linux-kernel - Re: Powertop shows events/0 waking at high rate due to ptys

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <4CA1367F.7040106@goop.org>
Date:	Mon, 27 Sep 2010 17:27:43 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg KH <gregkh@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Ian Jackson <Ian.Jackson@...citrix.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Subject: Re: Powertop shows events/0 waking at high rate due to ptys

 On 09/27/2010 11:15 AM, Alan Cox wrote:
> Really the line discipline should wake the work queue when it sets
> tty->receive_room non-zero, but while only n_tty currently uses that
> facility the existing code doesn't do it in any kind of race-free manner
> and sometimes is only saved by the polling picking it up.
>
> It's all really just a symptom of the fact that input and output buffers
> shouldn't be attached to the tty in the first place but to a struct
> representing the physical port. Fix that and the race conditions in
> serial output go away, as do the potential crashes and this wakeup stuff
> as well as a ton of locking in the irq/tx/rx paths. In several cases it
> also saves you an entire copy.
>
> Unfortunately while I got the tty port structures into lots of places
> needed the job never gone done.

OK, so it sounds like there's a basic design problem here which will
need some non-trivial work to fix.  In the meantime we'll need to look
at doing something to work around the issue, since it ends up consuming
a non-trivial amount of CPU in events/0.  I guess reducing HZ would be
the first, simplest thing to do, but changing xenconsoled to avoid
writing to readerless ptys might not be too hard.

Thanks,
    J
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/