Re: [PATCH 1/6] delayacct: Use sched_clock()

[Peter Zijlstra]

On Thu, May 06, 2021 at 09:59:11AM -0400, Johannes Weiner wrote:
> On Wed, May 05, 2021 at 12:59:41PM +0200, Peter Zijlstra wrote:
> > @@ -42,10 +42,9 @@ void __delayacct_tsk_init(struct task_st
> >   * Finish delay accounting for a statistic using its timestamps (@start),
> >   * accumalator (@total) and @count
> >   */
> > -static void delayacct_end(raw_spinlock_t *lock, u64 *start, u64 *total,
> > -			  u32 *count)
> > +static void delayacct_end(raw_spinlock_t *lock, u64 *start, u64 *total, u32 *count)
> >  {
> > -	s64 ns = ktime_get_ns() - *start;
> > +	s64 ns = local_clock() - *start;
> 
> I don't think this is safe. These time sections that have preemption
> and migration enabled and so might span multiple CPUs. local_clock()
> could end up behind *start, AFAICS.

Only if you have really crummy hardware, and in that case the drift is
bounded by around 1 tick. Also, this function actually checks: ns > 0.

And if you do have crummy hardware like that, ktime_get_ns() is the very
last thing you want to call at any frequency because it'll be the HPET.