linux-kernel - Re: [PATCH V2] clocksource: register cpu notifier to remove timer from dying CPU

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]

Date:	Wed, 7 May 2014 16:06:43 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	John Stultz <john.stultz@...aro.org>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	Linaro Networking <linaro-networking@...aro.org>,
	Arvind Chauhan <Arvind.Chauhan@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Jet Chen <jet.chen@...el.com>
Subject: Re: [PATCH V2] clocksource: register cpu notifier to remove timer
 from dying CPU

On 7 May 2014 15:38, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Wed, 9 Apr 2014, Viresh Kumar wrote:
>> This patch tries to fix this by registering cpu notifiers from clocksource core,
>> only when we start clocksource-watchdog. And if on the CPU_DEAD notification it
>> is found that dying CPU was the CPU on which this timer is queued on, then it is
>> removed from that CPU and queued to next CPU.
>
> Gah, no. We realy don't want more notifier crap.

Agreed, and could have used the generic ones, probably.

> It's perfectly fine for the watchdog timer to be moved around on cpu
> down.

Functionally? Yes. Then handler doesn't have any CPU specific stuff to
do here and so queuing it on any cpu is fine.

> And the timer itself is not pinned at all. add_timer_on() does
> not set the pinned bit.

The perception I had is this:
- mod_timer() is a more complicated form of add_timer() as it has to
tackle with migration and removal of timers as well. Otherwise they
should work in similar way.
- There is no PINNED bit which can be set, its just a parameter to
__mod_timer() to decide which CPU the timer should fire on.
- And by the 'name add_timer_on()', we must guarantee that timer
fires on the CPU its being added to, otherwise it may break things
for many. There might be users which want to run the handler
on a particular CPU due to some CPU-specific stuff they want to do.
And have used add_timer_on()...

But looking at your reply, it looks that add_timer_on() doesn't
guarantee that timer would fire on the CPU mentioned? Is that the
case for mod_timer_pinned() as well ?

And if that's the case what do we want should we do with these
timers (i.e. ones added with add_timer_on or mod_timer_pinned)
when we try to quiesce a cpu using cpuset.quiesce [1]?

--
viresh

[1] https://lkml.org/lkml/2014/4/4/99
--
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/