From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Petr Mladek <pmladek@suse.com>, Tejun Heo <tj@kernel.org>,
akpm@linux-foundation.org, linux-mm@kvack.org,
Cong Wang <xiyou.wangcong@gmail.com>,
Dave Hansen <dave.hansen@intel.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@kernel.org>,
Vlastimil Babka <vbabka@suse.cz>,
Peter Zijlstra <peterz@infradead.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Jan Kara <jack@suse.cz>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
rostedt@home.goodmis.org, Byungchul Park <byungchul.park@lge.com>,
Pavel Machek <pavel@ucw.cz>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup
Date: Tue, 16 Jan 2018 10:46:41 +0900 [thread overview]
Message-ID: <20180116014641.GA6607@jagdpanzerIV> (raw)
In-Reply-To: <20180115070637.1915ac20@gandalf.local.home>
On (01/15/18 07:06), Steven Rostedt wrote:
> > > Yep, but I'm still not convinced you are seeing an issue with a single
> > > printk.
> >
> > what do you mean by this?
>
> I'm not sure your issues happen because a single printk is locked up,
> but you have many printks in one area.
hm, need to think about it.
> > > An OOM does not do everything in one printk, it calls hundreds.
> > > Having hundreds of printks is an issue, especially in critical sections.
> >
> > unless your console_sem owner is preempted. as long as it is preempted
> > it doesn't really matter how many times we call printk from which CPUs
> > and from which sections, but what matters - who is going to print that all
> > out when console_sem is running again and how much time will it take.
> > that's what I'm saying.
>
> OK, if this is an issue, then we could do:
>
> preempt_disable();
> if (console_trylock_spinning())
> console_unlock();
> preempt_enable();
>
> Which would prevent any printks from being preempted, but allow for
> other console_lock owners to be so.
yes, non-preemptible printk->console_unlock() is good for a number of
reasons.
[..]
> > > > vprintk_emit()
> > > > {
> > > >
> > > > console_trylock_spinning(void)
> > > > {
> > > > printk_safe_enter_irqsave(flags);
> > > > while (READ_ONCE(console_waiter)) // spins as long as call_console_drivers() on other CPU
> > > > cpu_relax();
> > > > printk_safe_exit_irqrestore(flags);
> > > > ---> }
> > > > | // preemptible up until printk_safe_enter_irqsave() in console_unlock()
> > >
> > > Again, this means the waiter is not in a critical section. Why do we
> > > care?
> >
> > which is not what I was talking about. the point was that you said
>
> And would be fixed with the preempt_disable() I added above.
yes. and it's, basically, very close to a revert of the commit
I mentioned.
[..]
> > that is not true. we can have preemption "during" hand off. hand off,
> > thus, is a "delayed approach", by definition. so if you consider the
> > possibility of "if the machine were to crash in the transfer, we lost
> > all that data" and if you consider this to be important [otherwise you
> > wouldn't bring that up, would you] then the reality is that your patch
> > has the same problem as printk_kthread.
>
> With the preempt_disable() there really isn't a delay. I agree, we
> shouldn't let printk preempt (unless we have CONFIG_PREEMPT_RT enabled,
> but that's another story).
yes.
> > so very schematically, for hand-off it's something like
> >
> > if (... console_trylock_spinning()) // grabbed the ownership
> >
> > << ... preempted ... >>
> >
> > console_unlock();
>
> Which I think we should stop, with the preempt_disable().
yes.
> > for printk_kthread it's something like
> >
> > wake_up_process(printk_kthread);
> > up(console_sem);
> >
> >
> > in the later case we at least have console_sem unlocked. so any other CPU
> > that might do printk() can grab the lock and emit the logbuf messages. but
> > in case on hand-off, we have console_sem locked, so no printk() will be
> > able to emit the messages, we need that specific task to become running.
> >
> >
> > hence the following:
> >
> > [..]
