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    Subject[PATCH v17 2/3] /proc/pid/arch_status: Add AVX-512 usage elapsed time
    Date
    AVX-512 components use could cause core turbo frequency drop. So
    it's useful to expose AVX-512 usage elapsed time as a heuristic hint
    for the user space job scheduler to cluster the AVX-512 using tasks
    together.

    Tensorflow example:
    $ while [ 1 ]; do cat /proc/tid/arch_status | grep AVX512; sleep 1; done
    AVX512_elapsed_ms: 4
    AVX512_elapsed_ms: 8
    AVX512_elapsed_ms: 4

    This means that 4 milliseconds have elapsed since the AVX512 usage
    of tensorflow task was detected when the task was scheduled out.

    Or:
    $ cat /proc/tid/arch_status | grep AVX512
    AVX512_elapsed_ms: -1

    The number '-1' indicates that no AVX512 usage recorded before
    thus the task unlikely has frequency drop issue.

    User space tools may want to further check by:

    $ perf stat --pid <pid> -e core_power.lvl2_turbo_license -- sleep 1

    Performance counter stats for process id '3558':

    3,251,565,961 core_power.lvl2_turbo_license

    1.004031387 seconds time elapsed

    Non-zero counter value confirms that the task causes frequency drop.

    Signed-off-by: Aubrey Li <aubrey.li@linux.intel.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Andi Kleen <ak@linux.intel.com>
    Cc: Tim Chen <tim.c.chen@linux.intel.com>
    Cc: Dave Hansen <dave.hansen@intel.com>
    Cc: Arjan van de Ven <arjan@linux.intel.com>
    Cc: Alexey Dobriyan <adobriyan@gmail.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Linux API <linux-api@vger.kernel.org>
    ---
    arch/x86/include/asm/processor.h | 6 +++++
    arch/x86/kernel/fpu/xstate.c | 43 ++++++++++++++++++++++++++++++++
    2 files changed, 49 insertions(+)

    diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
    index 2bb3a648fc12..0728848473a2 100644
    --- a/arch/x86/include/asm/processor.h
    +++ b/arch/x86/include/asm/processor.h
    @@ -991,4 +991,10 @@ enum l1tf_mitigations {

    extern enum l1tf_mitigations l1tf_mitigation;

    +#ifdef CONFIG_PROC_PID_ARCH_STATUS
    +/* Add support for task architecture specific output in /proc/pid/arch_status */
    +void task_arch_status(struct seq_file *m, struct task_struct *task);
    +#define task_arch_status task_arch_status
    +#endif /* CONFIG_PROC_PID_ARCH_STATUS */
    +
    #endif /* _ASM_X86_PROCESSOR_H */
    diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
    index d7432c2b1051..a0dda11ab72e 100644
    --- a/arch/x86/kernel/fpu/xstate.c
    +++ b/arch/x86/kernel/fpu/xstate.c
    @@ -7,6 +7,7 @@
    #include <linux/cpu.h>
    #include <linux/mman.h>
    #include <linux/pkeys.h>
    +#include <linux/seq_file.h>

    #include <asm/fpu/api.h>
    #include <asm/fpu/internal.h>
    @@ -1243,3 +1244,45 @@ int copy_user_to_xstate(struct xregs_state *xsave, const void __user *ubuf)

    return 0;
    }
    +
    +#ifdef CONFIG_PROC_PID_ARCH_STATUS
    +/*
    + * Report the amount of time elapsed in millisecond since last AVX512
    + * use in the task.
    + */
    +static void avx512_status(struct seq_file *m, struct task_struct *task)
    +{
    + unsigned long timestamp = READ_ONCE(task->thread.fpu.avx512_timestamp);
    + long delta;
    +
    + if (!timestamp) {
    + /*
    + * Report -1 if no AVX512 usage
    + */
    + delta = -1;
    + } else {
    + delta = (long)(jiffies - timestamp);
    + /*
    + * Cap to LONG_MAX if time difference > LONG_MAX
    + */
    + if (delta < 0)
    + delta = LONG_MAX;
    + delta = jiffies_to_msecs(delta);
    + }
    +
    + seq_put_decimal_ll(m, "AVX512_elapsed_ms:\t", delta);
    + seq_putc(m, '\n');
    +}
    +
    +/*
    + * Report architecture specific information
    + */
    +void task_arch_status(struct seq_file *m, struct task_struct *task)
    +{
    + /*
    + * Report AVX512 state if the processor and build option supported.
    + */
    + if (cpu_feature_enabled(X86_FEATURE_AVX512F))
    + avx512_status(m, task);
    +}
    +#endif /* CONFIG_PROC_PID_ARCH_STATUS */
    --
    2.17.1
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