lkml.org 
[lkml]   [2019]   [Nov]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 1/6] sched/cputime: Support other fields on kcpustat_field()

* Frederic Weisbecker <frederic@kernel.org> wrote:

> Provide support for user, nice, guest and guest_nice fields through
> kcpustat_field().
>
> Whether we account the delta to a nice or not nice field is decided on
> top of the nice value snapshot taken at the time we call kcpustat_field().
> If the nice value of the task has been changed since the last vtime
> update, we may have inacurrate distribution of the nice VS unnice
> cputime.
>
> However this is considered as a minor issue compared to the proper fix
> that would involve interrupting the target on nice updates, which is
> undesired on nohz_full CPUs.
>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Cc: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Rik van Riel <riel@surriel.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Wanpeng Li <wanpengli@tencent.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> ---
> kernel/sched/cputime.c | 53 +++++++++++++++++++++++++++++++++---------
> 1 file changed, 42 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index e0cd20693ef5..b2cf544e2109 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -912,11 +912,21 @@ void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
> } while (read_seqcount_retry(&vtime->seqcount, seq));
> }
>
> +static u64 kcpustat_user_vtime(struct vtime *vtime)
> +{
> + if (vtime->state == VTIME_USER)
> + return vtime->utime + vtime_delta(vtime);
> + else if (vtime->state == VTIME_GUEST)
> + return vtime->gtime + vtime_delta(vtime);
> + return 0;
> +}
> +
> static int kcpustat_field_vtime(u64 *cpustat,
> - struct vtime *vtime,
> + struct task_struct *tsk,
> enum cpu_usage_stat usage,
> int cpu, u64 *val)
> {
> + struct vtime *vtime = &tsk->vtime;
> unsigned int seq;
> int err;
>
> @@ -946,9 +956,36 @@ static int kcpustat_field_vtime(u64 *cpustat,
>
> *val = cpustat[usage];
>
> - if (vtime->state == VTIME_SYS)
> - *val += vtime->stime + vtime_delta(vtime);
> -
> + /*
> + * Nice VS unnice cputime accounting may be inaccurate if
> + * the nice value has changed since the last vtime update.
> + * But proper fix would involve interrupting target on nice
> + * updates which is a no go on nohz_full.

Well, we actually already interrupt the target in both sys_nice() and
sys_setpriority() etc. syscall variants: we call set_user_nice() which
calls resched_curr() and the task is sent an IPI and runs through a
reschedule.

But ... I do agree that this kind of granularity of nice/non-nice
accounting doesn't really matter in practice: the changing of nice values
is a relatively low frequency operation on most systems.

But nevertheless the comment should probably be updated to reflect this.

Thanks,

Ingo

\
 
 \ /
  Last update: 2019-11-20 12:52    [W:0.059 / U:10.000 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site