linux-kernel - Re: [patch -rt 0/2] use simple waitqueue for kvm vcpu waitqueue (v3)

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:	Wed, 14 Jan 2015 18:35:27 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
CC:	Luiz Capitulino <lcapitulino@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, kvm@...r.kernel.org
Subject: Re: [patch -rt 0/2] use simple waitqueue for kvm vcpu waitqueue (v3)



On 14/01/2015 18:12, Marcelo Tosatti wrote:
> Against v3.14-rt branch of 
> git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-stable-rt.git
> 
> The problem:
> 
> On -RT, an emulated LAPIC timer instance has the following path:
> 
> 1) hard interrupt
> 2) ksoftirqd is scheduled
> 3) ksoftirqd wakes up vcpu thread
> 4) vcpu thread is scheduled
> 
> This extra context switch introduces unnecessary latency in the
> LAPIC path for a KVM guest.
> 
> The solution:
> 
> Allow waking up vcpu thread from hardirq context,
> thus avoiding the need for ksoftirqd to be scheduled.
> 
> Normal waitqueues make use of spinlocks, which on -RT
> are sleepable locks. Therefore, waking up a waitqueue
> waiter involves locking a sleeping lock, which
> is not allowed from hard interrupt context.
> 
> cyclictest command line:
> # cyclictest -m -n -q -p99 -l 1000000 -h60  -D 1m
> 
> This patch reduces the average latency in my tests from 14us to 11us.
> 
> v2: improve changelog (Rik van Riel)
> v3: limit (once) guest triggered printk and WARN_ON (Paolo Bonzini)
> 
> 

Looks good.

Reviewed-by: Paolo Bonzini <pbonzini@...hat.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/