From: Peter Zijlstra <peterz@infradead.org>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Yu Liao <liaoyu15@huawei.com>,
fweisbec@gmail.com, mingo@kernel.org, liwei391@huawei.com,
adobriyan@gmail.com, mirsad.todorovac@alu.unizg.hr,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC] tick/nohz: fix data races in get_cpu_idle_time_us()
Date: Wed, 1 Feb 2023 10:03:28 +0100 [thread overview]
Message-ID: <Y9oq4OSJXJvr56Oj@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <Y9mEB7LMaZ0dMQS/@lothringen>
On Tue, Jan 31, 2023 at 10:11:35PM +0100, Frederic Weisbecker wrote:
> On Tue, Jan 31, 2023 at 08:57:59PM +0100, Peter Zijlstra wrote:
> > On Tue, Jan 31, 2023 at 03:44:00PM +0100, Thomas Gleixner wrote:
> >
> > > P.S.: I hate the spinlock in the idle code path, but I don't have a
> > > better idea.
> >
> > seqcount? It would avoid the obvious interleave and put most of the onus
> > on the reader (which never happens anyway).
>
> Yep, and do the update locally only on idle exit. But note that neither
> seqcount nor spinlock will fix the nr_iowait_cpu() based thing. This counter
> can be decremented remotely even during the idle period so the reader
> can see an iowait period that may eventually be accounted as !iowait,
> or the reverse. Breaking the monotonicity and even coherency.
>
> That stuff is broken by design and this is the reason why it got never
> really fixed. The seqcount/spinlock would make it just a bit less worse.
Yeah, iowait is a random number generator, -EWONTFIX on that.
next prev parent reply other threads:[~2023-02-01 9:03 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-28 2:00 [PATCH RFC] tick/nohz: fix data races in get_cpu_idle_time_us() Yu Liao
2023-01-31 14:44 ` Thomas Gleixner
2023-01-31 18:35 ` Alexey Dobriyan
2023-01-31 19:59 ` Peter Zijlstra
2023-01-31 19:57 ` Peter Zijlstra
2023-01-31 21:11 ` Frederic Weisbecker
2023-02-01 9:03 ` Peter Zijlstra [this message]
2023-02-01 4:53 ` Hillf Danton
2023-02-01 12:02 ` Frederic Weisbecker
2023-02-01 14:01 ` Hillf Danton
2023-02-01 14:28 ` Frederic Weisbecker
2023-02-08 15:19 ` [PATCH] timers/nohz: Restructure and reshuffle struct tick_sched Frederic Weisbecker
2023-02-06 7:03 ` [PATCH RFC] tick/nohz: fix data races in get_cpu_idle_time_us() kernel test robot
2023-02-07 5:25 ` Mirsad Goran Todorovac
2023-02-07 8:03 ` Mirsad Goran Todorovac
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