Re: [RFC PATCH 09/13] sched/fair: core wide vruntime comparison

Aaron Lu
2020-04-15T04:08:17Z

On Wed, Apr 15, 2020 at 11:34:08AM +0800, Aaron Lu wrote:
> On Tue, Apr 14, 2020 at 03:56:24PM +0200, Peter Zijlstra wrote:
> > On Wed, Mar 04, 2020 at 04:59:59PM +0000, vpillai wrote:
> > > From: Aaron Lu <aaron.lu@linux.alibaba.com>
> > >
> > > This patch provides a vruntime based way to compare two cfs task's
> > > priority, be it on the same cpu or different threads of the same core.
> > >
> > > When the two tasks are on the same CPU, we just need to find a common
> > > cfs_rq both sched_entities are on and then do the comparison.
> > >
> > > When the two tasks are on differen threads of the same core, the root
> > > level sched_entities to which the two tasks belong will be used to do
> > > the comparison.
> > >
> > > An ugly illustration for the cross CPU case:
> > >
> > > cpu0 cpu1
> > > / | \ / | \
> > > se1 se2 se3 se4 se5 se6
> > > / \ / \
> > > se21 se22 se61 se62
> > >
> > > Assume CPU0 and CPU1 are smt siblings and task A's se is se21 while
> > > task B's se is se61. To compare priority of task A and B, we compare
> > > priority of se2 and se6. Whose vruntime is smaller, who wins.
> > >
> > > To make this work, the root level se should have a common cfs_rq min
> > > vuntime, which I call it the core cfs_rq min vruntime.
> > >
> > > When we adjust the min_vruntime of rq->core, we need to propgate
> > > that down the tree so as to not cause starvation of existing tasks
> > > based on previous vruntime.
> >
> > You forgot the time complexity analysis.
>
> This is a mistake and the adjust should be needed only once when core
> scheduling is initially enabled. It is an initialization thing and there
> is no reason to do it in every invocation of coresched_adjust_vruntime().

Correction...
I meant there is no need to call coresched_adjust_vruntime() in every
invocation of update_core_cfs_rq_min_vruntime().