Messages in this thread | | | Date | Sat, 11 Apr 2009 09:08:54 +0200 | From | Ingo Molnar <> | Subject | Re: iptables very slow after commit 784544739a25c30637397ace5489eeb6e15d7d49 |
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* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> I will nevertheless suggest the following egregious hack to > get a consistent sample of one counter for some other CPU: > > a. Disable interrupts > b. Atomically exchange the bottom 32 bits of the > counter with the value zero. > c. Atomically exchange the top 32 bits of the counter > with the value zero. > d. Concatenate the values obtained in (b) and (c), which > is the snapshot value.
Note, i have recently implemented full atomic64_t support on 32-bit x86, for the perfcounters code, based on the CMPXCHG8B instruction.
Which, while not the lightest of instructions, is still much better than the sequence above.
So i think a better approach would be to also add a dumb generic implementation for atomic64_t (using a global lock or so), and then generic code could just assume that atomic64_t always exists.
It is far nicer - and faster as well - as the hack above, even on 32-bit x86.
Ingo
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