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SubjectRe: [RFC PATCH v1 00/25] printk: new implementation
On (02/12/19 15:29), John Ogness wrote:
>
> 1. The printk buffer is protected by a global raw spinlock for readers
> and writers. This restricts the contexts that are allowed to
> access the buffer.

[..]

> 2. Because of #1, NMI and recursive contexts are handled by deferring
> logging/printing to a spinlock-safe context. This means that
> messages will not be visible if (for example) the kernel dies in
> NMI context and the irq_work mechanism does not survive.

panic() calls printk_safe_flush_on_panic(), which iterates all per-CPU
buffers and moves data to the main logbuf; so then we can flush pending
logbuf message

panic()
printk_safe_flush_on_panic();
console_flush_on_panic();

We don't really use irq_work mechanism for that.

> 3. Because of #1, when *not* using features such as PREEMPT_RT, large
> latencies exist when printing to slow consoles.

Because of #1? I'm not familiar with PREEMPT_RT; but logbuf spinlock
should be unlocked while we print messages to slow consoles
(call_consoles_drivers() is protected by console_sem, not logbuf
lock).

So it's

spin_lock_irqsave(logbuf);
vsprintf();
memcpy();
spin_unlock_irqrestore(logbuf);

console_trylock();
for (;;)
call_console_drivers();
// console_owner handover
console_unlock();

Do you see large latencies because of logbuf spinlock?

> 5. Printing to consoles is the responsibility of the printk caller
> and that caller may be required to print many messages that other
> printk callers inserted. Because of this there can be enormous
> variance in the runtime of a printk call.

That's complicated. Steven's console_owner handover patch makes
printk() more fair. We can have "winner takes it all" scenarios,
but significantly less often, IMO. Do you have any data that
suggest otherwise?

> 7. Loglevel INFO is handled the same as ERR. There seems to be an
> endless effort to get printk to show _all_ messages as quickly as
> possible in case of a panic (i.e. printing from any context), but
> at the same time try not to have printk be too intrusive for the
> callers. These are conflicting requirements that lead to a printk
> implementation that does a sub-optimal job of satisfying both
> sides.

Per my experience, fully preemptible "print it sometime maybe"
printk() does not work equally well for everyone.

-ss

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