2013/1/28 Christoph Lameter <c...@linux.com>: > 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.
Yeah I'll have a look at this. > >> >> 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. Ah ok. Thanks. -- 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/
