2013/1/10 Stanislaw Gruszka <sgrus...@redhat.com>: > On Wed, Jan 09, 2013 at 07:33:03PM +0100, Frederic Weisbecker wrote: >> On Mon, Jan 07, 2013 at 12:31:45PM +0100, Stanislaw Gruszka wrote: >> > We scale stime, utime values based on rtime (sum_exec_runtime converted >> > to jiffies). During scaling we multiple rtime * utime, what seems to be >> > fine, since both values are converted to u64, but is not. >> > >> > Let assume HZ is 1000 - 1ms tick. Process consist of 64 threads, run >> > for 1 day, threads utilize 100% cpu on user space. Machine has 64 cpus. >> > >> > Process rtime = utime will be 64 * 24 * 60 * 60 * 1000 jiffies, what is >> > 0x149970000. Multiplication rtime * utime result is 0x1a855771100000000, >> > which can not be covered in 64 bits. >> > >> > Result of overflow is stall of utime values visible in user space >> > (prev_utime in kernel), even if application still consume lot of CPU >> > time. >> > >> > Probably good fix for the problem, will be using 128 bit variable and >> > proper mul128 and div_u128_u64 primitives. While mul128 is on it's >> > way to kernel, there is no 128 bit division yet. I'm not sure, if we >> > want to add it to kernel. Perhaps we could also change the way how >> > stime and utime are calculated, but I don't know how, so I come with >> > the below solution for the problem. >> > >> > To avoid overflow patch change value we scale to min(stime, utime). This >> > is more like workaround, but will work for processes, which perform >> > mostly on user space or mostly on kernel space. Unfortunately processes, >> > which perform on kernel and user space equally, and additionally utilize >> > lot of CPU time, still will hit this overflow pretty quickly. However >> > such processes seems to be uncommon. >> > >> > Signed-off-by: Stanislaw Gruszka <sgrus...@redhat.com> >> >> I can easily imagine that overflow to happen with user time on intensive >> CPU bound loads, or may be guests. >> >> But can we easily reach the same for system time? Even on intensive I/O bound >> loads we shouldn't spend that much time in the kernel. Most of it probably >> goes >> to idle. >> >> What do you think? > > I think you are right :-) > >> If that assumption is right in most cases, the following patch should solve >> the >> issue: > > I'm fine with this patch, it achives the same effect as my patch, but is > simpler.
Cool! So I can add your acked-by, right? I'll send this patch to Ingo soon. 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/
