From: Christian Borntraeger <borntraeger@de.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>,
Pan Xinhui <xinhui@linux.vnet.ibm.com>
Cc: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>,
linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
virtualization@lists.linux-foundation.org,
linux-s390@vger.kernel.org,
xen-devel-request@lists.xenproject.org, kvm@vger.kernel.org,
xen-devel@lists.xenproject.org, x86@kernel.org,
benh@kernel.crashing.org, paulus@samba.org, mpe@ellerman.id.au,
mingo@redhat.com, paulmck@linux.vnet.ibm.com,
will.deacon@arm.com, kernellwp@gmail.com, jgross@suse.com,
pbonzini@redhat.com, bsingharora@gmail.com, boqun.feng@gmail.com,
rkrcmar@redhat.com, David.Laight@ACULAB.COM, dave@stgolabs.net,
konrad.wilk@oracle.com
Subject: Re: [PATCH v7 06/11] x86, paravirt: Add interface to support kvm/xen vcpu preempted check
Date: Wed, 16 Nov 2016 12:29:44 +0100 [thread overview]
Message-ID: <f8b9d1be-98a2-4d99-1ae1-c31c6fe13903@de.ibm.com> (raw)
In-Reply-To: <20161116102355.GP3142@twins.programming.kicks-ass.net>
On 11/16/2016 11:23 AM, Peter Zijlstra wrote:
> On Wed, Nov 16, 2016 at 12:19:09PM +0800, Pan Xinhui wrote:
>> Hi, Peter.
>> I think we can avoid a function call in a simpler way. How about below
>>
>> static inline bool vcpu_is_preempted(int cpu)
>> {
>> /* only set in pv case*/
>> if (pv_lock_ops.vcpu_is_preempted)
>> return pv_lock_ops.vcpu_is_preempted(cpu);
>> return false;
>> }
>
> That is still more expensive. It needs to do an actual load and makes it
> hard to predict the branch, you'd have to actually wait for the load to
> complete etc.
Out of curiosity, why is that hard to predict?
On s390 the branch prediction runs asynchronously ahead of the downstream
pipeline (e.g. search for "IBM z Systems Processor Optimization Primer" page 11).
given enough capacity, I would assume that modern x86 processors would do the same
and be able to predict this is as soon as it becomes hot (and otherwise you would
not notice the branch miss anyway). Is x86 behaving differently here?
> Also, it generates more code.
>
> Paravirt muck should strive to be as cheap as possible when ran on
> native hardware.
As I am interested in this series from the s390 point of view, this is
the only thing that block this series?
Is there a chance to add a static key around the paravirt ops somehow?
next prev parent reply other threads:[~2016-11-16 11:29 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-11-02 9:08 [PATCH v7 00/11] implement vcpu preempted check Pan Xinhui
2016-11-02 9:08 ` [PATCH v7 01/11] kernel/sched: introduce vcpu preempted check interface Pan Xinhui
2016-11-02 9:08 ` [PATCH v7 02/11] locking/osq: Drop the overload of osq_lock() Pan Xinhui
2016-11-02 9:08 ` [PATCH v7 03/11] kernel/locking: Drop the overload of {mutex, rwsem}_spin_on_owner Pan Xinhui
2016-11-02 9:08 ` [PATCH v7 04/11] powerpc/spinlock: support vcpu preempted check Pan Xinhui
2016-11-02 9:08 ` [PATCH v7 05/11] s390/spinlock: Provide vcpu_is_preempted Pan Xinhui
2016-11-02 9:08 ` [PATCH v7 06/11] x86, paravirt: Add interface to support kvm/xen vcpu preempted check Pan Xinhui
2016-11-15 15:47 ` Peter Zijlstra
2016-11-16 4:19 ` Pan Xinhui
2016-11-16 10:23 ` Peter Zijlstra
2016-11-16 11:29 ` Christian Borntraeger [this message]
2016-11-16 11:43 ` Peter Zijlstra
2016-11-17 5:16 ` Pan Xinhui
2016-11-02 9:08 ` [PATCH v7 07/11] KVM: Introduce kvm_write_guest_offset_cached Pan Xinhui
2016-11-02 9:08 ` [PATCH v7 08/11] x86, kvm/x86.c: support vcpu preempted check Pan Xinhui
2016-11-02 9:08 ` [PATCH v7 09/11] x86, kernel/kvm.c: " Pan Xinhui
2016-11-02 9:08 ` [PATCH v7 10/11] x86, xen: " Pan Xinhui
2016-11-02 9:08 ` [PATCH v7 11/11] Documentation: virtual: kvm: Support " Pan Xinhui
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=f8b9d1be-98a2-4d99-1ae1-c31c6fe13903@de.ibm.com \
--to=borntraeger@de.ibm.com \
--cc=David.Laight@ACULAB.COM \
--cc=benh@kernel.crashing.org \
--cc=boqun.feng@gmail.com \
--cc=bsingharora@gmail.com \
--cc=dave@stgolabs.net \
--cc=jgross@suse.com \
--cc=kernellwp@gmail.com \
--cc=konrad.wilk@oracle.com \
--cc=kvm@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-s390@vger.kernel.org \
--cc=linuxppc-dev@lists.ozlabs.org \
--cc=mingo@redhat.com \
--cc=mpe@ellerman.id.au \
--cc=paulmck@linux.vnet.ibm.com \
--cc=paulus@samba.org \
--cc=pbonzini@redhat.com \
--cc=peterz@infradead.org \
--cc=rkrcmar@redhat.com \
--cc=virtualization@lists.linux-foundation.org \
--cc=will.deacon@arm.com \
--cc=x86@kernel.org \
--cc=xen-devel-request@lists.xenproject.org \
--cc=xen-devel@lists.xenproject.org \
--cc=xinhui.pan@linux.vnet.ibm.com \
--cc=xinhui@linux.vnet.ibm.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).