tty: fix endless work loop when the buffer fills up
authorLinus Torvalds <torvalds@linux-foundation.org>
Mon, 4 Apr 2011 21:26:54 +0000 (14:26 -0700)
committerLinus Torvalds <torvalds@linux-foundation.org>
Mon, 4 Apr 2011 21:26:54 +0000 (14:26 -0700)
commita5660b41af6a28f8004e70eb261e1202ad55c5e3
tree5192bb81189953e2551ea7d66012b90cb5b1396b
parentd7c764c4c7b782c660b4600b0bff2e3509892a4d
tty: fix endless work loop when the buffer fills up

Commit f23eb2b2b285 ('tty: stop using "delayed_work" in the tty layer')
ended up causing hung machines on UP with no preemption, because the
work routine to flip the buffer data to the ldisc would endlessly re-arm
itself if the destination buffer had filled up.

With the delayed work, that only caused a timer-driving polling of the
tty state every timer tick, but without the delay we just ended up with
basically a busy loop instead.

Stop the insane polling, and instead make the code that opens up the
receive room re-schedule the buffer flip work.  That's what we should
have been doing anyway.

This same "poll for tty room" issue is almost certainly also the cause
of excessive kworker activity when idle reported by Dave Jones, who also
reported "flush_to_ldisc executing 2500 times a second" back in Nov 2010:

  http://lkml.org/lkml/2010/11/30/592

which is that silly flushing done every timer tick.  Wasting both power
and CPU for no good reason.

Reported-and-tested-by: Alexander Beregalov <a.beregalov@gmail.com>
Reported-and-tested-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Cc: Greg KH <gregkh@suse.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Dave Jones <davej@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
drivers/tty/n_tty.c
drivers/tty/tty_buffer.c