On my Ubuntu 18.04 machine uclamp is enabled by default by the way. 5.3 kernel
> though, so uclamp task group stuff not there yet. Should check how their server
> distro looks like.
>

Elsewhere in the thread, I showed some results based on 5.7 so uclamp
task group existed but I had it disabled. The uclamp related parts of
the kconfig were

# zgrep UCLAMP kconfig-5.7.0-rc7-with-clamp.txt.gz
CONFIG_UCLAMP_TASK=y
CONFIG_UCLAMP_BUCKETS_COUNT=5
# CONFIG_UCLAMP_TASK_GROUP is not set

--
Mel Gorman
SUSE Labs

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