linux-kernel - Re: [REPOST-v2] sched: Prevent wakeup to enter critical section needlessly

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]

Date:	Mon, 19 Nov 2012 16:34:04 +0100
From:	Ivo Sieben <meltedpianoman@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-serial@...r.kernel.org,
	Alan Cox <alan@...ux.intel.com>,
	Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [REPOST-v2] sched: Prevent wakeup to enter critical section needlessly

Hi

2012/11/19 Oleg Nesterov <oleg@...hat.com>:
>
> I am wondering if it makes sense unconditionally. A lot of callers do
>
>         if (waitqueue_active(q))
>                 wake_up(...);
>
> this patch makes the optimization above pointless and adds mb().
>
>
> But I won't argue.
>
> Oleg.
>

This patch solved an issue for me that I had with the TTY line
discipline idle handling:
Testing on a PREEMPT_RT system with TTY serial communication. Each
time the TTY line discipline is dereferenced the Idle handling wait
queue is woken up (see function put_ldisc in /drivers/tty/tty_ldisc.c)
However line discipline idle handling is not used very often so the
wait queue is empty most of the time. But still the wake_up() function
enters the critical section guarded by spin locks. This causes
additional scheduling overhead when a lower priority thread has
control of that same lock.

The /drivers/tty/tty_ldisc.c did not use the waitqueue_active() call
to check if the waitqueue was filled.... maybe I should solve this
problem the other way around: and make tty_ldisc.c first do the
waitqueue_active() call?

Regards,
Ivo
--
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/