From: Marc Zyngier <marc.zyngier@arm.com>
To: Zenghui Yu <yuzenghui@huawei.com>
Cc: kvm@vger.kernel.org, "Raslan, KarimAllah" <karahmed@amazon.de>,
"Saidi, Ali" <alisaidi@amazon.com>,
kvmarm@lists.cs.columbia.edu,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 7/9] KVM: arm/arm64: vgic-its: Cache successful MSI->LPI translation
Date: Wed, 26 Jun 2019 08:55:49 +0100 [thread overview]
Message-ID: <86a7e4pypm.wl-marc.zyngier@arm.com> (raw)
In-Reply-To: <7af32ebf-91a8-ef63-6108-4ca506fd364e@huawei.com>
On Tue, 25 Jun 2019 17:00:54 +0100,
Zenghui Yu <yuzenghui@huawei.com> wrote:
>
> Hi Marc,
>
> On 2019/6/25 20:31, Marc Zyngier wrote:
> > Hi Zenghui,
> >
> > On 25/06/2019 12:50, Zenghui Yu wrote:
> >> Hi Marc,
> >>
> >> On 2019/6/12 1:03, Marc Zyngier wrote:
> >>> On a successful translation, preserve the parameters in the LPI
> >>> translation cache. Each translation is reusing the last slot
> >>> in the list, naturally evincting the least recently used entry.
> >>>
> >>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> >>> ---
> >>> virt/kvm/arm/vgic/vgic-its.c | 86 ++++++++++++++++++++++++++++++++++++
> >>> 1 file changed, 86 insertions(+)
> >>>
> >>> diff --git a/virt/kvm/arm/vgic/vgic-its.c b/virt/kvm/arm/vgic/vgic-its.c
> >>> index 0aa0cbbc3af6..62932458476a 100644
> >>> --- a/virt/kvm/arm/vgic/vgic-its.c
> >>> +++ b/virt/kvm/arm/vgic/vgic-its.c
> >>> @@ -546,6 +546,90 @@ static unsigned long vgic_mmio_read_its_idregs(struct kvm *kvm,
> >>> return 0;
> >>> }
> >>> +static struct vgic_irq *__vgic_its_check_cache(struct
> >>> vgic_dist *dist,
> >>> + phys_addr_t db,
> >>> + u32 devid, u32 eventid)
> >>> +{
> >>> + struct vgic_translation_cache_entry *cte;
> >>> + struct vgic_irq *irq = NULL;
> >>> +
> >>> + list_for_each_entry(cte, &dist->lpi_translation_cache, entry) {
> >>> + /*
> >>> + * If we hit a NULL entry, there is nothing after this
> >>> + * point.
> >>> + */
> >>> + if (!cte->irq)
> >>> + break;
> >>> +
> >>> + if (cte->db == db &&
> >>> + cte->devid == devid &&
> >>> + cte->eventid == eventid) {
> >>> + /*
> >>> + * Move this entry to the head, as it is the
> >>> + * most recently used.
> >>> + */
> >>> + list_move(&cte->entry, &dist->lpi_translation_cache);
> >>
> >> Only for performance reasons: if we hit at the "head" of the list, we
> >> don't need to do a list_move().
> >> In our tests, we found that a single list_move() takes nearly (sometimes
> >> even more than) one microsecond, for some unknown reason...
> >
> > Huh... That's odd.
> >
> > Can you narrow down under which conditions this happens? I'm not sure if
> > checking for the list head would be more efficient, as you end-up
> > fetching the head anyway. Can you try replacing this line with:
> >
> > if (!list_is_first(&cte->entry, &dist->lpi_translation_cache))
> > list_move(&cte->entry, &dist->lpi_translation_cache);
> >
> > and let me know whether it helps?
>
> It helps. With this change, the overhead of list_move() is gone.
>
> We run 16 4-vcpu VMs on the host, each with a vhost-user nic, and run
> "iperf" in pairs between them. It's likely to hit at the head of the
> cache list in our tests.
