linux-kernel - Re: [patch 0/1] extending low-level markers

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Message-ID: <y0m4pjbxgxe.fsf@ton.toronto.redhat.com>
Date:	07 Aug 2007 15:50:37 -0400
From:	fche@...hat.com (Frank Ch. Eigler)
To:	Noah Watkins <nwatkins@...c.ku.edu>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	"systemtap@...rceware.org" <systemtap@...rceware.org>
Subject: Re: [patch 0/1] extending low-level markers

Noah Watkins <nwatkins@...c.ku.edu> writes:

> [...]
> The locks are aquired and released in each _start and _end marker, so
> the equilibrium is not a issue.

But it becomes an issue should preemption, or control flow upset such
as an early return or recursion, occurs between the start and end
markers.

> A more sane example is the insertion of values into a histogram. Instead
> of the instrumenation point logging the values and having the histogram
> constructed during a post-process step, data structures implemented the
> histogrm are associated with the instrumenation point. The lock protects
> this structure in a more intuitive way than the interval example.

A better way may be to do what we do in systemtap scripts: in a
interval-start type probe, store the start value in a table indexed by
thread-id and nesting-level; in the interval-end type probe, find the
corresponding value, compute the difference (as appropriate), and
store the result only then into your long-term lookup table.  This
approach requires *no locks* to survive between start & end: the
probes remain atomic.

> I think most of problems involving non atomicity of the
> connection/disconnection of probes can be accomplished by good house
> keeping and settings things up in phases which insure consistency.

A marker callback can take shortcuts, I guess, if it allows itself to
make assumptions about the ways it can be called.

I scanned over the thread, but still haven't seen a specific argument
for extra marker-type annotations that would relate to this issue.
What tool would need to scan the available markers, and how would
extra information allow it to do its job?

- FChE
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