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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 <mszeredi@suse.cz>

commit 53da1d9456fe7f87a920a78fdbdcf1225d197cb7 upstream.

This patch fixes bug #12208:

Bug-Entry : http://bugzilla.kernel.org/show_bug.cgi?id=12208
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
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 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 <mszeredi@suse.cz>
CC: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
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
read_lock(&tasklist_lock);
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();
read_unlock(&tasklist_lock);
+ preempt_enable_no_resched();
schedule();
} else {
/*



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