Re: [PATCH v3 0/2] sched: Optionally skip uclamp logic in fast path

[Lukasz Luba]

Hi Qais,

On 6/24/20 6:26 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 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().
>
> Thanks
>
> --
> Qais Yousef
>
> Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Ben Segall <bsegall@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> CC: Patrick Bellasi <patrick.bellasi@xxxxxxxxxx>
> Cc: Chris Redpath <chris.redpath@xxxxxxx>
> Cc: Lukasz Luba <lukasz.luba@xxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
>
> Qais Yousef (2):
>    sched/uclamp: Fix initialization of strut uclamp_rq
>    sched/uclamp: Protect uclamp fast path code with static key
>
>   kernel/sched/core.c | 75 +++++++++++++++++++++++++++++++++++++++------
>   1 file changed, 66 insertions(+), 9 deletions(-)


The results for this v3 series from mmtest netperf-udp (30x each UDP
size) are good.

                       v5.7-rc7-base-noucl    v5.7-rc7-ucl-tsk-nofix 
v5.7-rc7-ucl-tsk-grp-fix_v3
Hmean     send-64          62.15 (   0.00%)       59.65 *  -4.02%* 
65.83 *   5.93%*
Hmean     send-128        122.88 (   0.00%)      119.37 *  -2.85%* 
133.20 *   8.40%*
Hmean     send-256        244.85 (   0.00%)      234.26 *  -4.32%* 
264.01 *   7.83%*
Hmean     send-1024       919.24 (   0.00%)      880.67 *  -4.20%* 
1005.54 *   9.39%*
Hmean     send-2048      1689.45 (   0.00%)     1647.54 *  -2.48%* 
1845.64 *   9.25%*
Hmean     send-3312      2542.36 (   0.00%)     2485.23 *  -2.25%* 
2729.11 *   7.35%*
Hmean     send-4096      2935.69 (   0.00%)     2861.09 *  -2.54%* 
3161.16 *   7.68%*
Hmean     send-8192      4800.35 (   0.00%)     4680.09 *  -2.51%* 
5090.38 *   6.04%*
Hmean     send-16384     7473.66 (   0.00%)     7349.60 *  -1.66%* 
7786.42 *   4.18%*
Hmean     recv-64          62.15 (   0.00%)       59.65 *  -4.03%* 
65.82 *   5.91%*
Hmean     recv-128        122.88 (   0.00%)      119.37 *  -2.85%* 
133.20 *   8.40%*
Hmean     recv-256        244.84 (   0.00%)      234.26 *  -4.32%* 
264.01 *   7.83%*
Hmean     recv-1024       919.24 (   0.00%)      880.67 *  -4.20%* 
1005.54 *   9.39%*
Hmean     recv-2048      1689.44 (   0.00%)     1647.54 *  -2.48%* 
1845.06 *   9.21%*
Hmean     recv-3312      2542.36 (   0.00%)     2485.23 *  -2.25%* 
2728.74 *   7.33%*
Hmean     recv-4096      2935.69 (   0.00%)     2861.09 *  -2.54%* 
3160.74 *   7.67%*
Hmean     recv-8192      4800.35 (   0.00%)     4678.15 *  -2.55%* 
5090.36 *   6.04%*
Hmean     recv-16384     7473.63 (   0.00%)     7349.52 *  -1.66%* 
7786.25 *   4.18%*

I am happy to re-run v4 if there will be, but for now:

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

Regards,
Lukasz