Commit-ID: 84eef8ca758fa4a68c29f7d752376f6ca6872383
Gitweb: http://git.kernel.org/tip/84eef8ca758fa4a68c29f7d752376f6ca6872383
Author: Miklos Szeredi <miklos@xxxxxxxxxx>
AuthorDate: Mon, 23 Mar 2009 16:07:24 +0100
Committer: Ingo Molnar <mingo@xxxxxxx>
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@xxxxxxx>
Cc: oleg@xxxxxxxxxx
Cc: peterz@xxxxxxxxxxxxx
Cc: roland@xxxxxxxxxx
Cc: efault@xxxxxx
Cc: rjw@xxxxxxx
Cc: jdike@xxxxxxxxxxx
Cc: user-mode-linux-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: torvalds@xxxxxxxxxxxxxxxxxxxx
Cc: akpm@xxxxxxxxxxxxxxxxxxxx
Cc: <stable@xxxxxxxxxx>
LKML-Reference: <E1LllkK-0000zF-1h@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
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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