> > > > reverting 6b97a20d3a7909daa06625d4440c2c52d7bf08d7 may be the right
> > > > thing after all.
> >
> > this was cryptic and misleading. sorry.
> > some clarifications.
> >
> > what I meant was that with 6b97a20d3a7909daa06625d4440c2c52d7bf08d7
> > I think I badly broke printk() [some of paths]. I know what I tried
>
> I think adding the preempt_disable() would fix printk() but let non
> printk console_unlock() still preempt.
yes. might be a bit risky, but can try.
and yes, we still have console_lock() call sites, which can sleep
under console_sem, so scheduler still can mess up with us, but
that's a different story. agreed.
> > to fix (and you don't have to explain to me what a lock up is) with
> > that patch, but I don't think the patch ended up to be a clear win.
> > a very simple explanation would be:
> >
> > instead of having a direct nonpreemptible path
> >
> > logbuf -> for(;;) call_console_drivers -> happy user
> >
> > we now have
> >
> > logbuf -> for(;;) { call_console_drivers, scheduler ... ???} -> happy user
> >
> > which is a big change. with a non-zero potential for regressions.
> > and it didn't take long to find out that not all "happy users" were
> > exactly happy with the new scheme of things. glance through Tetsuo's
> > emails [see links in my another email], Tetsuo reported that printk can
> > stall for minutes now. basically, the worse the system state is the lower
> > printk throughput can be [down to zero chars in the worst case]. that's
> > why I think that my patch was a mistake. and that's why in my out-of-tree
> > patches I'm moving towards the non-preemptible path from logbuf through
> > console to a happy user [just like it used to be]. but, obviously, I can't
> > just restore preempt_disable()/preempt_enable() in vprintk_emit(). that's
> > why I bound console_unlock() to watchdog threshold and move towards the
> > batched non-preemptible print outs (enabling preemption and up()-ing the
> > console_sem at the end of each print out batch). this is not super good,
> > preemption is still here, but at least not after every line console_unlock()
> > prints. up() console_sem also increases chances that, for instance, systemd
> > or any other task that is sleeping in TASK_UNINTERRUPTIBLE on console_sem
> > now has a chance to be woken up sooner (not only after we flush all pending
> > logbuf messages and finally up() the console_sem).
>
> I rather try simpler approaches first (like adding the preempt_disable()
> on top of my patch) than an elaborate scheme of printk_kthreads.
ok, agreed.
-ss
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
prev parent reply other threads:[~2018-01-16 1:46 UTC|newest]
Thread overview: 140+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-10 13:24 [PATCH v5 0/2] printk: Console owner and waiter logic cleanup Petr Mladek
