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SubjectRe: [PATCH v6 0/2] sched: Optionally skip uclamp logic in fast path
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Hi Qais,

On 6/30/20 12:21 PM, Qais Yousef wrote:
> This series attempts to address the report that uclamp logic could be expensive
> sometimes and shows a regression in netperf UDP_STREAM under certain
> conditions.
>
> The first patch is a fix for how struct uclamp_rq is initialized which is
> required by the 2nd patch which contains the real 'fix'.
>
> Worth noting that the root cause of the overhead is believed to be system
> specific or related to potential certain code/data layout issues, leading to
> worse I/D $ performance.
>
> Different systems exhibited different behaviors and the regression did
> disappear in certain kernel version while attempting to reporoduce.
>
> More info can be found here:
>
> https://lore.kernel.org/lkml/20200616110824.dgkkbyapn3io6wik@e107158-lin/
>
> Having the static key seemed the best thing to do to ensure the effect of
> uclamp is minimized for kernels that compile it in but don't have a userspace
> that uses it, which will allow distros to distribute uclamp capable kernels by
> default without having to compromise on performance for some systems that could
> be affected.
>
> Changes in v6:
> * s/uclamp_is_enabled/uclamp_is_used/ + add comment
> * Improve the bailout condition for the case where we could end up with
> unbalanced call of uclamp_rq_dec_id()
> * Clarify some comments.
>

I've tried this v6 series with mmtest netperf-udp (30x each UDP
size) - the results are good.

v5.7-rc7-base-noucl v5.7-rc7-ucl-tsk-nofix
v5.7-rc7-ucl-tsk-grp-fix_v6
Hmean send-64 62.15 ( 0.00%) 59.65 * -4.02%*
65.87 * 5.99%*
Hmean send-128 122.88 ( 0.00%) 119.37 * -2.85%*
131.76 * 7.23%*
Hmean send-256 244.85 ( 0.00%) 234.26 * -4.32%*
259.87 * 6.14%*
Hmean send-1024 919.24 ( 0.00%) 880.67 * -4.20%*
975.48 * 6.12%*
Hmean send-2048 1689.45 ( 0.00%) 1647.54 * -2.48%*
1797.23 * 6.38%*
Hmean send-3312 2542.36 ( 0.00%) 2485.23 * -2.25%*
2665.69 * 4.85%*
Hmean send-4096 2935.69 ( 0.00%) 2861.09 * -2.54%*
3087.79 * 5.18%*
Hmean send-8192 4800.35 ( 0.00%) 4680.09 * -2.51%*
4966.50 * 3.46%*
Hmean send-16384 7473.66 ( 0.00%) 7349.60 * -1.66%*
7598.49 * 1.67%*
Hmean recv-64 62.15 ( 0.00%) 59.65 * -4.03%*
65.86 * 5.97%*
Hmean recv-128 122.88 ( 0.00%) 119.37 * -2.85%*
131.76 * 7.23%*
Hmean recv-256 244.84 ( 0.00%) 234.26 * -4.32%*
259.80 * 6.11%*
Hmean recv-1024 919.24 ( 0.00%) 880.67 * -4.20%*
975.42 * 6.11%*
Hmean recv-2048 1689.44 ( 0.00%) 1647.54 * -2.48%*
1797.18 * 6.38%*
Hmean recv-3312 2542.36 ( 0.00%) 2485.23 * -2.25%*
2665.53 * 4.84%*
Hmean recv-4096 2935.69 ( 0.00%) 2861.09 * -2.54%*
3087.79 * 5.18%*
Hmean recv-8192 4800.35 ( 0.00%) 4678.15 * -2.55%*
4966.48 * 3.46%*
Hmean recv-16384 7473.63 ( 0.00%) 7349.52 * -1.66%*
7597.90 * 1.66%*

You can add:

Tested-by: Lukasz Luba <lukasz.luba@arm.com>

Regards,
Lukasz

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