Re: [patch] fix uml slowness caused by ptrace preemption bug on host

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2009-03-19 at 23:23 +0100, Miklos Szeredi wrote:
> From: Miklos Szeredi <mszeredi@suse.cz>
> 
> This patch fixes bug #12208.
> 
> This turned out to be not a scheduler regression, but an already
> existing problem in ptrace being triggered by subtle scheduler
> changes.
> 
> The problem is this:
> 
>  - task A is ptracing task B
>  - task B stops on a trace event
>  - task A is woken up and preempts task B
>  - task A calls ptrace on task B, which does ptrace_check_attach()
>  - this calls wait_task_inactive(), which sees that task B is still on the runq
>  - task A goes to sleep for a jiffy
>  - ...
> 
> Since UML does lots of the above sequences, those jiffies quickly add
> up to make it slow as hell.
> 
> This patch solves this by not scheduling on preempt_enable() after
> ptrace_stop() has woken up the tracer.

Nice,.. however did you find this?

Ingo is looking at changing wait_task_inactive() to not be quite so
stupid. I'll let him respond with more details when he's done poking at
the code :-)