Le Mon, Mar 11, 2024 at 05:15:26PM +0000, Nicolas Saenz Julienne a écrit : > Hi Frederic, > > On Mon Feb 19, 2024 at 5:57 PM UTC, Nicolas Saenz Julienne wrote: > > Under certain extreme conditions, the tick-based cputime accounting may > > produce inaccurate data. For instance, guest CPU usage is sensitive to > > interrupts firing right before the tick's expiration. This forces the > > guest into kernel context, and has that time slice wrongly accounted as > > system time. This issue is exacerbated if the interrupt source is in > > sync with the tick, significantly skewing usage metrics towards system > > time. > > > > On CPUs with full dynticks enabled, cputime accounting leverages the > > context tracking subsystem to measure usage, and isn't susceptible to > > this sort of race conditions. However, this imposes a bigger overhead, > > including additional accounting and the extra dyntick tracking during > > user<->kernel<->guest transitions (RmW + mb). > > > > So, in order to get the best of both worlds, introduce a cputime > > configuration option that allows using the full dynticks accounting > > scheme on NOHZ & NOHZ_IDLE CPUs, while avoiding the expensive > > user<->kernel<->guest dyntick transitions. > > > > Signed-off-by: Nicolas Saenz Julienne <nsaenz@xxxxxxxxxx> > > Signed-off-by: Jack Allister <jalliste@xxxxxxxxxxxx> > > --- > > Would you be opposed to introducing a config option like this? Any > alternatives you might have in mind? I'm not opposed to the idea no. It is not the first time I hear about people using generic virt Cputime accounting for precise stime/utime measurements on benchmarks. But let me sit down and have a look at your patch. Once I find my way through performance regression reports and rcutorture splats anyway... Thanks!
