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Date:      Wed, 25 Mar 2015 12:27:49 +0100
From:      Hans Petter Selasky <hps@selasky.org>
To:        Gleb Smirnoff <glebius@FreeBSD.org>
Cc:        svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org
Subject:   Re: svn commit: r280495 - head/sys/kern
Message-ID:  <55129BB5.4000502@selasky.org>
In-Reply-To: <20150325111433.GT64665@FreeBSD.org>
References:  <201503250855.t2P8tZFp038467@svn.freebsd.org> <20150325105409.GS64665@FreeBSD.org> <551296C6.9070402@selasky.org> <20150325111433.GT64665@FreeBSD.org>

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On 03/25/15 12:14, Gleb Smirnoff wrote:
> On Wed, Mar 25, 2015 at 12:06:46PM +0100, Hans Petter Selasky wrote:
> H> On 03/25/15 11:54, Gleb Smirnoff wrote:
> H> > On Wed, Mar 25, 2015 at 08:55:35AM +0000, Hans Petter Selasky wrote:
> H> > H> Author: hselasky
> H> > H> Date: Wed Mar 25 08:55:34 2015
> H> > H> New Revision: 280495
> H> > H> URL: https://svnweb.freebsd.org/changeset/base/280495
> H> > H>
> H> > H> Log:
> H> > H>   Implement a simple OID number garbage collector. Given the increasing
> H> > H>   number of dynamically created and destroyed SYSCTLs during runtime it
> H> > H>   is very likely that the current new OID number limit of 0x7fffffff can
> H> > H>   be reached. Especially if dynamic OID creation and destruction results
> H> > H>   from automatic tests. Additional changes:
> H> > H>
> H> > H>   - Optimize the typical use case by decrementing the next automatic OID
> H> > H>   sequence number instead of incrementing it. This saves searching time
> H> > H>   when inserting new OIDs into a fresh parent OID node.
> H> > H>
> H> > H>   - Add simple check for duplicate non-automatic OID numbers.
> H> >
> H> > Why didn't you use alloc_unr(9) for that?
> H> >
> H>
> H> Hi Gleb,
> H>
> H> I thought about using alloc_unr(). The problem is that sysctls have no
> H> clear concept of freeing. For example in some existing code sysctl are
> H> unregistered and registered again assuming that the oid_number will be
> H> preserved. I didn't want touch those parts. Also, hence we are already
> H> traversing a list to insert an SYSCTL object in an ordered fashion, the
> H> benefit of alloc_unr() is not that big. What do you think?
>
> Is it possible to split the space into two halves: one for static OIDs and
> other for dynamic ones? The latter allocated via alloc_unr?
>

Yes, it is possible to do. One issue though is that the OID numbers 
shouldn't be recycled right away to preserve the old behaviour. It is 
not written anywhere, but I think there is some cleverness into the fact 
that the newoid value does not repeat instantly, as would happen using 
alloc_unr() to avoid races accessing OIDs from userspace? Does 
alloc_unr() support a sort of rotating allocation number mode within the 
defined range?

The OID numbers are only required to be unique per directory level from 
what I can see.

--HPS



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