linux-kernel - [tip:sched/urgent] sched, ptrace: fix UML and ptrace slowness

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Date:	Mon, 23 Mar 2009 16:39:34 GMT
From:	Miklos Szeredi <miklos@...redi.hu>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...hat.com,
	mszeredi@...e.cz, miklos@...redi.hu, stable@...nel.org,
	tglx@...utronix.de, mingo@...e.hu
Subject: [tip:sched/urgent] sched, ptrace: fix UML and ptrace slowness

Commit-ID:  84eef8ca758fa4a68c29f7d752376f6ca6872383
Gitweb:     http://git.kernel.org/tip/84eef8ca758fa4a68c29f7d752376f6ca6872383
Author:     Miklos Szeredi <miklos@...redi.hu>
AuthorDate: Mon, 23 Mar 2009 16:07:24 +0100
Committer:  Ingo Molnar <mingo@...e.hu>
CommitDate: Mon, 23 Mar 2009 17:37:38 +0100

sched, ptrace: fix UML and 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@...e.cz>
Cc: oleg@...hat.com
Cc: peterz@...radead.org
Cc: roland@...hat.com
Cc: efault@....de
Cc: rjw@...k.pl
Cc: jdike@...toit.com
Cc: user-mode-linux-devel@...ts.sourceforge.net
Cc: torvalds@...ux-foundation.org
Cc: akpm@...ux-foundation.org
Cc: <stable@...nel.org>
LKML-Reference: <E1LllkK-0000zF-1h@...az-ex.szeredi.hu>
Signed-off-by: Ingo Molnar <mingo@...e.hu>


---
 kernel/signal.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 2a74fe8..1c88144 100644
--- 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 {
 		/*
--
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