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From: John Ogness <john.ogness@linutronix.de>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Petr Mladek <pmladek@suse.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Andrea Parri <andrea.parri@amarulasolutions.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation
Date: Wed, 19 Jun 2019 00:12:43 +0200
Message-ID: <87imt2bl0k.fsf@linutronix.de> (raw)
In-Reply-To: <20190618045117.GA7419@jagdpanzerIV>

On 2019-06-18, Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> wrote:
>> +	struct prb_reserved_entry e;
>> +	char *s;
>> +
>> +	s = prb_reserve(&e, &rb, 32);
>> +	if (s) {
>> +		sprintf(s, "Hello, world!");
>> +		prb_commit(&e);
>> +	}
>
> A nit: snprintf().
>
> sprintf() is tricky, it may write "slightly more than was
> anticipated" bytes - all those string_nocheck(" disabled"),
> error_string("pK-error"), etc.

Agreed. Documentation should show good examples.

>> +Sample reader code::
>> +
>> +	DECLARE_PRINTKRB_ENTRY(entry, 128);
>> +	DECLARE_PRINTKRB_ITER(iter, &test_rb, &entry);
>> +	u64 last_seq = 0;
>> +	int len;
>> +	char *s;
>> +
>> +	prb_for_each_entry(&iter, len) {
>> +		if (entry.seq - last_seq != 1) {
>> +			printf("LOST %llu ENTRIES\n",
>> +				entry.seq - (last_seq + 1));
>> +		}
>> +		last_seq = entry.seq;
>> +
>> +		s = (char *)&entry.buffer[0];
>> +		if (len >= 128)
>> +			s[128 - 1] = 0;
>> +		printf("data: %s\n", s);
>> +	}
>
> How are we going to handle pr_cont() loops?
>
> print_modules()
>  preempt_disable();
>  list_for_each_entry_rcu(mod, &modules, list) {
>   pr_cont(" %s%s", mod->name, module_flags(mod, buf));
>  }
>  preempt_enable();

pr_cont() (in its current form) is not related to the printk buffer
because cont messages use their own separate struct cont buffer. And for
the initial integration of the new ringbuffer I would leave that as it
is. Which means initially, pr_cont() would still sit behind a raw
spinlock and pr_cont() from NMI context would be stored as individual
messages.

However, to remove the spinlock of the cont buffer and allow pr_cont()
to work from NMI context, I would like to introduce a separate lockless
ringbuffer instance for cont that contains all the cont pieces
(including the caller_id). As soon as the caller_id changes from the
oldest record in the cont ringbuffer, that caller would assemble the
full cont message, popping all the pieces from the ringbuffer (with a
single cmpxchg) and insert the message to the printk ringbuffer.

John Ogness

  reply index

Thread overview: 62+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-07 16:23 [RFC PATCH v2 0/2] printk: new " John Ogness
2019-06-07 16:23 ` [RFC PATCH v2 1/2] printk-rb: add a new printk " John Ogness
2019-06-18  4:51   ` Sergey Senozhatsky
2019-06-18 22:12     ` John Ogness [this message]
2019-06-25  6:45       ` Sergey Senozhatsky
2019-06-25  7:15         ` Sergey Senozhatsky
2019-06-25  8:44           ` John Ogness
2019-06-25  9:06             ` Petr Mladek
2019-06-25 10:03               ` Sergey Senozhatsky
2019-06-25 12:03                 ` John Ogness
2019-06-26  2:08                   ` Sergey Senozhatsky
2019-06-26  7:16                     ` John Ogness
2019-06-26  7:45                       ` Sergey Senozhatsky
2019-06-26  7:47                       ` Petr Mladek
2019-06-26  7:59                         ` Sergey Senozhatsky
2019-06-25  9:09             ` Sergey Senozhatsky
2019-06-18 11:12   ` Peter Zijlstra
2019-06-18 22:18     ` John Ogness
2019-06-18 11:22   ` Peter Zijlstra
2019-06-18 22:30     ` John Ogness
2019-06-19 10:46       ` Andrea Parri
2019-06-20 22:50         ` John Ogness
2019-06-21 12:16           ` Andrea Parri
2019-06-19 11:08       ` Peter Zijlstra
2019-06-18 11:47   ` Peter Zijlstra
2019-06-20 22:23     ` John Ogness
2019-06-26 22:40       ` Peter Zijlstra
2019-06-26 22:53         ` Peter Zijlstra
2019-06-28  9:50         ` John Ogness
2019-06-28 15:44           ` Peter Zijlstra
2019-06-28 16:07             ` Peter Zijlstra
2019-07-01 10:39             ` John Ogness
2019-07-01 14:10               ` Peter Zijlstra
2019-07-01 14:11               ` Peter Zijlstra
2019-06-29 21:05           ` Andrea Parri
2019-06-30  2:03             ` John Ogness
2019-06-30 14:08               ` Andrea Parri
2019-07-02 14:13                 ` John Ogness
2019-06-26 22:47       ` Peter Zijlstra
2019-06-21 14:05   ` Petr Mladek
2019-06-24  8:33     ` John Ogness
2019-06-24 14:09       ` Petr Mladek
2019-06-25 13:29         ` John Ogness
2019-06-26  8:29           ` Petr Mladek
2019-06-26  9:09             ` John Ogness
2019-06-26 21:16       ` Peter Zijlstra
2019-06-26 21:43         ` John Ogness
2019-06-27  8:28           ` Petr Mladek
2019-07-04 10:33     ` [PATCH POC] printk_ringbuffer: Alternative implementation of lockless printk ringbuffer Petr Mladek
2019-07-04 14:59       ` John Ogness
2019-07-08 15:23         ` Petr Mladek
2019-07-09  1:34           ` John Ogness
2019-07-09  9:06             ` Petr Mladek
2019-07-09 10:21               ` John Ogness
2019-07-09 11:58                 ` Petr Mladek
2019-08-14  3:46                   ` John Ogness
2019-06-24 13:55   ` [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation John Ogness
2019-06-25  8:55   ` Sergey Senozhatsky
2019-06-25  9:19     ` John Ogness
2019-06-07 16:23 ` [RFC PATCH v2 2/2] printk-rb: add test module John Ogness
2019-06-17 21:09 ` [RFC PATCH v2 0/2] printk: new ringbuffer implementation Thomas Gleixner
2019-06-18  7:15   ` Petr Mladek

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