From: Saravana Kannan <saravanak@google.com>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: David Dai <davidai@google.com>,
"Rafael J. Wysocki" <rafael@kernel.org>,
Rob Herring <robh+dt@kernel.org>,
Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
Conor Dooley <conor+dt@kernel.org>,
Sudeep Holla <sudeep.holla@arm.com>,
Quentin Perret <qperret@google.com>,
Masami Hiramatsu <mhiramat@google.com>,
Will Deacon <will@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Vincent Guittot <vincent.guittot@linaro.org>,
Marc Zyngier <maz@kernel.org>,
Oliver Upton <oliver.upton@linux.dev>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Pavan Kondeti <quic_pkondeti@quicinc.com>,
Gupta Pankaj <pankaj.gupta@amd.com>, Mel Gorman <mgorman@suse.de>,
kernel-team@android.com, linux-pm@vger.kernel.org,
devicetree@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 2/2] cpufreq: add virtual-cpufreq driver
Date: Fri, 4 Aug 2023 15:24:20 -0700 [thread overview]
Message-ID: <CAGETcx9BNQmV6eJDKBMxpU7VZeDh7SYHrRNH5g1TCFrY9cPBUA@mail.gmail.com> (raw)
In-Reply-To: <20230803055157.6buxd5lcxjo4wrbk@vireshk-i7>
On Wed, Aug 2, 2023 at 10:52 PM Viresh Kumar <viresh.kumar@linaro.org> wrote:
>
> On 02-08-23, 15:16, Saravana Kannan wrote:
> > This is mainly an optimization to reduce the latency of the "frequency
> > change" which has a huge impact on the performance (as can be seen
> > from the numbers in the cover letter).
> >
> > Setting this flag means that the vCPU thread triggering the MMIO
> > handling (on the host side) is the thread on which the host needs to
> > apply any uclamp settings. So this avoids the VMM having to look up
> > the right vCPU thread that corresponds to this CPU, and any
> > permissions issues wrt setting another threads uclamp, etc. This
> > becomes even more important if/when BPF support is added for handling
> > simple MMIO read/writes. Will Deacon has been working on the eBPF
> > part[1] and IIUC, not setting this flag adds a lot of extra overhead
> > on the BPF side.
> >
> > So, yeah, this flag is very helpful wrt reducing latency/simplifying
> > host side implementation and that's why we want it here.
> >
> > [1] - https://kvm-forum.qemu.org/2023/talk/AZKC77/
>
> Would be good to have a (big) comment in the code explaining that as
> it isn't obvious. Thanks.
Will do.
Thanks,
Saravana
next prev parent reply other threads:[~2023-08-04 22:25 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-07-31 17:46 [PATCH v3 0/2] Improve VM CPUfreq and task placement behavior David Dai
2023-07-31 17:46 ` [PATCH v3 1/2] dt-bindings: cpufreq: add bindings for virtual cpufreq David Dai
2023-07-31 18:12 ` Rob Herring
2023-08-05 19:38 ` Krzysztof Kozlowski
2023-08-08 23:31 ` Saravana Kannan
2023-08-09 6:28 ` Krzysztof Kozlowski
2023-07-31 17:46 ` [PATCH v3 2/2] cpufreq: add virtual-cpufreq driver David Dai
2023-07-31 22:02 ` Randy Dunlap
2023-07-31 23:46 ` David Dai
2023-08-01 9:36 ` Viresh Kumar
2023-08-02 22:16 ` Saravana Kannan
2023-08-03 5:51 ` Viresh Kumar
2023-08-04 22:24 ` Saravana Kannan [this message]
2023-08-03 16:50 ` David Dai
2023-08-04 4:42 ` Viresh Kumar
2023-08-01 9:45 ` Quentin Perret
2023-08-01 9:49 ` Quentin Perret
2023-08-04 22:23 ` Saravana Kannan
2023-08-03 4:18 ` Pavan Kondeti
2023-08-04 23:46 ` David Dai
2023-08-07 3:22 ` Pavan Kondeti
2023-08-24 23:55 ` David Dai
2023-08-12 2:55 ` kernel test robot
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