linux-kernel - Re: [PATCH 4/4] nohz: Fix iowait overcounting if iowait task migrates

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] [day] [month] [year] [list]

Message-ID: <536A3747.5040506@redhat.com>
Date:	Wed, 07 May 2014 15:38:15 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	linux-kernel@...r.kernel.org,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 4/4] nohz: Fix iowait overcounting if iowait task migrates

On 05/05/2014 08:06 PM, Denys Vlasenko wrote:
> On 04/29/2014 04:47 PM, Frederic Weisbecker wrote:
>> On Thu, Apr 24, 2014 at 08:45:58PM +0200, Denys Vlasenko wrote:
>>> +void tick_nohz_iowait_to_idle(int cpu)
>>> +{
>>> +	struct tick_sched *ts = tick_get_tick_sched(cpu);
>>> +	ktime_t now = ktime_get();
>>> +
>>> +	write_seqcount_begin(&ts->idle_sleeptime_seq);
>>> +	ts->iowait_exittime = now;
>>> +	write_seqcount_end(&ts->idle_sleeptime_seq);
>>
>> So now you have two concurrent updaters using the seqcount, which is
>> very dangerous as the counters aren't updated atomically.
>>
>> seqcount is only suitable when there is a single sequential updater.
>> Once you deal with concurrent updaters you need seqlock.
>>
>> And once you add seqlock in the hot scheduler path, you're hitting
>> a big scalability issue.
> 
> What I need here is merely an atomic store.
> The complication is, of course, that, ktime_t is not atomic[64]_t.
> 
> How do you think I can do an atomic store?

Ok, I think a have a version which uses atomic64_t
and is not looking ugly. Sending new patchset now...

--
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/