On Mon, 28 Jan 2013, Frederic Weisbecker wrote: > 2013/1/28 Christoph Lameter <c...@linux.com>: > > On Mon, 28 Jan 2013, Frederic Weisbecker wrote: > > > >> My last concern is the dependency on CONFIG_64BIT. We rely on cputime_t > >> being u64 for reasonable nanosec granularity implementation. And therefore > >> we need a single instruction fetch to read kernel cpustat for atomicity > >> requirement against concurrent incrementation, which only 64 bit archs > >> can provide. > > > > Most x86 cpus support cmpxchg8b on 32bit which can be abused for > > 64 bit reads (see cmpxchg64 in cmpxchg_32.h). Simply do a cmpxchg with > > zero and use whatever it returns. > > > > A percpu version that uses the instruction is called > > this_cpu_cmpxchg_double(). > > Yeah but we need to be able to do remote read. In fact atomic_read() > would do the trick.
Well yes, cmpxchg64() could do the same without the need for an atomic variable. > >> There is just no emergency though as this new option depends on the context > >> tracking subsystem that only x86-64 (and soon ppc64) implements yet. And > >> this set is complex enough already. I think we can deal with that later. > > > > Ok then this may not be that useful. > > What is not useful? The information about the 64 bit reads that I posted. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
