On Sat, May 10, 2014 at 12:57:15AM +0200, Thomas Gleixner wrote: > On Fri, 9 May 2014, Christoph Lameter wrote: > > On Fri, 9 May 2014, Thomas Gleixner wrote: > > > I understand why you want to get this done by a housekeeper, I just > > > did not understand why we need this whole move it around business is > > > required. > > > > This came about because of another objection against having it simply > > fixed to a processor. After all that processor may be disabled etc etc. > > I really regret that I did not pay more attention (though my cycle > constraints simply do not allow it). As far as I can see, the NO_HZ_FULL timekeeping CPU is always zero. If it can change in NO_HZ_FULL kernels, RCU will do some very strange things! One possible issue here is that Christoph's patch is unconditional. It takes effect for both NO_HZ_FULL and !NO_HZ_FULL. If I recall correctly, the timekeeping CPU -can- change in !NO_HZ_FULL kernels, which might be what Christoph was trying to take into account. > This is the typical overengineering failure: > > Before we even have a working proof that we can solve the massive > complex basic problem with the price of a dedicated housekeeper, we > try to make the housekeeper itself a moving target with the price of > making the problem exponential(unknown) instead of simply unknown. > > I really cannot figure out why a moving housekeeper would be a > brilliant idea at all, but I'm sure there is some magic use case in > some other disjunct universe. > > Whoever complained and came up with the NOT SO brilliant idea to make > the housekeeper a moving target, come please forth and explain: > > - How this can be done without having a working solution with a > dedicated housekeeper in the first place > > - How this can be done without knowing what implication it has w/o > seing the complexity of a dedicated housekeeper upfront. > > Keep it simple has always been and still is the best engineering > principle. If someone decides to make tick_do_timer_cpu non-constant in NO_HZ_FULL CPUs, they will break unless/until I make RCU deal with that sort of thing, at least for NO_HZ_FULL_SYSIDLE kernels. ;-) > We all know that we can do large scale overhauls in a very controlled > way if the need arises. But going for the most complex solution while > not knowing whether the least complex solution is feasible at all is > outright stupid or beyond. > > Unless someone comes up with a reasonable explantion for all of this I > put a general NAK on patches which are directed to kernel/time/* > > Correction: > > I'm taking patches right away which undo any damage which has been > applied w/o me noticing because I trusted the responsible developers / > maintainers. > > Preferrably those patches arrive before my return from LinuxCon Japan. I could easily have missed something, but as far as I know, there is nothing in the current kernel that allows tick_do_timer_cpu to move in NO_HZ_FULL kernels. Hmmm... Well, I -do- have a gratuitous ACCESS_ONCE() around one fetch of tick_do_timer_cpu that happens only in NO_HZ_FULL kernels. I will remove it. Thanx, Paul > Thanks, > > tglx > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>