Messages in this thread | | | Subject | Re: RCU latency regression in 2.6.16-rc1 | From | Lee Revell <> | Date | Sun, 29 Jan 2006 02:38:02 -0500 |
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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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