Messages in this thread | | | Date | Tue, 17 Mar 2009 00:44:54 -0400 | From | Mathieu Desnoyers <> | Subject | Re: cli/sti vs local_cmpxchg and local_add_return |
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* David Miller (davem@davemloft.net) wrote: > From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.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
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