On Sat, Mar 05, 2016 at 11:27:01AM +0100, Thomas Gleixner wrote:
Chris,
On Fri, 4 Mar 2016, Chris Friesen wrote:
First of all the subject line should contain a subsystem prefix,
i.e. "sched/cputime:"
The callers of steal_account_process_tick() expect it to return whether
the last jiffy was 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.
Sure, but what is the actual problem? The return value is boolean and tells
whether there was stolen time accounted or not.
Indeed the changelog should better explain the problem. So I think the issue is that
if the cputime has nsecs granularity and we have a tiny stolen time to account (lets say
a few nanosecs, in fact anything that is below a jiffy), we are not going to account the
tick on user/system.
But the fix doesn't look right to me because we are still accounting the steal time
if it is lower than a jiffy and that steal time will never be substracted to user/system
time if it never reach a jiffy.
Instead the fix should accumulate the steal time and account it only once it's worth
a jiffy and then substract it from system/user time accordingly.
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index b2ab2ff..d38e25f 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -262,7 +262,7 @@ static __always_inline bool steal_account_process_tick(void)
#ifdef CONFIG_PARAVIRT
if (static_key_false(¶virt_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;
@@ -272,11 +272,11 @@ static __always_inline bool steal_account_process_tick(void)
* based on jiffies). Lets cast the result to cputime
* 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;
+ return steal_jiffies;
}
#endif
return false;