Re: [PATCH v2 11/15] sched/deadline: Mark DL server as unthrottled before enqueue

From: Daniel Bristot de Oliveira <bristot@redhat.com>
To: "Joel Fernandes (Google)" <joel@joelfernandes.org>,
	linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Valentin Schneider <vschneid@redhat.com>
Cc: Suleiman Souhlal <suleiman@google.com>,
	Youssef Esmat <youssefesmat@google.com>,
	David Vernet <void@manifault.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	joseph.salisbury@canonical.com,
	Luca Abeni <luca.abeni@santannapisa.it>,
	Tommaso Cucinotta <tommaso.cucinotta@santannapisa.it>,
	Vineeth Pillai <vineeth@bitbyteword.org>,
	Shuah Khan <skhan@linuxfoundation.org>,
	Phil Auld <pauld@redhat.com>
Subject: Re: [PATCH v2 11/15] sched/deadline: Mark DL server as unthrottled before enqueue
Date: Fri, 5 Apr 2024 10:54:00 +0200	[thread overview]
Message-ID: <51540780-580f-47ff-ae0a-7d335c0702f7@redhat.com> (raw)
In-Reply-To: <20240313012451.1693807-12-joel@joelfernandes.org>

On 3/13/24 02:24, Joel Fernandes (Google) wrote:
> The DL server may not have had its timer started if start_dl_timer()
> returns 0 (say the zero-laxity time has already passed). In such cases,
> mark the DL task which is about to be enqueued as not throttled and
> cancel any previous timers, then do the enqueue.
> 
> This fixes the following crash:
> 
> [    9.263331] kernel BUG at kernel/sched/deadline.c:1765!
> [    9.282382] Call Trace:
> [    9.282767]  <TASK>
> [    9.283086]  ? __die_body+0x62/0xb0
> [    9.283602]  ? die+0x9b/0xc0
> [    9.284036]  ? do_trap+0xa3/0x170
> [    9.284528]  ? enqueue_dl_entity+0x45e/0x460
> [    9.285158]  ? enqueue_dl_entity+0x45e/0x460
> [    9.285791]  ? handle_invalid_op+0x65/0x80
> [    9.286392]  ? enqueue_dl_entity+0x45e/0x460
> [    9.287021]  ? exc_invalid_op+0x2f/0x40
> [    9.287585]  ? asm_exc_invalid_op+0x16/0x20
> [    9.288200]  ? find_later_rq+0x120/0x120
> [    9.288775]  ? fair_server_init+0x40/0x40
> [    9.289364]  ? enqueue_dl_entity+0x45e/0x460
> [    9.289989]  ? find_later_rq+0x120/0x120
> [    9.290564]  dl_task_timer+0x1d7/0x2f0
> [    9.291120]  ? find_later_rq+0x120/0x120
> [    9.291695]  __run_hrtimer+0x73/0x1b0
> [    9.292238]  hrtimer_interrupt+0x216/0x2c0
> [    9.292841]  __sysvec_apic_timer_interrupt+0x53/0x140
> [    9.293581]  sysvec_apic_timer_interrupt+0x2d/0x80
> [    9.294285]  asm_sysvec_apic_timer_interrupt+0x16/0x20
> 
> The crash can easily be reproduced by adding a 100ms delay as follows:
> 
> +int delay_inject_count;
> +
>  static void
>  enqueue_dl_entity(struct sched_dl_entity *dl_se, int flags)
>  {
> @@ -1827,6 +1830,12 @@ enqueue_dl_entity(struct sched_dl_entity *dl_se, int flags)
>                 setup_new_dl_entity(dl_se);
>         }
> 
> +       // 100ms delay every 20 enqueues.
> +       if (delay_inject_count++ > 20) {
> +               mdelay(100);
> +               delay_inject_count = 0;
> +       }
> +
>         /*
>          * If we are still throttled, eg. we got replenished but are a
>          * zero-laxity task and still got to wait, don't enqueue.

Makes sense, I am adding this in the defer patch v6 as it is a fix for it...

-- Daniel