On Tuesday 10 June 2008 09:54:09 Christoph Lameter wrote: > On Tue, 10 Jun 2008, Rusty Russell wrote: > > > > Yes, this should be fixed. I thought i386 had optimized versions > > > > pre-merge, but I was wrong (%gs for per-cpu came later, and noone > > > > cleaned up these naive versions). Did you want me to write them? > > > > > > How can that be fixed? You have no atomic instruction that calculates > > > the per cpu address in one go. > > > > Huh? "incl %fs:varname" does exactly this. > > Right that is what the cpu alloc patches do. So you could implement > cpu_local_inc on top of some of the cpu alloc patches. Or you could just implement it today as a standalone patch. > > > And as long as that is the case you need to > > > disable preempt. Otherwise you may increment the per cpu variable of > > > another processor because the process was rescheduled after the address > > > was calculated but before the increment was done. > > > > But of course, that is not a problem. You make local_t an atomic_t, and > > then it doesn't matter which CPU you incremented. > > But then the whole point of local_t is gone. Why not use atomic_t in the > first place? Because some archs can do better. > > By definition if the caller cared, they would have had premption > > disabled. > > There are numerous instances where the caller does not care about > preemption. Its just important that one per cpu counter is increment in > the least intrusive way. See f.e. the VM event counters. Yes, and that's exactly the point. The VM event counters are exactly a case where you should have used cpu_local_inc. Rusty. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
