2013/2/15 Kevin Hilman <[email protected]>: > Frederic Weisbecker <[email protected]> writes: > >> Ingo, >> >> Please pull the new full dynticks cputime accounting code that >> can be found at: >> >> git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git >> tags/full-dynticks-cputime-for-mingo >> >> 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. > > Actually, moderately recent 32-bit ARMs can do atomic 64-bit load/stores > too.
Does gcc automatically handle 64 bit store/loads in one way or does that require specific CPU instructions? > > Also, is it just kernel_cpustat increments that need protection? or do > the various reads of the task_struct's cputime fields also need > protection (hmm, thinking twice, maybe those are already sufficiently > protected by the vtime_seqlock?) At least the task stats are protected with vtime_seqlock (hopefully I haven't missed some). But per cpu or global stats are not. > >> It's probably no big deal to solve this issue. What we need is simply some >> atomic accessors. > > What about using the atomic64_* accessors? Those would just use the > native loads/stores on arches that have them, otherwise > CONFIG_GENERIC_ATOMIC64 provides some fallbacks. Yeah may be we can try this. > > To give it a try, below is a quick patch to convert kernel_cpustat to > atomic64. I only got as far as compile testing and basic boot testing > on a 32-bit ARM platform, but let me know if this is the right > direction. Ok, I'll comment on your second version. > >> 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. > > I've started working on the ARM version of the context_tracker, so > "later" is coming quickly and I will do what I can to help this along. Ok. Thanks. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
