linux-kernel - Re: cli/sti vs local_cmpxchg and local_add

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Date:	Tue, 17 Mar 2009 00:44:54 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	David Miller <davem@...emloft.net>
Cc:	paulmck@...ux.vnet.ibm.com, mingo@...e.hu,
	jwboyer@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	ltt-dev@...ts.casi.polymtl.ca
Subject: Re: cli/sti vs local_cmpxchg and local_add_return

* David Miller (davem@...emloft.net) wrote:
> From: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
> Date: Tue, 17 Mar 2009 00:10:16 -0400
> 
> > Thanks for running those tests. Actually, I did not expect good results
> > for sparc64 because the local_t primitives map to atomic_t. Looking at
> > sparc atomic_64.h, I notice that all atomic operations except cmpxchg
> > are done through function calls even when those functions only contain
> > few instructions.  Is there any particular reason for that ? These
> > function calls can be quite costly. We could easily inline those.
> 
> With all the memory barriers, cpu bug workarounds, et al.
> it's way too much to expand inline.
> 
> > And to "unleash" the full power of local_t, we should see if there are
> > variants of the atomic operations which are safe only on UP and if there
> > are some memory barriers currently embedded in the atomic_t ops we could
> > remove in a local_t version. Actually, all the
> > BACKOFF_SETUP/BACKOFF_SPIN is specific to SMP, and therefore the local_t
> > version probably does not need that because it touches specifically
> > per-cpu data. That could give very interesting results.
> > 
> > The reason why the results shows 0 cycles per loop is just because there
> > is less that a bus clock cycle per loop. But the total time (in bus
> > cycles) for the whole 20000 cycles gives us equivalent information.
> 
> I don't think it's worth it.  Rusty made similar tests not too long
> ago.
> 
> IRQ disabling/enabling on sparc64 is 9 cycles (each) and the atomic
> operation on the other hand is at least 35 cycles.

OK, so sparc64 should probably implement local_t with interrupt
disabling on the local CPU and two atomic aligned operations (1 read, 1
write) of 64-bits variables from/to memory, so we make sure that if a
remote CPU tries to simply read the information, it is never seen as
corrupted.

Note that any code doing "remote reads" and "write expected to be read
from a remote cpu" on local_t variables must provide its own memory
barriers.

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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