Patch "sched/cputime: Fix steal_account_process_tick() to always return jiffies" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

sched/cputime: Fix steal_account_process_tick() to always return jiffies

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
sched-cputime-fix-steal_account_process_tick-to-always-return-jiffies.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Chris Friesen <[email protected]>
Date: Sat, 5 Mar 2016 23:18:48 -0600
Subject: sched/cputime: Fix steal_account_process_tick() to always return jiffies

From: Chris Friesen <[email protected]>

commit f9c904b7613b8b4c85b10cd6b33ad41b2843fa9d upstream.

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.)

---
 kernel/sched/cputime.c |   14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

@@ -259,21 +259,21 @@ static __always_inline bool steal_accoun
 #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;

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/sched-cputime-fix-steal_account_process_tick-to-always-return-jiffies.patch