Re: [RFC PATCH v4 0/7] hw/arm/virt: Introduce cpu topology support

From: "wangyanan (Y)" <wangyanan55@huawei.com>
To: "Daniel P. Berrangé" <berrange@redhat.com>
Cc: Barry Song <song.bao.hua@hisilicon.com>,
	Peter Maydell <peter.maydell@linaro.org>,
	Andrew Jones <drjones@redhat.com>,
	"Michael S . Tsirkin" <mst@redhat.com>,
	wanghaibin.wang@huawei.com, qemu-devel@nongnu.org,
	yangyicong@huawei.com, Shannon Zhao <shannon.zhaosl@gmail.com>,
	qemu-arm@nongnu.org, Alistair Francis <alistair.francis@wdc.com>,
	prime.zeng@hisilicon.com, Paolo Bonzini <pbonzini@redhat.com>,
	yuzenghui@huawei.com, Igor Mammedov <imammedo@redhat.com>,
	zhukeqian1@huawei.com, David Gibson <david@gibson.dropbear.id.au>
Subject: Re: [RFC PATCH v4 0/7] hw/arm/virt: Introduce cpu topology support
Date: Tue, 22 Jun 2021 22:04:52 +0800	[thread overview]
Message-ID: <7fcc5f2d-cc84-3464-15cc-3bebb07f8190@huawei.com> (raw)
In-Reply-To: <YNHalhuNZhMa665J@redhat.com>

Hi Daniel,

On 2021/6/22 20:41, Daniel P. Berrangé wrote:
> On Tue, Jun 22, 2021 at 08:31:22PM +0800, wangyanan (Y) wrote:
>>
>> On 2021/6/22 19:46, Andrew Jones wrote:
>>> On Tue, Jun 22, 2021 at 11:18:09AM +0100, Daniel P. Berrangé wrote:
>>>> On Tue, Jun 22, 2021 at 05:34:06PM +0800, Yanan Wang wrote:
>>>>> Hi,
>>>>>
>>>>> This is v4 of the series [1] that I posted to introduce support for
>>>>> generating cpu topology descriptions to guest. Comments are welcome!
>>>>>
>>>>> Description:
>>>>> Once the view of an accurate virtual cpu topology is provided to guest,
>>>>> with a well-designed vCPU pinning to the pCPU we may get a huge benefit,
>>>>> e.g., the scheduling performance improvement. See Dario Faggioli's
>>>>> research and the related performance tests in [2] for reference. So here
>>>>> we go, this patch series introduces cpu topology support for ARM platform.
>>>>>
>>>>> In this series, instead of quietly enforcing the support for the latest
>>>>> machine type, a new parameter "expose=on|off" in -smp command line is
>>>>> introduced to leave QEMU users a choice to decide whether to enable the
>>>>> feature or not. This will allow the feature to work on different machine
>>>>> types and also ideally compat with already in-use -smp command lines.
>>>>> Also we make much stricter requirement for the topology configuration
>>>>> with "expose=on".
>>>> Seeing this 'expose=on' parameter feels to me like we're adding a
>>>> "make-it-work=yes" parameter. IMHO this is just something that should
>>>> be done by default for the current machine type version and beyond.
>>>> I don't see the need for a parameter to turnthis on, especially since
>>>> it is being made architecture specific.
>>>>
>>> I agree.
>>>
>>> Yanan, we never discussed an "expose" parameter in the previous versions
>>> of this series. We discussed a "strict" parameter though, which would
>>> allow existing command lines to "work" using assumptions of what the user
>>> meant and strict=on users to get what they mean or an error saying that
>>> they asked for something that won't work or would require unreasonable
>>> assumptions. Why was this changed to an "expose" parameter?
>> Yes, we indeed discuss a new "strict" parameter but not a "expose" in v2 [1]
>> of this series.
>> [1] https://patchwork.kernel.org/project/qemu-devel/patch/20210413080745.33004-6-wangyanan55@huawei.com/
>>
>> And in the discussion, we hoped things would work like below with "strict"
>> parameter:
>> Users who want to describe cpu topology should provide cmdline like
>>
>> -smp strict=on,cpus=4,sockets=2,cores=2,threads=1
>>
>> and in this case we require an more accurate -smp configuration and
>> then generate the cpu topology description through ACPI/DT.
>>
>> While without a strict description, no cpu topology description would
>> be generated, so they get nothing through ACPI/DT.
>>
>> It seems to me that the "strict" parameter actually serves as a knob to
>> turn on/off the exposure of topology, and this is the reason I changed
>> the name.
> Yes, the use of 'strict=on' is no better than expose=on IMHO.
>
> If I give QEMU a cli
>
>    -smp cpus=4,sockets=2,cores=2,threads=1
>
> then I expect that topology to be exposed to the guest. I shouldn't
> have to add extra flags to make that happen.
>
> Looking at the thread, it seems the concern was around the fact that
> the settings were not honoured historically and thus the CLI values
> could be garbage. ie  -smp cpus=4,sockets=8,cores=3,thread=9
This "-smp cpus=4,sockets=8,cores=3,threads=9" behaviors as a wrong
configuration, and the parsing function already report error for this case.

We hope more complete config like "-smp 4,sockets=2,cores=2,threads=1"
for exposure of topology, and the incomplete ones like "-smp 4,sockets=1"
or "-smp 4, cores=1" are not acceptable any more because we are starting
to expose the topology.
> A similar problem existed on x86 platforms. When we made that stricter
> we had cde that issued a warning for a few releases, essentially
> deprecating the config. EVentually it was turned into a fatal error.
> This gave applications time to fix their broken configs, while having
> correct configs "just work".
I understand this solution. Stop exposing topology for unqualified -smp
config and report a warning message at the transitional phase, and finally
incur an error for them.

BTW, just want to be sure, it this a common method in QEMU development
to solve this kind of compatibility issues?
> I'd suggest doing the same for arm. If the -smp args are semantically
> valid then expose the topology automatically (for new machine type).
> If the -smp args are semantically broken, then issue a warning. In
> a few releases time, turn this warning into an error.
So this topology feature will only work for the current machine type and
the following versions, right?

Thanks,
Yanan
.
> Regards,
> Daniel