From: Mike Mason <mmlnx@us.ibm.com>
To: Perry Cheng <perryche@us.ibm.com>
Cc: systemtap@sources.redhat.com
Subject: Re: Instrumenting context-switching
Date: Wed, 31 Oct 2007 16:34:00 -0000 [thread overview]
Message-ID: <4728AE84.2090608@us.ibm.com> (raw)
In-Reply-To: <OFC86D8062.3754ED5D-ON85257385.004AC02A-85257385.004B48D4@us.ibm.com>
Perry Cheng wrote:
> Instrumenting the context switch code has long been delicate. I can't
> seem to find any help on this particular topic on the wiki (specifically,
> the examples).
>
> In the past, following some hints from old docs and mailing list, it has
> been enough to instrument the __switch_to method to get the prev and next
> tasks. The method one level higher is switch_to which, being a macro, is
> not instrumentable. Lately, I've switched from an older i386 kernel to a
> new x86_64 kernel and now __switch_to no longer is instrumentable with
> kprobes. The higher-level context_switch itself does not seem probe-able
> because it is a static inline method. Even higherup, we have the call to
> context_switch from schedule but instrumenting that would require using a
> specific line number which seems rather fragile because of greater
> reliance of debugging code and susceptibility to kernel code change.
The scheduler tapset probes context_switch() on x86_64, but that doesn't help much. context_switch() is an inline and, thus, the entry parameters prev and next aren't accessible via SystemTap.
What kernel version are you using? If your using 2.6.24-rc1 then kernel markers are available. You can place a static marker (as shown in the patch below), rebuild and reboot the kernel, then access the probe point via a SystemTap script (as shown in the script below the patch). You'll need to use the latest SystemTap snapshot to get marker probe support: ftp://sources.redhat.com/pub/systemtap/snapshots/systemtap-20071027.tar.bz2.
Of course, this won't help if you're using earlier versions of the kernel or SystemTap.
- Mike
diff --git a/kernel/sched.c b/kernel/sched.c
index 3f6bd11..3281098 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1937,6 +1937,8 @@ context_switch(struct rq *rq, struct task_struct *prev,
spin_release(&rq->lock.dep_map, 1, _THIS_IP_);
#endif
+ trace_mark(sched_switch_to, "%p %p %p", rq, prev, next);
+
/* Here we just switch the register state and the stack. */
switch_to(prev, next, prev);
SCRIPT:
probe kernel.mark("sched_switch_to")
{
rq = $arg1
p = $arg2
n = $arg3
next_pid = task_pid(n)
prev_pid = task_pid(p)
next_name = task_execname(n)
prev_name = task_execname(p)
printf("%s (%d) switching to %s (%d)\n",
prev_name, prev_pid, next_name, next_pid)
}
>
> So, on an x86_64 kernel, how do I instrument this method?
>
> Perry
next prev parent reply other threads:[~2007-10-31 16:34 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-10-31 13:42 Perry Cheng
2007-10-31 16:34 ` Mike Mason [this message]
2007-10-31 16:48 ` Masami Hiramatsu
2007-10-31 19:04 ` Stone, Joshua I
2007-10-31 19:19 ` Stone, Joshua I
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