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Subject[patch 82/88] fix ptrace slowness
2.6.28-stable review patch.  If anyone has any objections, please let us know.


From: Miklos Szeredi <>

commit 53da1d9456fe7f87a920a78fdbdcf1225d197cb7 upstream.

This patch fixes bug #12208:

Bug-Entry :
Subject : uml is very slow on 2.6.28 host

This turned out to be not a scheduler regression, but an already
existing problem in ptrace being triggered by subtle scheduler

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 rescheduling in read_unlock() after
ptrace_stop() has woken up the tracer.

Thanks to Oleg Nesterov and Ingo Molnar for the feedback.

Signed-off-by: Miklos Szeredi <>
Signed-off-by: Linus Torvalds <>
Signed-off-by: Greg Kroah-Hartman <>

kernel/signal.c | 8 ++++++++
1 file changed, 8 insertions(+)

--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1552,7 +1552,15 @@ static void ptrace_stop(int exit_code, i
if (may_ptrace_stop()) {
do_notify_parent_cldstop(current, CLD_TRAPPED);
+ /*
+ * Don't want to allow preemption here, because
+ * sys_ptrace() needs this task to be inactive.
+ *
+ * XXX: implement read_unlock_no_resched().
+ */
+ preempt_disable();
+ preempt_enable_no_resched();
} else {

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