Re: kprobe example watching block device access

[William Cohen <wcohen at redhat dot com>]

Frank Ch. Eigler wrote:
> Hi -
>
> wcohen wrote:
>
>
> > I have an example module working tracking the number of 
> > generic_make_request() calls and sectors transferred on a per device 
> > basis. It takes advantage of the current serial nature of the kprobe 
> > execution. [...]

Did you have suggestions on the intrumentation itself regardless of it 
being written in C?
> To me, the remarkable aspects about this example are:
>
> - how much shorter and more robust this should be in systemtap script
>   rather than C.  It should be only a handful of lines total:
>
>     #! /usr/bin/stap
>     probe kernel::function("generic_make_request") {
>         count_generic_make_request ++;
>         sectors_transferred += /*bio_sectors(bio)*/;
>         bin_bdev[bio] = /*bio->bi_bdev*/;
>         bin_bd_dev[bio] = /*bio->bi_bdev->bd_dev*/;
>         ++bin_count[bio];
>         bin_sectors[bio] += /*bio_sectors(bio)*/;
>     }
>     global bin_bdev, bin_bd_dev, bin_count, bin_sectors;
>     global count_generic_make_request, sectors_transferred;


Yes, much of the crud in the C version would go away: no code for 
setup/shutdown and associative arrays handle more concisely. The 
mechanism to get the data out is pretty weak, have to unload the module 
to get the data.

> - how important it is to be able to dereference C data structures
>   (the struct bio *) from within such a script.  I don't have a 
>   clear idea yet about how best to express such operations, either
>   specialized or generalized.  (Thus the /* */ stuff above.)
>   Someone still needs to dwelve into the whole "provider" concept,
>   which is closely related.

Doing the concrete examples is helping to see what works and doesn't 
work in the instrumentation. I am going to try to implement some of the 
other ones suggested by Ingo.

One place I cheated in the kprobebio example was using the knowledge 
that kprobes are currently sequential. There isn't any locking for the 
associative array used in the example it is relying on the kprobe lock.

-Will