Messages in this thread Patch in this message |  | | | Subject | [patch] fix ptrace slowness | | From | Miklos Szeredi <> | | Date | Mon, 23 Mar 2009 16:07:24 +0100 |
| |
This one incorporates comments from Oleg and Ingo. Please apply to 2.6.29 and 2.6.2[78]-stable trees.
Thanks, Miklos ---- From: Miklos Szeredi <mszeredi@suse.cz>
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 --- kernel/signal.c | 8 ++++++++ 1 file changed, 8 insertions(+) Index: linux.git/kernel/signal.c =================================================================== --- linux.git.orig/kernel/signal.c 2009-03-20 09:41:04.000000000 +0100 +++ linux.git/kernel/signal.c 2009-03-23 15:40:57.000000000 +0100 @@ -1575,7 +1575,15 @@ 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 { /*
|  |