Re: ptrace performance (was: [Bug #12208] uml is very slow on2.6.28 host)

[Ingo Molnar]

* Michael Riepe <michael.riepe@xxxxxxxxxxxxxx> 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@xxxxxxxxxx>
> > 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 <dd command as above>
> 
> 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;
 		}
 
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