> With this change, the sys% utilization of vhostdpfwd threads will
> decrease by about 10%. But I don't know the reason exactly (I haven't
> found any clues in code yet, in implementation of list_move...).
list_move is rather simple, and shouldn't be too hard to execute
quickly. The only contention I can imagine is that as the cache line
is held by multiple CPUs, the update to the list pointers causes an
invalidation to be sent to other CPUs, leading to a slower update.
But it remains that 500ns is a pretty long time (that's 1000 cycles on
a 2GHz CPU). It'd be interesting to throw perf at this and see shows
up. It would give us a clue about what is going on here.
Thanks,
M.
--
Jazz is not dead, it just smells funny.
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next prev parent reply other threads:[~2019-06-26 7:56 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-11 17:03 [PATCH v2 0/9] KVM: arm/arm64: vgic: ITS translation cache Marc Zyngier
2019-06-11 17:03 ` [PATCH v2 1/9] KVM: arm/arm64: vgic: Add LPI translation cache definition Marc Zyngier
2019-06-12 8:16 ` Julien Thierry
2019-06-12 8:49 ` Julien Thierry
2019-06-12 9:52 ` Marc Zyngier
2019-06-12 10:58 ` Julien Thierry
2019-06-12 12:28 ` Julien Thierry
2019-07-23 12:43 ` Auger Eric
2019-06-11 17:03 ` [PATCH v2 2/9] KVM: arm/arm64: vgic: Add __vgic_put_lpi_locked primitive Marc Zyngier
2019-06-11 17:03 ` [PATCH v2 3/9] KVM: arm/arm64: vgic-its: Add MSI-LPI translation cache invalidation Marc Zyngier
2019-07-23 12:39 ` Auger Eric
2019-06-11 17:03 ` [PATCH v2 4/9] KVM: arm/arm64: vgic-its: Invalidate MSI-LPI translation cache on specific commands Marc Zyngier
2019-07-01 12:38 ` Auger Eric
2019-07-22 10:54 ` Marc Zyngier
2019-07-23 12:25 ` Auger Eric
2019-07-23 12:43 ` Marc Zyngier
2019-07-23 12:47 ` Auger Eric
2019-07-23 12:50 ` Marc Zyngier
2019-06-11 17:03 ` [PATCH v2 5/9] KVM: arm/arm64: vgic-its: Invalidate MSI-LPI translation cache on disabling LPIs Marc Zyngier
2019-07-23 15:09 ` Auger Eric
2019-06-11 17:03 ` [PATCH v2 6/9] KVM: arm/arm64: vgic-its: Invalidate MSI-LPI translation cache on vgic teardown Marc Zyngier
2019-07-23 15:10 ` Auger Eric
2019-06-11 17:03 ` [PATCH v2 7/9] KVM: arm/arm64: vgic-its: Cache successful MSI->LPI translation Marc Zyngier
2019-06-25 11:50 ` Zenghui Yu
2019-06-25 12:31 ` Marc Zyngier
2019-06-25 16:00 ` Zenghui Yu
2019-06-26 3:54 ` Zenghui Yu
2019-06-26 7:55 ` Marc Zyngier [this message]
2019-07-23 15:21 ` Auger Eric
2019-06-11 17:03 ` [PATCH v2 8/9] KVM: arm/arm64: vgic-its: Check the LPI translation cache on MSI injection Marc Zyngier
2019-07-23 15:10 ` Auger Eric
2019-07-23 15:45 ` Marc Zyngier
2019-07-24 7:41 ` Auger Eric
2019-06-11 17:03 ` [PATCH v2 9/9] KVM: arm/arm64: vgic-irqfd: Implement kvm_arch_set_irq_inatomic Marc Zyngier
2019-07-23 15:14 ` Auger Eric
2019-07-25 8:24 ` Marc Zyngier
2019-07-23 11:14 ` [PATCH v2 0/9] KVM: arm/arm64: vgic: ITS translation cache Andre Przywara
2019-07-25 8:50 ` Marc Zyngier
2019-07-25 10:01 ` Andre Przywara
2019-07-25 15:37 ` Marc Zyngier
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