Re: [PATCH v4 0/2] sched: Optionally skip uclamp logic in fast path

[Lukasz Luba <lukasz.luba@arm.com>]

Hi Qais,

On 6/25/20 4:43 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 v4:
> 	* Fix broken boosting of RT tasks when static key is disabled.
> 
> Changes in v3:
> 	* Avoid double negatives and rename the static key to uclamp_used
> 	* Unconditionally enable the static key through any of the paths where
> 	  the user can modify the default uclamp value.
> 	* Use C99 named struct initializer for struct uclamp_rq which is easier
> 	  to read than the memset().
> 
> Changes in v2:
> 	* Add more info in the commit message about the result of perf diff to
> 	  demonstrate that the activate/deactivate_task pressure is reduced in
> 	  the fast path.
> 
> 	* Fix sparse warning reported by the test robot.
> 
> 	* Add an extra commit about using static_branch_likely() instead of
> 	  static_branc_unlikely().
> 


I've tried this v4 series with mmtest netperf-udp (30x each UDP
size) - results are good (just double checking and making sure
the tag indicating that v4 was tested can be applied).

                       v5.7-rc7-base-noucl    v5.7-rc7-ucl-tsk-nofix 
v5.7-rc7-ucl-tsk-grp-fix_v4
Hmean     send-64          62.15 (   0.00%)       59.65 *  -4.02%* 
65.86 *   5.97%*
Hmean     send-128        122.88 (   0.00%)      119.37 *  -2.85%* 
131.75 *   7.22%*
Hmean     send-256        244.85 (   0.00%)      234.26 *  -4.32%* 
259.33 *   5.92%*
Hmean     send-1024       919.24 (   0.00%)      880.67 *  -4.20%* 
979.49 *   6.55%*
Hmean     send-2048      1689.45 (   0.00%)     1647.54 *  -2.48%* 
1805.21 *   6.85%*
Hmean     send-3312      2542.36 (   0.00%)     2485.23 *  -2.25%* 
2658.30 *   4.56%*
Hmean     send-4096      2935.69 (   0.00%)     2861.09 *  -2.54%* 
3083.08 *   5.02%*
Hmean     send-8192      4800.35 (   0.00%)     4680.09 *  -2.51%* 
4984.22 *   3.83%*
Hmean     send-16384     7473.66 (   0.00%)     7349.60 *  -1.66%* 
7703.88 *   3.08%*
Hmean     recv-64          62.15 (   0.00%)       59.65 *  -4.03%* 
65.85 *   5.96%*
Hmean     recv-128        122.88 (   0.00%)      119.37 *  -2.85%* 
131.74 *   7.21%*
Hmean     recv-256        244.84 (   0.00%)      234.26 *  -4.32%* 
259.33 *   5.92%*
Hmean     recv-1024       919.24 (   0.00%)      880.67 *  -4.20%* 
979.46 *   6.55%*
Hmean     recv-2048      1689.44 (   0.00%)     1647.54 *  -2.48%* 
1805.17 *   6.85%*
Hmean     recv-3312      2542.36 (   0.00%)     2485.23 *  -2.25%* 
2657.67 *   4.54%*
Hmean     recv-4096      2935.69 (   0.00%)     2861.09 *  -2.54%* 
3082.58 *   5.00%*
Hmean     recv-8192      4800.35 (   0.00%)     4678.15 *  -2.55%* 
4982.49 *   3.79%*
Hmean     recv-16384     7473.63 (   0.00%)     7349.52 *  -1.66%* 
7701.53 *   3.05%*

You can add my:

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

If anyone would like to see some other tests, please let me know,
maybe I can setup something.

Regards,
Lukasz