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SubjectRe: [PATCH] Read-Copy Update 2.5.34
On Wed, Sep 11, 2002 at 01:24:21PM +0200, Ingo Molnar wrote:
>
> i dont really understand why it has to change the scheduler. You want a
> facility to force a reschedule on any given CPU, correct?
>

Hi Ingo,

Yes, like force_cpu_reschedule().

Apart from that RCU adds some minor things in the scheduler code -

1. Per-CPU context switch counter increment in the fast path
2. For preemptive kernels, a conditional branch in the voluntary
context switch code path checking whether the current task may
have had an involuntary context switch earlier [rcu_preempt_put()].
3. Adding a field to the task structure - cpu_preempt_cntr.
4. RCU checking hook into the scheduler_tick(), but this is not
in the fast path.

#2 and #3 are necessary to support call_rcu_preempt() which
allows preemption-safe reads of data protected by RCU. A preempted
task may still contain references to RCU protected data and
the RCU grace period needs to be prolonged until all such tasks
before the update do voluntary context switches.

I did some reflex benchmarking to make sure that I didn't introduce
any false sharing by mistake in scheduler fast path and the results
look comparable -

(4CPU P3 Xeon with 1MB L2 + 1GB RAM)

vanilla-2.5.34 rcu_poll-2.5.34
-------------- ---------------
80 , 40 , 1.593 1.569
112 , 40 , 1.544 1.554
144 , 40 , 1.595 1.552
176 , 40 , 1.568 1.605
198 , 40 , 1.562 1.577
230 , 40 , 1.563 1.583
244 , 40 , 1.671 1.638
Not sure how reliable these numbers are.

Thanks
--
Dipankar Sarma <dipankar@in.ibm.com> http://lse.sourceforge.net
Linux Technology Center, IBM Software Lab, Bangalore, India.
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