Re: [PATCH 2/3] sched,fair: Fix local starvation

[Matt Fleming]

On Tue, 10 May, at 07:43:16PM, Peter Zijlstra wrote:
> Mike reported that the recent commit 3a47d5124a95 ("sched/fair: Fix
> fairness issue on migration") broke interactivity and the signal
> starve test.
> 
> The problem is that I assumed ENQUEUE_WAKING was only set when we do a
> cross-cpu wakeup (migration), which isn't true. This means we now
> destroy the vruntime history of tasks and wakeup-preemption suffers.
> 
> Cure this by making my assumption true, only call
> sched_class::task_waking() when we do a cross-cpu wakeup. This avoids
> the indirect call in the case we do a local wakeup.
> 
> Cc: Pavan Kondeti <pkondeti@xxxxxxxxxxxxxx>
> Cc: Ben Segall <bsegall@xxxxxxxxxx>
> Cc: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
> Cc: Morten Rasmussen <morten.rasmussen@xxxxxxx>
> Cc: Paul Turner <pjt@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: byungchul.park@xxxxxxx
> Cc: Andrew Hunter <ahh@xxxxxxxxxx>
> Fixes: 3a47d5124a95 ("sched/fair: Fix fairness issue on migration")
> Reported-by: Mike Galbraith <mgalbraith@xxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  kernel/sched/core.c |   29 +++++++++++++++++++++--------
>  kernel/sched/fair.c |   41 ++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 61 insertions(+), 9 deletions(-)
 
This patch appears to cause a regression for hackbench -pipe of
between ~8% and ~10% with groups >= NR_CPU.

I haven't probed much yet, but it looks like the vruntime of tasks has
gone nuts.