linux-kernel - Re: [RFC PATCH] LTTng instrumentation mm (updated)

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Message-Id: <1196448122.19681.16.camel@localhost>
Date:	Fri, 30 Nov 2007 10:42:02 -0800
From:	Dave Hansen <haveblue@...ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, mbligh@...gle.com
Subject: Re: [RFC PATCH] LTTng instrumentation mm (updated)

On Fri, 2007-11-30 at 12:05 -0500, Mathieu Desnoyers wrote:
> 
> 
> Given a trace including :
> - Swapfiles initially used
> - multiple swapon/swapoff
> - swap in/out events
> 
> We would like to be able to tell which swap file the information has
> been written to/read from at any given time during the trace.

Oh, tracing is expected to be on at all times?  I figured someone would
encounter a problem, then turn it on to dig down a little deeper, then
turn it off.

As for why I care what is in /proc/swaps.  Take a look at this:

struct swap_info_struct *
get_swap_info_struct(unsigned type)
{
        return &swap_info[type];
}

Then, look at the proc functions: 

static void *swap_next(struct seq_file *swap, void *v, loff_t *pos)
{
        struct swap_info_struct *ptr;
        struct swap_info_struct *endptr = swap_info + nr_swapfiles;

        if (v == SEQ_START_TOKEN)
                ptr = swap_info;
...

I guess if that swap_info[] has any holes, we can't relate indexes in
there right back to /proc/swaps, but maybe we should add some
information so that we _can_.

-- Dave

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