* Rusty Russell (rusty@xxxxxxxxxxxxxxx) wrote: > On Tuesday 23 December 2008 05:13:28 Mathieu Desnoyers wrote: > > > I can be convinced, but I'll need more than speculation. Assuming > > > local_long_atomic_t, can you produce a patch which uses it somewhere else? > > > > I had this patch applying over Christoph Lameter's vm tree last > > February. It did accelerate the slub fastpath allocator by using > > cmpxchg_local rather than disabling interrupts. cmpxchg_local is not > > using the local_t type, but behaves similarly to local_cmpxchg. > > > > http://lkml.org/lkml/2008/2/28/568 > > OK, I'll buy that. So we split local_t into a counter and an atomic type. > > > I know that > > local_counter_long_t and local_atomic_long_t are painful to write, but > > that would follow the current atomic_t vs atomic_long_t semantics. Hm ? > > OK, I've looked at how they're used, to try to figure out whether long > is the right thing. Counters generally want to be long, but I was in doubt > about atomics; yet grep shows that atomic_long_t is quite popular. Then > I hit struct nfs_iostats which would want a u64 and a long. I don't think > we want local_counter_u64 etc. > > Just thinking out loud, perhaps a new *type* is the wrong direction? How > about a set of macros which take a fundamental type, such as: > > DECLARE_LOCAL_COUNTER(type, name); > local_counter_inc(type, addr); > ... > DECLARE_LOCAL_ATOMIC(type, name); > local_atomic_add_return(type, addr); > > This allows pointers, u32, u64, long, etc. If a 32-bit arch can't do 64-bit > local_counter_inc easily, at least the hairy 64-bit code can be eliminated at > compile time. > > Or maybe that's overdesign? > Rusty. Yeah, I also thought of this, but I am not sure every architecture provides primitives to modify u16 or u8 data atomically like x86 does. But yes, I remember hearing Christoph Lameter being interested to use unsigned char or short atomic counters for the vm allocator in the past. The rationale was mostly that he wanted to keep a counter in a very small data type, expecting to "poll" the counter periodically (e.g. every X counter increment) and sum the total somewhere else. So I think it would be the right design in the end if we want to allow wider use of such atomic primitives for counters w/o interrupts disabled. And I would propose we use a BUILD_BUG_ON() when the architecture does not support an atomic operation on a specific type. We should also document which type sizes are supported portably and which are architecture-specific. Or, as you say, maybe it's overdesign ? If we have to pick something simple, just supporting "long" would be a good start. Mathieu -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- To unsubscribe from this list: send the line "unsubscribe linux-ia64" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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