Re: [PATCH] rtnetlink: add rtnl_lock debug log

From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
To: Rocco Yue <rocco.yue@mediatek.com>
Cc: Peter Enderborg <peter.enderborg@sony.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Vitor Massaru Iha <vitor@massaru.org>,
	Sedat Dilek <sedat.dilek@gmail.com>,
	Wei Yang <richard.weiyang@gmail.com>,
	Cong Wang <cong.wang@bytedance.com>, Di Zhu <zhudi21@huawei.com>,
	Stephen Hemminger <stephen@networkplumber.org>,
	Francis Laniel <laniel_francis@privacyrequired.com>,
	Roopa Prabhu <roopa@cumulusnetworks.com>,
	Andrii Nakryiko <andrii@kernel.org>,
	linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-mediatek@lists.infradead.org, wsd_upsream@mediatek.com,
	"David S . Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Matthias Brugger <matthias.bgg@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	peterz@infradead.org
Subject: Re: [PATCH] rtnetlink: add rtnl_lock debug log
Date: Fri, 30 Apr 2021 20:48:02 +0900	[thread overview]
Message-ID: <27fa19c9-81b2-3604-033c-b7fe5d14b620@i-love.sakura.ne.jp> (raw)
In-Reply-To: <20210429070237.3012-1-rocco.yue@mediatek.com>

On 2021/04/29 16:02, Rocco Yue wrote:
> +static void rtnl_print_btrace(struct timer_list *unused)
> +{
> +	pr_info("----------- %s start -----------\n", __func__);
> +	pr_info("%s[%d][%c] hold rtnl_lock more than 2 sec, start time: %llu\n",
> +		rtnl_instance.task->comm,
> +		rtnl_instance.pid,
> +		task_state_to_char(rtnl_instance.task),
> +		rtnl_instance.start_time);
> +	stack_trace_print(rtnl_instance.addrs, rtnl_instance.nr_entries, 0);

Do we want to print same traces every 2 seconds? 

Since it is possible to stall for e.g. 30 seconds, printing either only upon
first call to rtnl_print_btrace() for each stalling duration or only upon
end of stalling duration (i.e. from rtnl_relase_btrace()) is better?

> +	show_stack(rtnl_instance.task, NULL, KERN_DEBUG);

Why KERN_DEBUG ?

If you retrieve the output via dmesg, KERN_DEBUG would be fine.
But for syzkaller (which counts on printk() messages being printed to
consoles), KERN_INFO (or default) is expected.

> +	pr_info("------------ %s end -----------\n", __func__);
> +}
> +
> +static void rtnl_relase_btrace(void)
> +{
> +	rtnl_instance.end_time = sched_clock();
> +

You should del_timer_sync() here than

> +	if (rtnl_instance.end_time - rtnl_instance.start_time > 2000000000ULL) {
> +		pr_info("rtnl_lock is held by [%d] from [%llu] to [%llu]\n",
> +			rtnl_instance.pid,
> +			rtnl_instance.start_time,
> +			rtnl_instance.end_time);
> +	}
> +
> +	del_timer(&rtnl_chk_timer);

here in order to make sure that end message is printed only after
rtnl_print_btrace() messages are printed.

> +}
> +#endif
> +