Re: [RFC][PATCH 1/7] Resource counters

Herbert Poetzl <herbert@xxxxxxxxxxxx> writes:

On Sun, Mar 11, 2007 at 01:00:15PM -0600, Eric W. Biederman wrote:
Herbert Poetzl <herbert@xxxxxxxxxxxx> writes:


Linux-VServer does the accounting with atomic counters,
so that works quite fine, just do the checks at the
beginning of whatever resource allocation and the
accounting once the resource is acquired ...

Atomic operations versus locks is only a granularity thing.
You still need the cache line which is the cost on SMP.

Are you using atomic_add_return or atomic_add_unless or
are you performing you actions in two separate steps
which is racy? What I have seen indicates you are using
a racy two separate operation form.

yes, this is the current implementation which
is more than sufficient, but I'm aware of the
potential issues here, and I have an experimental
patch sitting here which removes this race with
the following change:

- doesn't store the accounted value but
limit - accounted (i.e. the free resource)
- uses atomic_add_return()
- when negative, an error is returned and
the resource amount is added back

changes to the limit have to adjust the 'current'
value too, but that is again simple and atomic

best,
Herbert

PS: atomic_add_unless() didn't exist back then
(at least I think so) but that might be an option
too ...

I think as far as having this discussion if you can remove that race
people will be more willing to talk about what vserver does.

That said anything that uses locks or atomic operations (finer grained locks)
because of the cache line ping pong is going to have scaling issues on large
boxes.

So in that sense anything short of per cpu variables sucks at scale. That said
I would much rather get a simple correct version without the complexity of
per cpu counters, before we optimize the counters that much.

Eric
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