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    Subject[PATCH 5.15 237/917] fs/proc/uptime.c: Fix idle time reporting in /proc/uptime
    Date
    From: Josh Don <joshdon@google.com>

    [ Upstream commit a130e8fbc7de796eb6e680724d87f4737a26d0ac ]

    /proc/uptime reports idle time by reading the CPUTIME_IDLE field from
    the per-cpu kcpustats. However, on NO_HZ systems, idle time is not
    continually updated on idle cpus, leading this value to appear
    incorrectly small.

    /proc/stat performs an accounting update when reading idle time; we
    can use the same approach for uptime.

    With this patch, /proc/stat and /proc/uptime now agree on idle time.
    Additionally, the following shows idle time tick up consistently on an
    idle machine:

    (while true; do cat /proc/uptime; sleep 1; done) | awk '{print $2-prev; prev=$2}'

    Reported-by: Luigi Rizzo <lrizzo@google.com>
    Signed-off-by: Josh Don <joshdon@google.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Eric Dumazet <edumazet@google.com>
    Link: https://lkml.kernel.org/r/20210827165438.3280779-1-joshdon@google.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    fs/proc/stat.c | 4 ++--
    fs/proc/uptime.c | 14 +++++++++-----
    include/linux/kernel_stat.h | 1 +
    3 files changed, 12 insertions(+), 7 deletions(-)

    diff --git a/fs/proc/stat.c b/fs/proc/stat.c
    index 6561a06ef9059..4fb8729a68d4e 100644
    --- a/fs/proc/stat.c
    +++ b/fs/proc/stat.c
    @@ -24,7 +24,7 @@

    #ifdef arch_idle_time

    -static u64 get_idle_time(struct kernel_cpustat *kcs, int cpu)
    +u64 get_idle_time(struct kernel_cpustat *kcs, int cpu)
    {
    u64 idle;

    @@ -46,7 +46,7 @@ static u64 get_iowait_time(struct kernel_cpustat *kcs, int cpu)

    #else

    -static u64 get_idle_time(struct kernel_cpustat *kcs, int cpu)
    +u64 get_idle_time(struct kernel_cpustat *kcs, int cpu)
    {
    u64 idle, idle_usecs = -1ULL;

    diff --git a/fs/proc/uptime.c b/fs/proc/uptime.c
    index 5a1b228964fb7..deb99bc9b7e6b 100644
    --- a/fs/proc/uptime.c
    +++ b/fs/proc/uptime.c
    @@ -12,18 +12,22 @@ static int uptime_proc_show(struct seq_file *m, void *v)
    {
    struct timespec64 uptime;
    struct timespec64 idle;
    - u64 nsec;
    + u64 idle_nsec;
    u32 rem;
    int i;

    - nsec = 0;
    - for_each_possible_cpu(i)
    - nsec += (__force u64) kcpustat_cpu(i).cpustat[CPUTIME_IDLE];
    + idle_nsec = 0;
    + for_each_possible_cpu(i) {
    + struct kernel_cpustat kcs;
    +
    + kcpustat_cpu_fetch(&kcs, i);
    + idle_nsec += get_idle_time(&kcs, i);
    + }

    ktime_get_boottime_ts64(&uptime);
    timens_add_boottime(&uptime);

    - idle.tv_sec = div_u64_rem(nsec, NSEC_PER_SEC, &rem);
    + idle.tv_sec = div_u64_rem(idle_nsec, NSEC_PER_SEC, &rem);
    idle.tv_nsec = rem;
    seq_printf(m, "%lu.%02lu %lu.%02lu\n",
    (unsigned long) uptime.tv_sec,
    diff --git a/include/linux/kernel_stat.h b/include/linux/kernel_stat.h
    index 44ae1a7eb9e39..69ae6b2784645 100644
    --- a/include/linux/kernel_stat.h
    +++ b/include/linux/kernel_stat.h
    @@ -102,6 +102,7 @@ extern void account_system_index_time(struct task_struct *, u64,
    enum cpu_usage_stat);
    extern void account_steal_time(u64);
    extern void account_idle_time(u64);
    +extern u64 get_idle_time(struct kernel_cpustat *kcs, int cpu);

    #ifdef CONFIG_VIRT_CPU_ACCOUNTING_NATIVE
    static inline void account_process_tick(struct task_struct *tsk, int user)
    --
    2.33.0


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