linux-kernel - Re: [PATCH] net/skbuff: silence warnings under memory pressure

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Date:   Mon, 9 Sep 2019 10:10:18 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>, davem@...emloft.net,
        Eric Dumazet <eric.dumazet@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        Qian Cai <cai@....pw>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH] net/skbuff: silence warnings under memory pressure

On (09/06/19 17:32), Petr Mladek wrote:
> > [..]
> > > I mean, really, do we need to keep calling wake up if it
> > > probably never even executed?
> > 
> > I guess ratelimiting you are talking about ("if it probably never even
> > executed") would be to check if we have already called wake up on the
> > log_wait ->head. For that we need to, at least, take log_wait spin_lock
> > and check that ->head is still in TASK_INTERRUPTIBLE; which is (quite,
> > but not exactly) close to what wake_up_interruptible() does - it doesn't
> > wake up the same task twice, it bails out on `p->state & state' check.
> 
> I have just realized that only sleeping tasks are in the waitqueue.
> It is already handled by waitqueue_active() check.

Yes.

> I am afraid that we could not ratelimit the wakeups. The userspace
> loggers might then miss the last lines for a long.

That's my concern as well.

> We could move wake_up_klogd() back to console_unlock(). But it might
> end up with a back-and-forth games according to who is currently
> complaining.

We still don't need irq_work, tho.

If we can do
	printk()->console_unlock()->up()->try_to_wake_up()
then we can also do
	printk()           ->             try_to_wake_up()

It's LOGLEVEL_SCHED which tells us if we can try_to_wake_up()
or cannot.

> Sigh, I still suggest to ratelimit the warning about failed
> allocation.

Hard to imagine how many printk()-s we will have to ratelimit.
To imagine NET maintainers being OK with this is even harder.

	-ss