Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

Sergey Senozhatsky
2017-06-30T11:57:11Z

On (06/30/17 19:18), Tetsuo Handa wrote:
> Sergey Senozhatsky wrote:
> > if (!alloc_cpumask_var(&cpus_allowed, GFP_KERNEL)) {
> > wake_up_process(printk_kthread);
> > return true;
> > }
>
> Please avoid memory allocations when trying to print something.
> __GFP_DIRECT_RECLAIM allocations (e.g. GFP_KERNEL) can sleep for
> unpredictable duration. Allocations without __GFP_NOWARN will cause
> e.g. memory allocation failure messages. Even with __GFP_NOWARN,
> some messages might be still printed (e.g. serious problem).

wow... dammit, what a stupid mistake. you are 100% right, thanks!
it's sooo unsafe and dumb, console_unlock() and, thus, offloading
can happen from IRQ.

thanks again.

> > I'm still thinking about Steven's proposals; but we will need offloading
> > anyways, so the bits we are talking about here are important regardless
> > the direction printk design will take, I think.
>
> Is there a chance that printk() waits for only data queued by that printk()
> call (exception will be printk() from NMI).

hm, I don't think this can be done easily... consider

console_lock();
printk();
printk();
... -> this guys will wait forever. nothing
flushes the logbuf.
printk();
console_unlock();

> If we carry penalty for printk() (charge delay according to amount of
> data queued by that printk()), users will stop doing stupid flooding
> with printk() based on an assumption that offloaded kernel thread will
> manage magically with guarantee of being printed out (i.e. users has
> to become careful).

ratelimiting is on my list. but it's a bit tricky... what should happen
if one does

CPU1

-> IRQ
printk()
console_unlock();
printk()
console_unlock();
...
printk()
ratelimit();
console_unlock();

-> NMI panic()

need to think more.

-ss