linux-kernel - Re: [PATCH v2] KVM: halt-polling: poll if emulated lapic timer will fire soon

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: <CALzav=cAk1bnVLQY9NCH9EGYcTK1-D6Y1FGDBTY+0usGz1ETZA@mail.gmail.com>
Date:	Mon, 23 May 2016 11:04:41 -0700
From:	David Matlack <dmatlack@...gle.com>
To:	Yang Zhang <yang.zhang.wz@...il.com>
Cc:	Wanpeng Li <kernellwp@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm list <kvm@...r.kernel.org>,
	Wanpeng Li <wanpeng.li@...mail.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [PATCH v2] KVM: halt-polling: poll if emulated lapic timer will
 fire soon

On Sun, May 22, 2016 at 6:26 PM, Yang Zhang <yang.zhang.wz@...il.com> wrote:
> On 2016/5/21 2:37, David Matlack wrote:
>>
>> It's not obvious to me why polling for a timer interrupt would improve
>> context switch latency. Can you explain a bit more?
>
>
> We have a workload which using high resolution timer(less than 1ms) inside
> guest. It rely on the timer to wakeup itself. Sometimes the timer is
> expected to fired just after the VCPU is blocked due to execute halt
> instruction. But the thread who is running in the CPU will turn off the
> hardware interrupt for long time due to disk access. This will cause the
> timer interrupt been blocked until the interrupt is re-open.

Does this happen on the idle thread (swapper)? If not, halt-polling
may not help; it only polls if there are no other runnable threads.

> For optimization, we let VCPU to poll for a while if the next timer will
> arrive soon before schedule out. And the result shows good when running
> several workloads inside guest.

Thanks for the explanation, I appreciate it.

>
> --
> best regards
> yang