linux-kernel - question about logic of steal_account_process

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Message-ID: <56D9E72D.5060308@windriver.com>
Date:	Fri, 4 Mar 2016 13:51:09 -0600
From:	Chris Friesen <chris.friesen@...driver.com>
To:	lkml <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: question about logic of steal_account_process_tick() ?

I'm trying to wrap my head around how steal_account_process_tick() interacts 
with account_process_tick().

Suppose we have CONFIG_VIRT_CPU_ACCOUNTING_GEN=y and CONFIG_NO_HZ_IDLE, with a 
cpu hog on cpu0 to prevent it going idle.

As I understand it, account_process_tick() will be called once per tick to 
decide whether that tick should be allocated against user/system/idle.  However, 
it first calls steal_account_process_tick() and then returns if that returns a 
nonzero value.

The thing is, steal_account_process_tick() returns units of cputime, which I 
think is nanoseconds on x86_64.  So if we have a tiny amount of stolen time it 
seems like that will prevent a whole tick from being accounted into 
user/system/idle.

I feel like I must be missing something here, can someone tell me what it is?

Chris