Message-ID: <56DBBDB8.40305@mail.usask.ca>
Date: Sat, 05 Mar 2016 23:18:48 -0600
From: Chris Friesen <cbf123@mail.usask.ca>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.3.0
MIME-Version: 1.0
To: Frederic Weisbecker <fweisbec@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>
CC: John Stultz <john.stultz@linaro.org>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        Rik van Riel <riel@redhat.com>
Subject: [PATCH v2] sched/cputime: steal_account_process_tick() should return
 jiffies
References: <56DA1339.5030601@mail.usask.ca> <alpine.DEB.2.11.1603051121550.3686@nanos> <20160305131856.GA4441@lerouge> <56DBAF6D.5010503@mail.usask.ca>
In-Reply-To: <56DBAF6D.5010503@mail.usask.ca>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2052
Lines: 56

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.friesen@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;