Re: [rfc] migrate lagg to an rmlock

On Aug 24, 2013, at 12:14 PM, Alfred Perlstein <bright_at_mu.org> wrote:

> On 8/24/13 10:47 AM, Robert N. M. Watson wrote:
>> On 24 Aug 2013, at 17:36, Alfred Perlstein wrote:
>> 
>>>> We should distinguish "lock contention" from "line contention". When acquiring a rwlock on multiple CPUs concurrently, the cache lines used to implement the lock are contended, as they must bounce between caches via the cache coherence protocol, also referred to as "contention".  In the if_lagg code, I assume that the read-only acquire of the rwlock (and perhaps now rmlock) is for data stability rather than mutual exclusion -- e.g., to allow processing to completion against a stable version of the lagg configuration. As such, indeed, there should be no lock contention unless a configuration update takes place, and any line contention is a property of the locking primitive rather than data model.
>>>> 
>>>> There are a number of other places in the kernel where migration to an rmlock makes sense -- however, some care must be taken for four reasons: (1) while read locks don't experience line contention, write locking becomes observably e.g., rmlocks might not be suitable for tcbinfo; (2) rmlocks, unlike rwlocks, more expensive so is not suitable for all rwlock line contention spots -- implement reader priority propagation, so you must reason about; and (3) historically, rmlocks have not fully implemented WITNESS so you may get less good debugging output.  if_lagg is a nice place to use rmlocks, as reconfigurations are very rare, and it's really all about long-term data stability.
>>> Robert, what do you think about a quick swap of the ifnet structures to counter before 10.x?
>> Could you be more specific about the proposal you're making?
>> 
>> Robert
> 
> The lagg patch referred to in the thread seems to indicate that zero locking is needed if we just switched to counter(9), that makes me wonder if we could do better with locking in other places if we switched to counter(9) while we have the chance.
> 
> This is the thread:
> 
> http://lists.freebsd.org/pipermail/svn-src-all/2013-April/067570.html
> 
>> /    
> />/    Perfect solution would be to convert ifnet(9) to counters(9), but this
> />/    requires much more work, and unfortunately ABI change, so temporarily
> />/    patch lagg(4) manually.
> />/    />/    We store counters in the softc, and once per second push their values
> />/    to legacy ifnet counters./
> 

Some sort of gatekeeper semantic is needed to ensure that configuration changes to the lagg state don't cause incorrect behavior to the data path.  It's not about protecting the integrity of counters.  This can be done several ways, but right now it's via a read/write semantic.

Scott