[PATCH v2] sched/cputime: steal_account_process_tick() should return jiffies

[Chris Friesen]

The callers of steal_account_process_tick() expect it to return
whether a jiffy should be considered stolen or not.

Currently the return value of steal_account_process_tick() is in
units of cputime, which vary between either jiffies or nsecs
depending on CONFIG_VIRT_CPU_ACCOUNTING_GEN.

If cputime has nsecs granularity and there is a tiny amount of
stolen time (a few nsecs, say) then we will consider the entire
tick stolen and will not account the tick on user/system/idle,
causing /proc/stats to show invalid data.

The fix is to change steal_account_process_tick() to accumulate
the stolen time and only account it once it's worth a jiffy.

(Thanks to Frederic Weisbecker for suggestions to fix a bug in my
first version of the patch.)

Signed-off-by: Chris Friesen <chris.frie...@windriver.com>
---
 kernel/sched/cputime.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index b2ab2ff..ab2b5fb 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -262,21 +262,21 @@ static __always_inline bool 
steal_account_process_tick(void)
 #ifdef CONFIG_PARAVIRT
        if (static_key_false(&paravirt_steal_enabled)) {
                u64 steal;
-               cputime_t steal_ct;
+               unsigned long steal_jiffies;
 
                steal = paravirt_steal_clock(smp_processor_id());
                steal -= this_rq()->prev_steal_time;
 
                /*
-                * cputime_t may be less precise than nsecs (eg: if it's
-                * based on jiffies). Lets cast the result to cputime
+                * steal is in nsecs but our caller is expecting steal
+                * time in jiffies. Lets cast the result to jiffies
                 * granularity and account the rest on the next rounds.
                 */
-               steal_ct = nsecs_to_cputime(steal);
-               this_rq()->prev_steal_time += cputime_to_nsecs(steal_ct);
+               steal_jiffies = nsecs_to_jiffies(steal);
+               this_rq()->prev_steal_time += jiffies_to_nsecs(steal_jiffies);
 
-               account_steal_time(steal_ct);
-               return steal_ct;
+               account_steal_time(jiffies_to_cputime(steal_jiffies));
+               return steal_jiffies;
        }
 #endif
        return false;