Re: [patch 9/9] LTTng instrumentation - swap

Ingo Molnar
2009-03-24T18:52:45Z

* Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:

> +DECLARE_TRACE(swap_in,
> + TPPROTO(struct page *page, swp_entry_t entry),
> + TPARGS(page, entry));
> +DECLARE_TRACE(swap_out,
> + TPPROTO(struct page *page),
> + TPARGS(page));
> +DECLARE_TRACE(swap_file_open,
> + TPPROTO(struct file *file, char *filename),
> + TPARGS(file, filename));
> +DECLARE_TRACE(swap_file_close,
> + TPPROTO(struct file *file),
> + TPARGS(file));

These are more complete than the pagecache tracepoints, but still
incomplete to make a comprehensive picture about swap activities.

Firstly, the swap_file_open/close events seem quite pointless. Most
systems enable swap during bootup and never close it. These
tracepoints just wont be excercised in practice.

Also, to _really_ help with debugging VM pressure problems, the
whole LRU state-machine should be instrumented, and linked up with
pagecache instrumentation via page frame numbers and (inode,offset)
[file] and (pgd,addr) [anon] pairs.

Not just the fact that something got swapped out is interesting, but
also the whole decision chain that leads up to it. The lifetime of a
page how it jumps between the various stages of eviction and LRU
scores.

a minor nit:

> +DECLARE_TRACE(swap_file_open,
> + TPPROTO(struct file *file, char *filename),
> + TPARGS(file, filename));

there's no need to pass in the filename - it can be deducted in the
probe from struct file.

a small inconsistency:

> +DECLARE_TRACE(swap_in,
> + TPPROTO(struct page *page, swp_entry_t entry),
> + TPARGS(page, entry));
> +DECLARE_TRACE(swap_out,
> + TPPROTO(struct page *page),
> + TPARGS(page));

you pass in swp_entry to trace_swap_in(), which encodes the offset -
but that parameter is not needed, the page already represents the
offset at that stage in do_swap_page(). (the actual data is not read
in yet from swap, but the page is already linked up in the
swap-cache and has the offset available - which a probe can
recover.)

So this suffices:

DECLARE_TRACE(swap_in,
TPPROTO(struct page *page),
TPARGS(page));

DECLARE_TRACE(swap_out,
TPPROTO(struct page *page),
TPARGS(page));

And here again i'd like to see actual meaningful probe contents via
a TRACE_EVENT() construct. That shows and proves that it's all part
of a comprehensive framework, and the data that is recovered is
understood and put into a coherent whole - upstream. That makes it
immediately useful to the built-in tracers, and will also cause
fewer surprises downstream.

Ingo