linux-kernel - Re: [PATCH 0/3] cpuidle: updates related to tick_broadcast

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: <2602092.6h7ufczYGd@vostro.rjw.lan>
Date:	Tue, 12 May 2015 01:31:14 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>, peterz@...radead.org,
	tglx@...utronix.de, rafael.j.wysocki@...el.com,
	rlippert@...gle.com, linux-pm@...r.kernel.org,
	linus.walleij@...aro.org, linux-kernel@...r.kernel.org,
	mingo@...hat.com, sudeep.holla@....com,
	linuxppc-dev@...ts.ozlabs.org, Kevin Hilman <khilman@...aro.org>,
	Lina Iyer <lina.iyer@...aro.org>,
	Ulf Hansson <ulf.hansson@...aro.org>
Subject: Re: [PATCH 0/3] cpuidle: updates related to tick_broadcast_enter() failures

On Monday, May 11, 2015 07:40:41 PM Daniel Lezcano wrote:
> On 05/10/2015 01:15 AM, Rafael J. Wysocki wrote:
> > On Saturday, May 09, 2015 10:33:05 PM Rafael J. Wysocki wrote:
> >> On Saturday, May 09, 2015 10:11:41 PM Rafael J. Wysocki wrote:
> >>> On Saturday, May 09, 2015 11:19:16 AM Preeti U Murthy wrote:
> >>>> Hi Rafael,
> >>>>
> >>>> On 05/08/2015 07:48 PM, Rafael J. Wysocki wrote:
> >>>
> >>> [cut]
> >>>
> >>>>>>
> >>>>>> +	/* Take note of the planned idle state. */
> >>>>>> +	idle_set_state(smp_processor_id(), target_state);
> >>>>>
> >>>>> And I wouldn't do this either.
> >>>>>
> >>>>> The behavior here is pretty much as though the driver demoted the state chosen
> >>>>> by the governor and we don't call idle_set_state() again in those cases.
> >>>>
> >>>> Why is this wrong?
> >>>
> >>> It is not "wrong", but incomplete, because demotions done by the cpuidle driver
> >>> should also be taken into account in the same way.
> >>>
> >>> But I'm seeing that the recent patch of mine that made cpuidle_enter_state()
> >>> call default_idle_call() was a mistake, because it might confuse find_idlest_cpu()
> >>> significantly as to what state the CPU is in.  I'll drop that one for now.
> >>
> >> OK, done.
> >>
> >> So after I've dropped it I think we need to do three things:
> >> (1) Move the idle_set_state() calls to cpuidle_enter_state().
> >> (2) Make cpuidle_enter_state() call default_idle_call() again, but this time
> >>      do that *before* it has called idle_set_state() for target_state.
> >> (3) Introduce demotion as per my last patch.
> >>
> >> Let me cut patches for that.
> >
> > Done as per the above and the patches follow in replies to this messge.
> >
> > All on top of the current linux-next branch of the linux-pm.git tree.
> 
> IMO the resulting code is more and more confusing.

Why is it confusing?

What part of it is confusing?

Patches [1-2/3] simply replace https://patchwork.kernel.org/patch/6326761/
and I'm not sure why that would be confusing.

Patch [3/3] simply causes cpuidle_enter_state() to pick up a more suitable
state if tick_broadcast_enter() fails instead of returning an error code
in that case.  What exactly is confusing in that?

> Except I miss something, the tick_broadcast_enter can fail only if the 
> local timer of the current cpu is used as a broadcast timer (which is 
> the case today for PPC only).

well, why does this matter?

> The correct fix would be to tie this local timer with the cpu power 
> domain and disable the idle state powering down this domain like it was 
> done for the renesas cpuidle driver.
> 
> IOW, the cpu power domain is in use (because of its local timer), so we 
> shouldn't shut it down.
> 
> No ?

Sorry, I'm not sure what you're talking about.

The problem at hand is that tick_broadcast_enter() can fail and we need to
handle that.  If we can prevent it from ever failing, that would be awesome,
but quite honestly I don't see how to do that ATM.

> I am aware this is not easily fixable because the genpd framework is 
> incomplete and has some restrictions but I believe it is worth to have a 
> discussion. Add Kevin and Ulf in Cc.

So I'm going to queue up these patches for 4.2 and we can have a discussion
just fine regardless.


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
--
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/