Re: [PATCH 1/6] writeback: initial tracing support

Dave Chinner
2010-05-28T00:42:15Z

On Thu, May 27, 2010 at 02:32:33PM -0700, Andrew Morton wrote:
> On Tue, 25 May 2010 20:54:07 +1000
> Dave Chinner <david@fromorbit.com> wrote:
>
> > From: From: Jens Axboe <jens.axboe@oracle.com>
> >
> > Trace queue/sched/exec parts of the writeback loop.
>
> It would be most useful if this patchset's description provided sample
> tracing output, so we can see what the patch is actually providing us.

This is just a forward port of Jen's patch. I guess I'll have to
clean it up some more...

>
> > -#define inode_to_bdi(inode) ((inode)->i_mapping->backing_dev_info)
> > -
> > -/*
> > - * We don't actually have pdflush, but this one is exported though /proc...
> > - */
> > -int nr_pdflush_threads;
> > -
> > /*
> > * Passed into wb_writeback(), essentially a subset of writeback_control
> > */
> > @@ -63,6 +57,16 @@ struct bdi_work {
> > unsigned long state; /* flag bits, see WS_* */
> > };
> >
> > +#define CREATE_TRACE_POINTS
> > +#include <trace/events/writeback.h>
> > +
> > +#define inode_to_bdi(inode) ((inode)->i_mapping->backing_dev_info)
>
> Could/should be implemented in C.

OK.

>
> > +/*
> > + * We don't actually have pdflush, but this one is exported though /proc...
> > + */
> > +int nr_pdflush_threads;
>
> So this is always zero now?

I guess so. I'd forgotten that this was in the original patch....

> We don't want to keep it forever. Add a
> printk_once("nr_pdflush_threads is deprecated") when someone reads it,
> remove it in 2014.

OK.

> > --- /dev/null
> > +++ b/include/trace/events/writeback.h
> > @@ -0,0 +1,171 @@
> > +#undef TRACE_SYSTEM
> > +#define TRACE_SYSTEM writeback
> > +
> > +#if !defined(_TRACE_WRITEBACK_H) || defined(TRACE_HEADER_MULTI_READ)
>
> Seems wrong. If you define TRACE_HEADER_MULTI_READ then include this
> header twice, things explode. Which negates the purpose of
> _TRACE_WRITEBACK_H.

Every other trace event header does this, so if it's wrong then the
same mistake is in at least 30 files now. I don't know enough about
the tracing code to know why this is done, and I'm not keen to
address such a mistake here...

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com