linux-kernel - Re: [PATCH v5 0/7] Thermal: Introduce the Hardware Feedback Interface for thermal and performance management

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Date:   Sun, 30 Jan 2022 16:23:02 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Len Brown <len.brown@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Amit Kucheria <amitk@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Lukasz Luba <lukasz.luba@....com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ricardo Neri <ricardo.neri@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 0/7] Thermal: Introduce the Hardware Feedback Interface
 for thermal and performance management

On Thu, Jan 27, 2022 at 8:33 PM Ricardo Neri
<ricardo.neri-calderon@...ux.intel.com> wrote:
>
> Hi,
>
> This is v5 of this patchset after having incorporated the feedback from
> reviewers. Please find v1, v2, v3, and v4 in [1], [2], [3], and [4],
> respectively.
>
> The Intel Hardware Feedback Interface (HFI) [5] provides information about
> the performance and energy efficiency of each CPU in the system. It uses a
> table that is shared between hardware and the operating system. The
> contents of the table may be updated as a result of changes in the
> operating conditions of the system (e.g., reaching a thermal limit) or the
> action of external factors (e.g., changes in the thermal design power).
>
> The information that HFI provides are specified as numeric, unit-less
> capabilities relative to other CPUs in the system. These capabilities have
> a range of [0-255] where higher numbers represent higher capabilities.
> Energy efficiency and performance are reported in separate capabilities.
> If either the performance or energy capabilities efficiency of a CPU are 0,
> the hardware recommends to not schedule any tasks on such CPU for
> performance, energy efficiency or thermal reasons, respectively.
>
> The kernel or user space may use the information from the HFI to modify
> task placement and/or adjust power limits. This patchset focuses on the
> user space. The thermal notification framework is extended to relay
> updates of CPU capacity. Thus, a userspace daemon can affinitize workloads
> to certain CPUs and/or offline CPUs whose capabilities are zero.
>
> The frequency of HFI updates is specific to each processor model. On some
> systems, there is just a single HFI update at boot. On other systems, there
> may be updates every tens of milliseconds. In order to not overwhelm
> userspace with too many updates, they are limited to one update every
> CONFIG_HZ jiffies.
>
> Thanks and BR,
> Ricardo
>
> [1]. https://lore.kernel.org/lkml/20211106013312.26698-1-ricardo.neri-calderon@linux.intel.com/
> [2]. https://lore.kernel.org/lkml/20211220151438.1196-1-ricardo.neri-calderon@linux.intel.com/
> [3]. https://lore.kernel.org/lkml/20220106025059.25847-8-ricardo.neri-calderon@linux.intel.com/
> [4]. https://lore.kernel.org/lkml/20220108034743.31277-1-ricardo.neri-calderon@linux.intel.com/
> [5]. https://www.intel.com/sdm
>
> Changes since v4:
>  ++ Unchanged patches: 1, 2.
>  * Reworded description hfi_instance::hdr and hfi_instance::data.
>    (Patch 3; Srinivas)
>  * Call intel_hfi_online() before enabling the thermal vector at the
>    local APIC is enabled. This makes sure that a CPU has an associated
>    HFI instance when an HFI event happens. Reworded the commit message to
>    reflect this change. (Patch 4; Srinivas)
>  * Set hfi_instances to NULL if we fail to allocate memory for
>    hfi_instance::cpus. (Patch 4; Srinivas)
>  * Delayed initialization of local variables until after the check for a
>    non-NULL hfi_instances in intel_hfi_online(). (Patch 4; Srinivas)
>  * Optimized the error path in init_hfi_init() to not needlessly
>    free the memory of cpumasks that have not been allocated. (Patch 4;
>    Srinivas)
>  * Removed pointless checks for X86_FEATURE_HFI in
>    intel_hfi_[on|off]line(). It is sufficient to check for a non-NULL
>    hfi_instances or a CPU's hfi_instance. (Patch 4)
>  * Added a dedicated (i.e., not system_wq) workqueue to process HFI updates.
>    Reworded commit message accordingly. (Patch 5; Rafael)
>  * Repurposed comment on possibly receiving an HFI event with a NULL
>    hfi_cpu_info::instance. Patch 4 fixed this potential issue. Instead,
>    add a debug statement. (Patch 5; Srinivas)
>  * Wrapped check for NULL hfi_cpu_info::instance in the unlikely macro.
>    (Patch 5; Srinivas)
>  * Renamed struct cpu_capability as struct thermal_genl_cpu_caps.
>    (Patch 6; Rafael)
>  * Removed automatic variable ret from
>    thermal_genl_event_cpu_capability_change() and instead always return
>    -EMSGSIZE on error. (Patch 6; Rafael)
>  * Reworked parsing of HFI capabilities into chunks of
>    HFI_MAX_THERM_NOTIFY_COUNT CPUs at a time to reduce the time
>    we spend with interrupts disabled. (Patch7; Srinivas)
>  * Protected hfi_instance::cpus when iterating over them. (Patch 7;
>    Rafael)

All patches in the series look good to me now, so I will be queuing it
up for 5.18 unless there are any objections or concerns.

Thanks!