2018-01-10 13:24 ` [PATCH v5 1/2] printk: Add console owner and waiter logic to load balance console writes Petr Mladek
2018-01-10 16:50 ` Steven Rostedt
2018-01-12 16:54 ` Steven Rostedt
2018-01-12 17:11 ` Steven Rostedt
2018-01-17 19:13 ` Rasmus Villemoes
2018-01-17 19:33 ` Steven Rostedt
2018-01-19 9:51 ` Sergey Senozhatsky
2018-01-18 22:03 ` Pavel Machek
2018-01-19 0:20 ` Steven Rostedt
2018-01-17 2:19 ` Byungchul Park
2018-01-17 4:54 ` Byungchul Park
2018-01-17 7:34 ` Byungchul Park
2018-01-17 12:04 ` Petr Mladek
2018-01-18 1:53 ` Byungchul Park
2018-01-18 1:57 ` Byungchul Park
2018-01-18 2:19 ` Steven Rostedt
2018-01-18 4:01 ` Byungchul Park
2018-01-18 15:21 ` Steven Rostedt
2018-01-19 2:37 ` Byungchul Park
2018-01-19 3:27 ` Steven Rostedt
2018-01-22 2:31 ` Byungchul Park
2018-01-10 13:24 ` [PATCH v5 2/2] printk: Hide console waiter logic into helpers Petr Mladek
2018-01-10 17:52 ` Steven Rostedt
2018-01-11 12:03 ` Petr Mladek
2018-01-12 15:37 ` Steven Rostedt
2018-01-12 16:08 ` Petr Mladek
2018-01-12 16:36 ` Steven Rostedt
2018-01-15 16:08 ` Petr Mladek
2018-01-16 5:05 ` Sergey Senozhatsky
2018-01-10 14:05 ` [PATCH v5 0/2] printk: Console owner and waiter logic cleanup Tejun Heo
2018-01-10 16:29 ` Petr Mladek
2018-01-10 17:02 ` Tejun Heo
2018-01-10 18:21 ` Peter Zijlstra
2018-01-10 18:30 ` Tejun Heo
2018-01-10 18:41 ` Peter Zijlstra
2018-01-10 19:05 ` Tejun Heo
2018-01-11 5:15 ` Sergey Senozhatsky
2018-01-10 18:22 ` Steven Rostedt
2018-01-10 18:36 ` Tejun Heo
2018-01-10 18:40 ` Mathieu Desnoyers
2018-01-11 7:36 ` Sergey Senozhatsky
2018-01-11 11:24 ` Petr Mladek
2018-01-11 13:19 ` Sergey Senozhatsky
2018-01-24 9:36 ` Peter Zijlstra
2018-01-24 18:46 ` Tejun Heo
2018-05-09 8:58 ` Sergey Senozhatsky
2018-01-10 18:54 ` Steven Rostedt
2018-01-11 5:10 ` Sergey Senozhatsky
2018-01-10 18:05 ` Steven Rostedt
2018-01-10 18:12 ` Tejun Heo
2018-01-10 18:14 ` Tejun Heo
2018-01-10 18:45 ` Steven Rostedt
2018-01-10 18:41 ` Steven Rostedt
2018-01-10 18:57 ` Tejun Heo
2018-01-10 19:17 ` Steven Rostedt
2018-01-10 19:34 ` Tejun Heo
2018-01-10 19:44 ` Steven Rostedt
2018-01-10 22:44 ` Tejun Heo
2018-01-11 5:35 ` Sergey Senozhatsky
2018-01-11 4:58 ` Sergey Senozhatsky
2018-01-11 9:34 ` Petr Mladek
2018-01-11 10:38 ` Sergey Senozhatsky
2018-01-11 11:50 ` Petr Mladek
2018-01-11 16:29 ` Steven Rostedt
2018-01-12 1:30 ` Steven Rostedt
2018-01-12 2:55 ` Steven Rostedt
2018-01-12 4:20 ` Steven Rostedt
2018-01-16 19:44 ` Tejun Heo
2018-01-17 9:12 ` Petr Mladek
2018-01-17 15:15 ` Tejun Heo
2018-01-17 17:12 ` Steven Rostedt
2018-01-17 18:42 ` Steven Rostedt
2018-01-19 18:20 ` Steven Rostedt
2018-01-20 7:14 ` Sergey Senozhatsky
2018-01-20 15:49 ` Steven Rostedt
2018-01-21 14:15 ` Sergey Senozhatsky
2018-01-21 21:04 ` Steven Rostedt
2018-01-22 8:56 ` Sergey Senozhatsky
2018-01-22 10:28 ` Sergey Senozhatsky
