linux-kernel - Re: [PATCH 2/3] sched: Correctly handle nohz ticks cpu load accounting

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: <20160419140157.GA12400@lerouge>
Date:	Tue, 19 Apr 2016 16:01:59 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Byungchul Park <byungchul.park@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>, Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 2/3] sched: Correctly handle nohz ticks cpu load
 accounting

On Tue, Apr 19, 2016 at 09:01:00AM +0900, Byungchul Park wrote:
> On Mon, Apr 18, 2016 at 03:35:06PM +0200, Frederic Weisbecker wrote:
> > On Mon, Apr 18, 2016 at 06:17:21PM +0900, Byungchul Park wrote:
> > > On Wed, Apr 13, 2016 at 03:56:51PM +0200, Frederic Weisbecker wrote:
> > > > @@ -4645,11 +4674,11 @@ void cpu_load_update_nohz(int active)
> > > >  void cpu_load_update_active(struct rq *this_rq)
> > > >  {
> > > >  	unsigned long load = weighted_cpuload(cpu_of(this_rq));
> > > > -	/*
> > > > -	 * See the mess around cpu_load_update_idle() / cpu_load_update_nohz().
> > > > -	 */
> > > > -	this_rq->last_load_update_tick = jiffies;
> > > > -	__cpu_load_update(this_rq, load, 1, 1);
> > > > +
> > > > +	if (tick_nohz_tick_stopped())
> > > > +		cpu_load_update_nohz(this_rq, READ_ONCE(jiffies), load);
> > > > +	else
> > > > +		cpu_load_update_periodic(this_rq, load);
> > > 
> > > Considering it further, I wonder if needing it.
> > > (Sorry if I missed something.)
> > > 
> > > Case 1. tickless -> (scheduler_tick) -> tickless
> > > 
> > > 	I am not sure for this case if the rq's load can be changed or not,
> > > 	especially, if the rq's load can be changed *at this point*.
> > > 	Please remind that the load[0] is set here.
> > 
> > load[0] won't change because it's set by cpu_load_update_nohz_start().
> > But all the other load[idx] need to be decayed further.
> 
> Ah. Right. Sched tick will be handled even in the case 1...
> 
> I like your patches. But I am still wondering if the sched tick handling is
> necessary even in the case 1. Of course it's another problem though.

Right, we could indeed ignore those ticks happening in dynticks/idle and just wait
for the end of the dynticks frame that calls cpu_load_update_stop(). In fact
the first version of this patchset did that but Thomas didn't seem to like it.

That said more local updates means less need for remote updates through
cpu_load_update_idle().