linux-kernel - Re: [RT BUG] Stall caused by eventpoll, rwlocks and CFS bandwidth controller

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Message-ID: <534df953-3cfb-4b3d-8953-5ed9ef24eabc@amd.com>
Date: Mon, 14 Apr 2025 21:51:02 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Jan Kiszka <jan.kiszka@...mens.com>, Aaron Lu <ziqianlu@...edance.com>
CC: Valentin Schneider <vschneid@...hat.com>,
	<linux-rt-users@...r.kernel.org>, Sebastian Andrzej Siewior
	<bigeasy@...utronix.de>, <linux-kernel@...r.kernel.org>, Thomas Gleixner
	<tglx@...utronix.de>, Juri Lelli <juri.lelli@...hat.com>, Clark Williams
	<williams@...hat.com>, "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>,
	Andreas Ziegler <ziegler.andreas@...mens.com>, Felix Moessbauer
	<felix.moessbauer@...mens.com>, Florian Bezdeka <florian.bezdeka@...mens.com>
Subject: Re: [RT BUG] Stall caused by eventpoll, rwlocks and CFS bandwidth
 controller

On 4/14/2025 8:20 PM, K Prateek Nayak wrote:
>>
>> BTW, does anyone already have a test case that produces the lockup issue
>> with one or two simple programs and some hectic CFS bandwidth settings?
> 
> This is your cue to grab a brown paper bag since what I'm about to paste
> below is probably lifetime without parole in the RT land but I believe
> it gets close to the scenario described by Valentin:
> 
> (Based on v6.15-rc1; I haven't yet tested this with Aaron's series yet)

I tried this with Aaron's series [1] and I did not run into any rcu
stalls yet. following are dmesg logs:

     [  122.853909] sched_cfs_period_timer: Started on CPU248
     [  122.853912] sched_cfs_period_timer: Finished on CPU248
     [  123.726232] dumb_ways_to_die: Started on CPU248 with 50000000 loops
     [  123.726574] dumb_ways_to_die: Queuing timer on CPU248
     [  123.726577] dumb_ways_to_die: Waking up RT kthread on CPU248
     [  125.768969] RT kthread: Started on CPU248
     [  125.769050] deadlock_timer: Started on CPU248

     # Fair task runs, drops rwlock, is preempted

     [  126.666709] RT kthread: Finished on CPU248

     # RT kthread finishes

     [  126.666737] deadlock_timer: Finished on CPU248

     # ktimerd function finishes and unblocks replenish

     [  126.666740] sched_cfs_period_timer: Started on CPU248
     [  126.666741] sched_cfs_period_timer: Finished on CPU248

     # cfs task runs prctl() to completion and is throttled

     [  126.666762] dumb_ways_to_die: Finished on CPU248 with 50000000 loops

     # cfs_bandwidth continues to catch up on slack accumulated

     [  126.851820] sched_cfs_period_timer: Started on CPU248
     [  126.851825] sched_cfs_period_timer: Finished on CPU248

[1] https://lore.kernel.org/all/20250409120746.635476-1-ziqianlu@bytedance.com/

-- 
Thanks and Regards,
Prateek