Re: [PATCH] psi: avoid divide-by-zero crash inside virtual machines
From: Johannes Weiner
Date: Thu Feb 14 2019 - 15:53:51 EST
On Thu, Feb 14, 2019 at 11:58:55AM -0800, Andrew Morton wrote:
> On Thu, 14 Feb 2019 14:31:57 -0500 Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> > --- a/kernel/sched/psi.c
> > +++ b/kernel/sched/psi.c
> > @@ -322,7 +322,7 @@ static bool update_stats(struct psi_group *group)
> > expires = group->next_update;
> > if (now < expires)
> > goto out;
> > - if (now - expires > psi_period)
> > + if (now - expires >= psi_period)
> > missed_periods = div_u64(now - expires, psi_period);
> > /*
> It seems appropriate to use time_after64() and friends in this code.
These timestamps are all sourced from sched_clock(), which is defined
to be monotonic and never wrap in practice. From the "sched_clock()"
section in Documentation/timers/timekeeping.txt:
"This function shall return the number of nanoseconds since
the system was started."
"The sched_clock() function may wrap only on unsigned long
long boundaries, i.e. after 64 bits. Since this is a
nanosecond value this will mean it wraps after circa 585
years. (For most practical systems this means "never".)"
As far as readability goes, I have to say I find the naked comparisons
a bit easier to understand (and I'm glad we can use those here since
the code is already complicated):
if (now < expires)
if (time_before64(now, expires))
These macros always have me double check the argument order.