linux-kernel - Re: [PATCH RFC 2/4] Add yield hypercall for KVM guests

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: <4C56DF5D.807@goop.org>
Date:	Mon, 02 Aug 2010 08:08:13 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Avi Kivity <avi@...hat.com>
CC:	vatsa@...ux.vnet.ibm.com, Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...hat.com>, linux-kernel@...r.kernel.org,
	npiggin@...e.de, kvm@...r.kernel.org, bharata@...ibm.com,
	Balbir Singh <balbir@...ibm.com>,
	Jan Beulich <JBeulich@...ell.com>
Subject: Re: [PATCH RFC 2/4] Add yield hypercall for KVM guests

  On 08/02/2010 01:32 AM, Avi Kivity wrote:
>  On 07/26/2010 08:19 PM, Jeremy Fitzhardinge wrote:
>>  On 07/25/2010 11:14 PM, Srivatsa Vaddagiri wrote:
>>> Add KVM hypercall for yielding vcpu timeslice.
>>
>> Can you do a directed yield?
>>
>
> A problem with directed yield is figuring out who to yield to.  One 
> idea is to look for a random vcpu that is not running and donate some 
> runtime to it.  In the best case, it's the lock holder and we cause it 
> to start running.  Middle case it's not the lock holder, but we lose 
> enough runtime to stop running, so at least we don't waste cpu.  Worst 
> case we continue running not having woken the lock holder.  Spin 
> again, yield again hoping to find the right vcpu.

That can help with lockholder preemption, but on unlock you need to wake 
up exactly the right vcpu - the next in the ticket queue - in order to 
avoid burning masses of cpu.  If each cpu records what lock it is 
spinning on and what its ticket is in a percpu variable, then the 
unlocker can search for the next person to kick.

     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/