Inserting delays with systemtap

["Bryn M. Reeves" <bmr at redhat dot com>]

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Hi Folks,

Recently I found it useful to use systemtap to add a configurable
delay inside a critical section of module code. This was done to widen
a (very difficult to hit) race window for testing/verification purposes.

I got this working with a bit of embedded C:

# cat cdc-acm-test.stp
#!/usr/bin/stap -DSTP_NO_OVERLOAD -g

%{
#include <linux/delay.h>
%}

function mdelay(ms:long) %{
 /* guru */
 mdelay(THIS->ms);
%}

probe begin {
 printf("%s\n", "cdc-acm-test: inserting delay probe in tasklet fn")
}

probe end {
 printf("%s\n", "cdc-acm-test: removing delay probe from tasklet fn")
 printf("%s\n", "cdc-acm-test: done")
}

probe module("cdc_acm").statement("*@drivers/usb/class/cdc-acm.c:341") {
 mdelay($1)
}

The DWARF probe is necessary as we need to land in the middle of a
section protected by a spinlock (that's taken from a tasklet without
disabling interrupts..).

Would it be worth having something like this in a tapset? The
requirement to use STP_NO_OVERLOAD for long-running delays (I needed
200ms to reliably hit the lockup in cdc-acm) might make this less
useful but I found the technique very useful for this particular case.

Cheers,
Bryn.
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