Re: RCU latency regression in 2.6.16-rc1

[Dipankar Sarma]

On Sat, Jan 28, 2006 at 01:51:23PM -0500, Lee Revell wrote:
> On Sat, 2006-01-28 at 13:00 -0500, Lee Revell wrote:
> > OK, now we are making progress.
> 
> I spoke too soon, it's not fixed:
> 
> preemption latency trace v1.1.5 on 2.6.16-rc1
> --------------------------------------------------------------------
>  latency: 4183 us, #3676/3676, CPU#0 | (M:rt VP:0, KP:0, SP:0 HP:0)
>     -----------------
> evolutio-2877  0d.s.   97us : local_bh_enable (rt_run_flush)
> evolutio-2877  0d.s.   98us : local_bh_enable (rt_run_flush)
> evolutio-2877  0d.s.   99us : local_bh_enable (rt_run_flush)
> evolutio-2877  0d.s.  100us : local_bh_enable (rt_run_flush)
> evolutio-2877  0d.s.  101us : local_bh_enable (rt_run_flush)
> 
> [ etc ]
>  
> evolutio-2877  0d.s. 4079us : local_bh_enable (rt_run_flush)
> evolutio-2877  0d.s. 4080us : local_bh_enable (rt_run_flush)

I am not sure if I am interpreting the latency trace right,
but it seems that there is a difference between the problem
you were seeing earlier and now.

In one of your earlier traces, I saw  -

  <idle>-0     0d.s.  182us : dst_destroy (dst_rcu_free)
  <idle>-0     0d.s.  183us : ipv4_dst_destroy (dst_destroy)

[ etc - zillions of dst_rcu_free()s deleted ]

  <idle>-0     0d.s. 13403us : dst_rcu_free (__rcu_process_callbacks)
  <idle>-0     0d.s. 13403us : dst_destroy (dst_rcu_free)

This points to latency increase caused by lots and lots of
RCU callbacks doing dst_rcu_free(). Do you still see those ?

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.

Thanks
Dipankar
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