Re: [RFC&PATCH] Alternative RCU implementation

[Dipankar Sarma]

On Mon, Aug 30, 2004 at 01:13:41PM -0400, Jim Houston wrote:
> On Sun, 2004-08-29 at 20:43, Paul E. McKenney wrote:
> > Are these critical realtime processes user-mode only, or do they
> > also execute kernel code?  If they are user-mode only, a much more
> > straightforward approach might be to have RCU pretend that they do
> > not exist.
> > 
> > This approach would have the added benefit of keeping rcu_read_unlock()
> > atomic-instruction free.  In some cases, the overhead of the atomic
> > exchange would overwhelm that of the read-side RCU critical section.
> > 
> > Taking this further, if the realtime CPUs are not allowed to execute in
> > the kernel at all, you can avoid overhead from smp_call_function() and
> > the like -- and avoid confusing those parts of the kernel that expect to
> > be able to send IPIs and the like to the realtime CPU (or do you leave
> > IPIs enabled on the realtime CPU?).
> 
> Our customers applications vary but, in general, the realtime processes
> will do the usual system calls to synchronize with other processes and
> do I/O.
> 
> I considered tracking the user<->kernel mode transitions extending the
> idea of the nohz_cpu_mask.  I gave up on this idea mostly because it
> required hooking into assembly code.  Just extending the idea of this
> bitmap has its own scaling issues.  We may have several cpus running
> realtime processes. The obvious answer is to keep the information in
> a per-cpu variable and pay the price of polling this from another cpu.

Tracking user<->kernel transitions and putting smarts in scheduler
about RCU is the right way to go IMO.

Anything that poll other cpus and has read-side overheads will likely
not be scalable. I think that is not the right way to go about solving
the issue of dependency on local timer interrupt. It is a worthy goal
and we need to do this anyway, but we need to do it right without
hurting the current advantages as much as possible.


> I know that I'm questioning one of your design goals for RCU by adding
> overhead to the read-side.  I have read everything I could find on RCU.
> My belief is that the cost of the xchg() instruction is small 
> compared to the cache benifit of freeing memory more quickly.
> I think it's more interesting to look at the impact of the xchg() at the
> level of an entire system call.  Adding 30 nanoseconds to a open/close
> path that tasks 3 microseconds seems reasonable.  It is hard to measure
> the benefit of reusing the a dcache entry more quickly.
> 
> I would be interested in suggestions for testing.  I would be very
> interested to hear how my patch does on a large machine.

I will get you some numbers on a large machine. But I remain opposed
to this approach. I believe it can be done without the read-side
overheads. 


> I'm also trying to figure out if I need the call_rcu_bh() changes.
> Since my patch will recognize a grace periods as soon as any 
> pending read-side critical sections complete, I suspect that I
> don't need this change.

Except that under a softirq flood, a reader in a different read-side
critical section may get delayed a lot holding up RCU. Let me know
if I am missing something here.

Thanks
Dipankar
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