Re: ptrace performance (was: [Bug #12208] uml is very slow on 2.6.28 host)

Ingo Molnar
2009-03-21T15:22:17Z

* Michael Riepe <michael.riepe-gM/Ye1E23mwN+BqQ9rBEUg@public.gmane.org> wrote:

> Disclaimer: I'm not using UML, but these problems may be related.
>
> > Bug-Entry : http://bugzilla.kernel.org/show_bug.cgi?id=12208
> > Subject : uml is very slow on 2.6.28 host
> > Submitter : Miklos Szeredi <miklos-sUDqSbJrdHQHWmgEVkV9KA@public.gmane.org>
> > Date : 2008-12-12 9:35 (93 days old)
> > References : http://marc.info/?l=linux-kernel&m=122907463518593&w=4
>
> The other day I noticed a dramatic ptrace slowdown between 2.6.27 and
> 2.6.28.x (verified with 2.6.28.8). In particular, a command like
>
> dd if=/dev/zero of=/dev/null bs=1024k count=1024
>
> will normally report a throughput in the GB/s range. On 2.6.27, this is
> also true if you run
>
> strace -o /dev/null
>
> which is only a little slower. But if I do the same on 2.6.28.x, I
> get a throughput of about 100 MB/s or less, i.e. less than 10%. I
> tried the commands on three different machines (an Athlon64 3000+,
> a Core Duo T2400 and an Atom 330), and they all behave similar.
> The more system calls a program uses, the worse the slowdown (try
> the dd command with bs=16k and count=65536, for example - but
> don't hold your breath).
>
> Interestingly, the CPUs are mostly idle while the command is
> executing on 2.6.28.x, but there is a high (system) load on
> 2.6.27. Therefore, I suspect that it's a scheduling or maybe timer
> problem that was introduced between 2.6.27 and 2.6.28. I haven't
> had the time to check the rc kernels yet; perhaps someone else can
> run a quick check to verify that it's gone in the latest
> 2.6.29-rc.

that's almost certainly due to the wait_task_inactive() change. Does
the patch below improve things?

Ingo

diff --git a/kernel/sched.c b/kernel/sched.c
index 3e827b8..2d60f23 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2119,7 +2119,8 @@ unsigned long wait_task_inactive(struct task_struct *p, long match_state)
* yield - it could be a while.
*/
if (unlikely(on_rq)) {
- schedule_timeout_uninterruptible(1);
+ cpu_relax();
+ cond_resched();
continue;
}