Instrumenting context-switching

[Mike Mason]

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