| From 53da1d9456fe7f87a920a78fdbdcf1225d197cb7 Mon Sep 17 00:00:00 2001 |
| From: Miklos Szeredi <mszeredi@suse.cz> |
| Date: Mon, 23 Mar 2009 16:07:24 +0100 |
| Subject: fix ptrace slowness |
| |
| 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 { |
| /* |
