From: David Woodhouse <dwmw2@infradead.org>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Wanpeng Li <kernellwp@gmail.com>,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v7 1/2] KVM: LAPIC: Make lapic timer unpinned
Date: Wed, 09 Mar 2022 16:03:54 +0100 [thread overview]
Message-ID: <0e8f1e1a9653d95332cc8f0d461c3015ae20d03e.camel@infradead.org> (raw)
In-Reply-To: <YiiOZaQCf1K653MS@fuller.cnet>
[-- Attachment #1: Type: text/plain, Size: 2624 bytes --]
On Wed, 2022-03-09 at 08:24 -0300, Marcelo Tosatti wrote:
> On Wed, Mar 09, 2022 at 10:26:51AM +0100, David Woodhouse wrote:
> > On Sat, 2019-07-06 at 09:26 +0800, Wanpeng Li wrote:
> > > From: Wanpeng Li <wanpengli@tencent.com>
> > >
> > > Commit 61abdbe0bcc2 ("kvm: x86: make lapic hrtimer pinned") pinned the
> > > lapic timer to avoid to wait until the next kvm exit for the guest to
> > > see KVM_REQ_PENDING_TIMER set. There is another solution to give a kick
> > > after setting the KVM_REQ_PENDING_TIMER bit, make lapic timer unpinned
> > > will be used in follow up patches.
> > >
> > > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > > Cc: Radim Krčmář <rkrcmar@redhat.com>
> > > Cc: Marcelo Tosatti <mtosatti@redhat.com>
> > > Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> > > ---
> > > arch/x86/kvm/lapic.c | 8 ++++----
> > > arch/x86/kvm/x86.c | 6 +-----
> > > 2 files changed, 5 insertions(+), 9 deletions(-)
> >
> > ...
> >
> >
> > > @@ -2510,7 +2510,7 @@ void __kvm_migrate_apic_timer(struct kvm_vcpu *vcpu)
> > >
> > > timer = &vcpu->arch.apic->lapic_timer.timer;
> > > if (hrtimer_cancel(timer))
> > > - hrtimer_start_expires(timer, HRTIMER_MODE_ABS_PINNED);
> > > + hrtimer_start_expires(timer, HRTIMER_MODE_ABS);
> > > }
> > >
> > > /*
> >
> > Wait, in that case why are we even bothering to cancel and restart the
> > timer? I thought the whole point of that was to pin it to the *new* CPU
> > that this vCPU is running on.
> >
> > If not, can't we just kill __kvm_migrate_apic_timer() off completely
> > not?
>
> Current code looks like this:
>
> void __kvm_migrate_apic_timer(struct kvm_vcpu *vcpu)
> {
> struct hrtimer *timer;
>
> if (!lapic_in_kernel(vcpu) ||
> kvm_can_post_timer_interrupt(vcpu)) <----------
> return;
>
> timer = &vcpu->arch.apic->lapic_timer.timer;
> if (hrtimer_cancel(timer))
> hrtimer_start_expires(timer, HRTIMER_MODE_ABS_HARD);
> }
>
> Yeah, should be HRTIMER_MODE_ABS_PINNED AFAICS.
No, it's *intentionally* not pinned any more, since this patch that I'm
replying to, which became commit 4d151bf3b89. It doesn't *have* to run
on the same physical CPU, because of the epiphany that it can just call
kvm_vcpu_kick() after making the request.
But if it's a recurring timer it's still *best* for it to run on the
same physical CPU, just for cache locality reasons. So I think I was
wrong; the migration *isn't* pointless. It's still a valid
optimisation; it's just not *mandatory* any more.
[-- Attachment #2: smime.p7s --]
[-- Type: application/pkcs7-signature, Size: 5965 bytes --]
next prev parent reply other threads:[~2022-03-09 15:03 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-06 1:26 [PATCH v7 0/2] KVM: LAPIC: Implement Exitless Timer Wanpeng Li
2019-07-06 1:26 ` [PATCH v7 1/2] KVM: LAPIC: Make lapic timer unpinned Wanpeng Li
2022-03-09 9:26 ` David Woodhouse
2022-03-09 11:24 ` Marcelo Tosatti
2022-03-09 15:03 ` David Woodhouse [this message]
2019-07-06 1:26 ` [PATCH v7 2/2] KVM: LAPIC: Inject timer interrupt via posted interrupt Wanpeng Li
2019-07-17 16:27 ` Paolo Bonzini
[not found] ` <TY2PR02MB41600B4C6B9FF4A9F8CD957880F30@TY2PR02MB4160.apcprd02.prod.outlook.com>
2019-07-11 14:03 ` [PATCH v7 0/2] KVM: LAPIC: Implement Exitless Timer Paolo Bonzini
2019-07-17 0:30 ` Wanpeng Li
2019-08-21 4:16 ` Wanpeng Li
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=0e8f1e1a9653d95332cc8f0d461c3015ae20d03e.camel@infradead.org \
--to=dwmw2@infradead.org \
--cc=kernellwp@gmail.com \
--cc=kvm@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mtosatti@redhat.com \
--cc=pbonzini@redhat.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).