Re: [RFC PATCH v3 00/16] Core scheduling v3

[Aaron Lu]

On Mon, Jul 22, 2019 at 06:26:46PM +0800, Aubrey Li wrote:
> The granularity period of util_avg seems too large to decide task priority
> during pick_task(), at least it is in my case, cfs_prio_less() always picked
> core max task, so pick_task() eventually picked idle, which causes this change
> not very helpful for my case.
> 
>  <idle>-0     [057] dN..    83.716973: __schedule: max: sysbench/2578
> ffff889050f68600
>  <idle>-0     [057] dN..    83.716974: __schedule:
> (swapper/5/0;140,0,0) ?< (mysqld/2511;119,1042118143,0)
>  <idle>-0     [057] dN..    83.716975: __schedule:
> (sysbench/2578;119,96449836,0) ?< (mysqld/2511;119,1042118143,0)
>  <idle>-0     [057] dN..    83.716975: cfs_prio_less: picked
> sysbench/2578 util_avg: 20 527 -507 <======= here===
>  <idle>-0     [057] dN..    83.716976: __schedule: pick_task cookie
> pick swapper/5/0 ffff889050f68600

I tried a different approach based on vruntime with 3 patches following.

When the two tasks are on the same CPU, no change is made, I still route
the two sched entities up till they are in the same group(cfs_rq) and
then do the vruntime 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. The smaller vruntime wins.

To make this work, the root level ses on both CPU should have a common
cfs_rq min vuntime, which I call it the core cfs_rq min vruntime.

This is mostly done in patch2/3.

Test:
1 wrote an cpu intensive program that does nothing but while(1) in
  main(), let's call it cpuhog;
2 start 2 cgroups, with one cgroup's cpuset binding to CPU2 and the
  other binding to cpu3. cpu2 and cpu3 are smt siblings on the test VM;
3 enable cpu.tag for the two cgroups;
4 start one cpuhog task in each cgroup;
5 kill both cpuhog tasks after 10 seconds;
6 check each cgroup's cpu usage.

If the task is scheduled fairly, then each cgroup's cpu usage should be
around 5s.

With v3, the cpu usage of both cgroups are sometimes 3s, 7s; sometimes
1s, 9s.

With the 3 patches applied, the numbers are mostly around 5s, 5s.

Another test is starting two cgroups simultaneously with cpu.tag set,
with one cgroup running: will-it-scale/page_fault1_processes -t 16 -s 30,
the other running: will-it-scale/page_fault2_processes -t 16 -s 30.
With v3, like I said last time, the later started page_fault processes
can't start running. With the 3 patches applied, both running at the
same time with each CPU having a relatively fair score:

output line of 16 page_fault1 processes in 1 second interval:
min:105225 max:131716 total:1872322

output line of 16 page_fault2 processes in 1 second interval:
min:86797 max:110554 total:1581177

Note the value in min and max, the smaller the gap is, the better the
faireness is.

Aubrey,

I haven't been able to run your workload yet...