2018-01-22 10:36 ` Sergey Senozhatsky
2018-01-23 6:40 ` Sergey Senozhatsky
2018-01-23 7:05 ` Sergey Senozhatsky
2018-01-23 7:31 ` Sergey Senozhatsky
2018-01-23 14:56 ` Steven Rostedt
2018-01-23 15:21 ` Sergey Senozhatsky
2018-01-23 15:41 ` Steven Rostedt
2018-01-23 15:43 ` Tejun Heo
2018-01-23 16:12 ` Sergey Senozhatsky
2018-01-23 16:13 ` Steven Rostedt
2018-01-23 17:21 ` Tejun Heo
2018-04-23 5:35 ` Sergey Senozhatsky
2018-01-23 16:01 ` Sergey Senozhatsky
2018-01-23 16:24 ` Steven Rostedt
2018-01-24 2:11 ` Sergey Senozhatsky
2018-01-24 2:52 ` Steven Rostedt
2018-01-24 4:44 ` Sergey Senozhatsky
2018-01-23 17:22 ` Tejun Heo
2018-01-20 12:19 ` Tejun Heo
2018-01-20 14:51 ` Steven Rostedt
2018-01-17 20:05 ` Tejun Heo
2018-01-18 5:43 ` Sergey Senozhatsky
2018-01-18 11:51 ` Petr Mladek
2018-01-18 5:42 ` Sergey Senozhatsky
2018-01-12 3:12 ` Sergey Senozhatsky
2018-01-12 2:56 ` Sergey Senozhatsky
2018-01-12 3:21 ` Steven Rostedt
2018-01-12 10:05 ` Sergey Senozhatsky
2018-01-12 12:21 ` Steven Rostedt
2018-01-12 12:55 ` Petr Mladek
2018-01-13 7:31 ` Sergey Senozhatsky
2018-01-15 8:51 ` Petr Mladek
2018-01-15 9:48 ` Sergey Senozhatsky
2018-01-16 5:16 ` Sergey Senozhatsky
2018-01-16 9:08 ` Petr Mladek
2018-01-15 12:08 ` Steven Rostedt
2018-01-16 4:51 ` Sergey Senozhatsky
2018-01-13 7:28 ` Sergey Senozhatsky
2018-01-15 10:17 ` Petr Mladek
2018-01-15 11:50 ` Petr Mladek
2018-01-16 6:10 ` Sergey Senozhatsky
2018-01-16 9:36 ` Petr Mladek
2018-01-16 10:10 ` Sergey Senozhatsky
2018-01-16 16:06 ` Steven Rostedt
2018-01-16 5:23 ` Sergey Senozhatsky
2018-01-15 12:06 ` Steven Rostedt
2018-01-15 14:45 ` Petr Mladek
2018-01-16 2:23 ` Sergey Senozhatsky
2018-01-16 4:47 ` Sergey Senozhatsky
2018-01-16 10:19 ` Petr Mladek
2018-01-17 2:24 ` Sergey Senozhatsky
2018-01-16 15:45 ` Steven Rostedt
2018-01-17 2:18 ` Sergey Senozhatsky
2018-01-17 13:04 ` Petr Mladek
2018-01-17 15:24 ` Steven Rostedt
2018-01-18 4:31 ` Sergey Senozhatsky
2018-01-18 15:22 ` Steven Rostedt
2018-01-16 10:13 ` Petr Mladek
2018-01-17 6:29 ` Sergey Senozhatsky
2018-01-16 1:46 ` Sergey Senozhatsky [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20180116014641.GA6607@jagdpanzerIV \
--to=sergey.senozhatsky.work@gmail.com \
--cc=akpm@linux-foundation.org \
--cc=byungchul.park@lge.com \
--cc=dave.hansen@intel.com \
--cc=hannes@cmpxchg.org \
--cc=jack@suse.cz \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mathieu.desnoyers@efficios.com \
--cc=mgorman@suse.de \
--cc=mhocko@kernel.org \
--cc=pavel@ucw.cz \
--cc=penguin-kernel@I-love.SAKURA.ne.jp \
--cc=peterz@infradead.org \
--cc=pmladek@suse.com \
--cc=rostedt@goodmis.org \
--cc=rostedt@home.goodmis.org \
--cc=sergey.senozhatsky@gmail.com \
--cc=tj@kernel.org \
--cc=torvalds@linux-foundation.org \
--cc=vbabka@suse.cz \
--cc=xiyou.wangcong@gmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).