Re: [PATCH] steal_account_process_tick() should return jiffies

[Chris Friesen]

On 03/05/2016 07:19 AM, Frederic Weisbecker wrote:
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.

Yes, this is exactly it.  Because of this, if CONFIG_VIRT_CPU_ACCOUNTING_GEN is 
enabled in a guest then the idle/system/user stats in /proc/stat can show odd 
values, and "top" shows nothing for user/system even if CPU hogs are running.

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.

Yes, on reflection you are correct, and the patch looks pretty close, except 
that account_steal_time() is still expecting units of cputime.  I'll send a 
followup patch.

> Something like that:

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(&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;
@@ -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;