Messages in this thread Patch in this message | | | Date | Wed, 29 Apr 2009 15:07:51 -0700 | From | Greg KH <> | Subject | [patch 52/58] fix ptrace slowness |
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2.6.27-stable review patch. If anyone has any objections, please let us know.
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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 @@ -1549,7 +1549,15 @@ 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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