linux-kernel - Re: [RFC] sched: implement the exclusive wait queue as a LIFO queue

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: <y2r412e6f7f1004280642n49b8d6f2vcd08774531cb59da@mail.gmail.com>
Date:	Wed, 28 Apr 2010 21:42:12 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Jamie Lokier <jamie@...reable.org>
Cc:	David Howells <dhowells@...hat.com>,
	Yong Zhang <yong.zhang@...driver.com>,
	Xiaotian Feng <xtfeng@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Roland Dreier <rolandd@...co.com>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <dada1@...mosbay.com>,
	Christoph Lameter <cl@...ux.com>,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Takashi Iwai <tiwai@...e.de>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] sched: implement the exclusive wait queue as a LIFO queue

On Wed, Apr 28, 2010 at 9:21 PM, Jamie Lokier <jamie@...reable.org> wrote:
> Changli Gao wrote:
>>
>> fs/eventpoll.c: 1443.
>>                 wait.flags |= WQ_FLAG_EXCLUSIVE;
>>                 __add_wait_queue(&ep->wq, &wait);
>
> The same thing about assumptions applies here.  The userspace process
> may be waiting for an epoll condition to get access to a resource,
> rather than being a worker thread interchangeable with others.

Oh, the lines above are the current ones. So the assumptions applies
and works here.

>
> For example, userspace might be using a pipe as a signal-safe lock, or
> signal-safe multi-token semaphore, and epoll to wait for that pipe.
>
> WQ_FLAG_EXCLUSIVE means there is no point waking all tasks, to avoid a
> pointless thundering herd.  It doesn't mean unfairness is ok.

The users should not make any assumption about the waking up sequence,
neither LIFO nor FIFO.

>
> The LIFO idea _might_ make sense for interchangeable worker-thread
> situations - including userspace.  It would make sense for pipe
> waiters, socket waiters (especially accept), etc.

Yea, and my following patches are for socket waiters.

>
> Do you have any measurements which showing the LIFO mode performing
> better than FIFO, and by how much?
>

I didn't do any test yet. But some work done by LSE project years ago
showed that it is better.

http://lse.sourceforge.net/io/aionotes.txt

" Also in view of
better cache utilization the wake queue mechanism is LIFO by default.
(A new exclusive LIFO wakeup option has been introduced for this purpose)"

-- 
Regards，
Changli Gao(xiaosuo@...il.com)
--
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/