[PATCH] sched/rt: fix incorrect schedstats for rt thread

[Peter Zijlstra]

On Thu, Jan 08, 2026 at 11:13:07AM +0800, Dengjun Su wrote:
> For RT thread, only 'set_next_task_rt' will call
> 'update_stats_wait_end_rt' to update schedstats information.
> However, during the RT migration process,
> 'update_stats_wait_start_rt' will be called twice, which
> will cause the values of wait_max and wait_sum to be incorrect.

Right, that looses time. Also note that I think dl has the same issue.

> The specific output as follows:
> $ cat /proc/6046/task/6046/sched | grep wait
> wait_start                                   :             0.000000
> wait_max                                     :        496717.080029
> wait_sum                                     :       7921540.776553
> 
> Add 'update_stats_wait_end_rt' in 'update_stats_dequeue_rt' to
> update schedstats information when dequeue_task.

This needs a few more words on why this is correct -- notably it took me
a little time to find the 'task_on_rq_migrating()' case in
__update_stats_wait_end() which makes this not actually 'end'.

But then the corresponding clause in __update_stats_wait_start() gives
me a headache:

 'wait_start > prev_wait_start'

I mean, wtf. Should that not equally be using task_on_rq_migrating() ?

Can you please take a hard look at all that and fix up things all-round?