linux-kernel - Re: [RFC PATCH v3 2/3] sched: Avoid placing RT threads on cores handling long softirqs

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Message-ID: <20221005154647.vok6unu3vnzxp74d@wubuntu>
Date:   Wed, 5 Oct 2022 16:46:47 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'John Stultz' <jstultz@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Connor O'Brien <connoro@...gle.com>,
        John Dias <joaodias@...gle.com>, Rick Yiu <rickyiu@...gle.com>,
        John Kacur <jkacur@...hat.com>,
        Chris Redpath <chris.redpath@....com>,
        Abhijeet Dharmapurikar <adharmap@...cinc.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "kernel-team@...roid.com" <kernel-team@...roid.com>,
        "J . Avila" <elavila@...gle.com>
Subject: Re: [RFC PATCH v3 2/3] sched: Avoid placing RT threads on cores
 handling long softirqs

On 10/04/22 09:50, David Laight wrote:

[...]

> > That's fair. Digging through the patch history in the Android trees,
> > the first pass was for all softirqs but then restricted to remove
> > known short-running ones.
> > From the bug history and what I can directly reproduce, the net and
> > block softirqs have definitely caused trouble, but I don't see a
> > specific example from TASKLET,  so I'm ok dropping that for now, and
> > should we get specific evidence we can argue for it in a future patch.
> > 
> > So I'll drop TASKLET from the list here. Thanks for the suggestion!
> 
> I've also seen the code that finally frees memory freed under rcu
> take a long time.
> That was a workload sending a lot of UDP/RTP from a raw socket using
> IP_HDRINC - each send allocated a structure (fib?) that was freed from
> the rcu (timer?) softint callback.

I'm assuming this is a network driver that using RCU callback to free some
memory?

> 
> But, actually, one of the biggest causes of RT wakeup latency
> was a normal thread looping without a cond_resched() call.
> In my case some graphics driver doing page flushes of the
> display memory.

Were these drivers that cause these problem in-tree or out-of-tree?


Thanks

--
Qais Yousef