Re: RCU latency regression in 2.6.16-rc1

[Lee Revell]

On Sat, 2006-01-28 at 20:52 +0100, Eric Dumazet wrote:
> > Your new trace shows that we are held up in in rt_run_flush(). 
> > I guess we need to investigate why we spend so much time in rt_run_flush(),
> > because of a big route table or the lock acquisitions.
> 
> Some machines have millions of entries in their route cache.
> 
> I suspect we cannot queue all them (or only hash heads as your
> previous patch) by RCU. Latencies and/or OOM can occur.
> 
> What can be done is :
> 
> in rt_run_flush(), allocate a new empty hash table, and exchange the
> hash tables.
> 
> Then wait a quiescent/grace RCU period (may be the exact term is not
> this one, sorry, I'm not RCU expert)
> 
> Then free all the entries from the old hash table (direclty of course,
> no need for RCU grace period), and free the hash table.
> 
> As the hash table can be huge, we might need allocate it at boot time,
> just in case a flush is needed (it usually is :) ). If we choose
> dynamic allocation and this allocation fails, then fallback to what is
> done today.
> 

No problem, I'm not a networking expert...

Ingo's response to these traces was that softirq preemption, which
simply offloads all softirq processing to softirqd and has been tested
in the -rt patchset for over a year, is the easiest solution.  Any
thoughts on that?  Personally, I'd rather fix the very few problematic
softirqs, than take such a drastic step - this softirq appears to be one
of the last obstacles to being able to meet a 1ms soft RT constraint
with the mainline kernel.

Thanks for looking at this; I'd be glad to test any patches...

Lee


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