linux-kernel - Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenarios in PLE handler

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: <50604988.2030506@linux.vnet.ibm.com>
Date:	Mon, 24 Sep 2012 17:22:40 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Avi Kivity <avi@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, Jiannan Ouyang <ouyang@...pitt.edu>,
	chegu vinod <chegu_vinod@...com>,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Gleb Natapov <gleb@...hat.com>,
	Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenarios
 in PLE handler

On 09/24/2012 05:04 PM, Peter Zijlstra wrote:
> On Fri, 2012-09-21 at 17:29 +0530, Raghavendra K T wrote:
>> In some special scenarios like #vcpu<= #pcpu, PLE handler may
>> prove very costly, because there is no need to iterate over vcpus
>> and do unsuccessful yield_to burning CPU.
>
> What's the costly thing? The vm-exit, the yield (which should be a nop
> if its the only task there) or something else entirely?
>
Both vmexit and yield_to() actually,

because unsuccessful yield_to() overall is costly in PLE handler.

This is because when we have large guests, say 32/16 vcpus, and one
vcpu is holding lock, rest of the vcpus waiting for the lock, when they
do PL-exit, each of the vcpu try to iterate over rest of vcpu list in
the VM and try to do directed yield (unsuccessful). (O(n^2) tries).

this results is fairly high amount of cpu burning and double run queue
lock contention.

(if they were spinning probably lock progress would have been faster).
As Avi/Chegu Vinod had felt it is better to avoid vmexit itself, which
seems little complex to achieve currently.





--
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/