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? Nicolas
