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 <[email protected]> Cc: Vincent Guittot <[email protected]> Cc: Dietmar Eggemann <[email protected]> Cc: Steven Rostedt <[email protected]> Cc: Ben Segall <[email protected]> Cc: Mel Gorman <[email protected]> CC: Patrick Bellasi <[email protected]> Cc: Chris Redpath <[email protected]> Cc: Lukasz Luba <[email protected]> Cc: [email protected] 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(-) -- 2.17.1
