linux-kernel - Re: [patch v5] mm: lru_cache_disable: replace work queue synchronization with synchronize

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Message-Id: <20220311164043.e5e4f3bcac2b472792085f47@linux-foundation.org>
Date:   Fri, 11 Mar 2022 16:40:43 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Minchan Kim <minchan@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [patch v5] mm: lru_cache_disable: replace work queue
 synchronization with synchronize_rcu

On Fri, 11 Mar 2022 09:35:49 +0100 Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:

> + sched division
> 
> On 2022-03-10 18:23:26 [-0800], Andrew Morton wrote:
> > On Thu, 10 Mar 2022 10:22:12 -0300 Marcelo Tosatti <mtosatti@...hat.com> wrote:
> > > On systems that run FIFO:1 applications that busy loop,
> > > any SCHED_OTHER task that attempts to execute
> > > on such a CPU (such as work threads) will not
> > > be scheduled, which leads to system hangs.
> …
> > 
> > Permitting a realtime thread to hang the entire system warrants a
> > -stable backport, I think.  That's just rude.
> 
> I'm not sure if someone is not willingly breaking the system. Based on
> my experience, a thread with an elevated priority (that FIFO, RR or DL)
> should not hog the CPU. A normal user (!root && !CAP_SYS_NICE) can't
> increase the priority of the task.
> To avoid a system hangup there is sched_rt_runtime_us which ensures that
> all RT threads are throttled once the RT class exceed a certain amount
> of runtime. This has been relaxed a little on systems with more CPUs so
> that the RT runtime can be shared but this sharing (RT_RUNTIME_SHARE)
> has been disabled by default a while ago. That safe switch
> (sched_rt_runtime_us) can be disabled and is usually disabled on RT
> system since the RT tasks usually run longer especially in corner cases.

Does all this apply if the kernel is non-preemptible?

Marcelo, do you know how the offending system bypassed the failsafe?