linux-kernel - Re: [PATCH v3] sched/deadline: Fix bad accounting of nr

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <53048849.3000601@gmail.com>
Date:	Wed, 19 Feb 2014 11:32:41 +0100
From:	Juri Lelli <juri.lelli@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3] sched/deadline: Fix bad accounting of nr_running

On 02/19/2014 09:46 AM, Peter Zijlstra wrote:
> On Tue, Feb 18, 2014 at 09:50:12PM -0500, Steven Rostedt wrote:
>>  
>>> Rationale for this odd behavior is that, when a task is throttled, it
>>> is removed only from the dl_rq, but we keep it on_rq (as this is not
>>> a "full dequeue", that is the task is not actually sleeping). But, it
>>> is also true that, while throttled a task behaves like it is sleeping
>>> (e.g., its timer will fire on a new CPU if the old one is dead). So,
>>> Steven's fix sounds also semantically correct.
>>
>> Actually, it seems that I was hitting it again, but this time getting a
>> negative number. OK, after looking at the code a bit more, I think we
>> should update the runqueue nr_running only when the task is officially
>> enqueued and dequeued, and all accounting within, will not touch that
>> number.

This is a different way to get the same result (mildly tested on my box):

---
 kernel/sched/deadline.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 0dd5e09..675dad3 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -837,7 +837,8 @@ static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags)
        if (!task_current(rq, p) && p->nr_cpus_allowed > 1)
                enqueue_pushable_dl_task(rq, p);

-       inc_nr_running(rq);
+       if (!(flags & ENQUEUE_REPLENISH))
+               inc_nr_running(rq);
 }

 static void __dequeue_task_dl(struct rq *rq, struct task_struct *p, int flags)
--

We touch nr_running only when we don't enqueue back as a consequence
of a replenishment.

> 
> But if the task is throttled it should still very much decrement the
> number. There's places that very much rely on nr_running be exactly the
> number of runnable tasks.
> 

This is a different thing, and V2 seemed to implement this behavior
(that's why I said it looked semantically correct).

Thanks,

- Juri
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/