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    Subject[PATCH v4 20/24] sched/fair: Introduce sched_smt_siblings_idle()
    Date
    X86 needs to know the idle state of the SMT siblings of a CPU to improve
    the accuracy of IPCC classification. X86 implements support for IPC classes
    in the thermal HFI driver.

    Rename is_core_idle() as sched_smt_siblings_idle() and make it available
    outside the scheduler code.

    Cc: Ben Segall <bsegall@google.com>
    Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
    Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
    Cc: Len Brown <len.brown@intel.com>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: Perry Yuan <Perry.Yuan@amd.com>
    Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
    Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Cc: Tim C. Chen <tim.c.chen@intel.com>
    Cc: Valentin Schneider <vschneid@redhat.com>
    Cc: Zhao Liu <zhao1.liu@linux.intel.com>
    Cc: x86@kernel.org
    Cc: linux-kernel@vger.kernel.org
    Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
    ---
    is_core_idle() is no longer an inline function after this patch. To rule
    out performance degradation, I compared the execution time of the inline
    and non-inline versions on a 4-socket Cascade Lake system using the NUMA
    stressor of stress-ng.

    I used this test command:

    $ stress-ng --numa 1500 -t 10m

    During the test, is_core_idle() was called ~200,000 times. To measure the
    execution time, I recorded the value of the TSC counter before and after
    calling is_core_idle() and calculated the difference.
    value.

    I arbitrarily removed outliers (defined as any delta larger than 5000
    counts). This required removing ~40 samples.

    The table below summarizes the difference in execution time. All values
    are expressed in TSC counts, except for the standard deviation, expressed
    as a percentage of the average.

    Average Median Std(%) Mode
    TSCdelta inline 668.76 626 67.24 42
    TSCdelta non-inline 677.64 624 67.67 46

    The metrics show that both the inline and non-inline versions exhibit
    similar performance characteristics.
    ---
    Changes since v3:
    * None

    Changes since v2:
    * Brought back this previously dropped patch.
    * Profiled inline vs non-inline is_core_idle(). I found not major penalty.
    * Merged is_core_idle() and sched_smt_siblings_idle() into a single
    function. (Dietmar)

    Changes since v1:
    * Dropped this patch.
    ---
    include/linux/sched.h | 2 ++
    kernel/sched/fair.c | 13 ++++++++++---
    2 files changed, 12 insertions(+), 3 deletions(-)

    diff --git a/include/linux/sched.h b/include/linux/sched.h
    index 719147460ca8..986344ebf2f6 100644
    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -2461,4 +2461,6 @@ static inline void sched_core_fork(struct task_struct *p) { }

    extern void sched_set_stop_task(int cpu, struct task_struct *stop);

    +extern bool sched_smt_siblings_idle(int cpu);
    +
    #endif
    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index da3e009eef42..a52589a6c561 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -1064,7 +1064,14 @@ update_stats_curr_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
    * Scheduling class queueing methods:
    */

    -static inline bool is_core_idle(int cpu)
    +/**
    + * sched_smt_siblings_idle - Check whether SMT siblings of a CPU are idle
    + * @cpu: The CPU to check
    + *
    + * Returns true if all the SMT siblings of @cpu are idle or @cpu does not have
    + * SMT siblings. The idle state of @cpu is not considered.
    + */
    +bool sched_smt_siblings_idle(int cpu)
    {
    #ifdef CONFIG_SCHED_SMT
    int sibling;
    @@ -1767,7 +1774,7 @@ static inline int numa_idle_core(int idle_core, int cpu)
    * Prefer cores instead of packing HT siblings
    * and triggering future load balancing.
    */
    - if (is_core_idle(cpu))
    + if (sched_smt_siblings_idle(cpu))
    idle_core = cpu;

    return idle_core;
    @@ -9652,7 +9659,7 @@ static bool sched_use_asym_prio(struct sched_domain *sd, int cpu)
    if (!sched_smt_active())
    return true;

    - return sd->flags & SD_SHARE_CPUCAPACITY || is_core_idle(cpu);
    + return sd->flags & SD_SHARE_CPUCAPACITY || sched_smt_siblings_idle(cpu);
    }

    /**
    --
    2.25.1
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