fix ptrace slowness

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>

diff --git a/kernel/signal.c b/kernel/signal.c
index 2a74fe8..1c88144 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1575,7 +1575,15 @@ static void ptrace_stop(int exit_code, int clear_code, siginfo_t *info)
        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 {
                /*