Re: [PATCH] sched: Folding nohz load accounting more accurate

Charles Wang
2012-06-18T10:07:13Z

On Saturday, June 16, 2012 01:39 AM, Peter Zijlstra wrote:

> Wednesday I ended up with something like the below.. but I haven't
> gotten round to trying Doug's latest testing method, nor did I really
> read the email I'm now replying to.
>
> I think it does something like what Wang described... every time I try
> and write comments related to why it does this I get stuck though.
>
> I ran out of time again for this week, I'll try and prod at it a little
> more next week (and try and catch up with the thread).
>
> In the meantime I thought I might as well post this.. who knows somebody
> might be bored over the weekend, it might actually work, or not :-)
>
> ---
> kernel/sched/core.c | 77 +++++++++++++++++++++++++++++++++++----------------
> 1 file changed, 53 insertions(+), 24 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index ca07ee0..4101a0e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2198,26 +2198,49 @@ calc_load(unsigned long load, unsigned long exp, unsigned long active)
> *
> * When making the ILB scale, we should try to pull this in as well.
> */
> -static atomic_long_t calc_load_tasks_idle;
> +static atomic_long_t calc_load_idle[2];
> +static int calc_load_idx;
> +
> +static inline int calc_load_write_idx(void)
> +{
> + int idx = calc_load_idx;
> +
> + /*
> + * See calc_global_nohz(), if we observe the new index, we also
> + * need to observe the new update time.
> + */
> + smp_rmb();
> +
> + if (!time_before(jiffies, calc_load_update))
> + idx++;
> +
> + return idx & 1;
> +}
> +
> +static inline int calc_load_read_idx(void)
> +{
> + return calc_load_idx & 1;
> +}
>
> void calc_load_account_idle(struct rq *this_rq)
> {
> long delta;
> + int idx;
>
> delta = calc_load_fold_active(this_rq);
> - if (delta)
> - atomic_long_add(delta, &calc_load_tasks_idle);
> + if (delta) {
> + idx = calc_load_write_idx();
> + atomic_long_add(delta, &calc_load_idle[idx]);
> + }
> }
>
> static long calc_load_fold_idle(void)
> {
> + int idx = calc_load_read_idx();
> long delta = 0;
>
> - /*
> - * Its got a race, we don't care...
> - */
> - if (atomic_long_read(&calc_load_tasks_idle))
> - delta = atomic_long_xchg(&calc_load_tasks_idle, 0);
> + if (atomic_long_read(&calc_load_idle[idx]))
> + delta = atomic_long_xchg(&calc_load_idle[idx], 0);
>
> return delta;
> }
> @@ -2313,26 +2336,32 @@ static void calc_global_nohz(void)
> if (delta)
> atomic_long_add(delta, &calc_load_tasks);
>
> - /*
> - * It could be the one fold was all it took, we done!
> - */
> - if (time_before(jiffies, calc_load_update + 10))
> - return;
> + if (!time_before(jiffies, calc_load_update + 10)) {
> + /*
> + * Catch-up, fold however many we are behind still
> + */
> + delta = jiffies - calc_load_update - 10;
> + n = 1 + (delta / LOAD_FREQ);
>
> - /*
> - * Catch-up, fold however many we are behind still
> - */
> - delta = jiffies - calc_load_update - 10;
> - n = 1 + (delta / LOAD_FREQ);
> + active = atomic_long_read(&calc_load_tasks);
> + active = active > 0 ? active * FIXED_1 : 0;
>
> - active = atomic_long_read(&calc_load_tasks);
> - active = active > 0 ? active * FIXED_1 : 0;
> + avenrun[0] = calc_load_n(avenrun[0], EXP_1, active, n);
> + avenrun[1] = calc_load_n(avenrun[1], EXP_5, active, n);
> + avenrun[2] = calc_load_n(avenrun[2], EXP_15, active, n);
>
> - avenrun[0] = calc_load_n(avenrun[0], EXP_1, active, n);
> - avenrun[1] = calc_load_n(avenrun[1], EXP_5, active, n);
> - avenrun[2] = calc_load_n(avenrun[2], EXP_15, active, n);
> + calc_load_update += n * LOAD_FREQ;
> + }
>
> - calc_load_update += n * LOAD_FREQ;
> + /*
> + * Flip the idle index...
> + *
> + * Make sure we first write the new time then flip the index, so that
> + * calc_load_write_idx() will see the new time when it reads the new
> + * index, this avoids a double flip messing things up.
> + */
> + smp_wmb();
> + calc_load_idx++;
> }
> #else
> void calc_load_account_idle(struct rq *this_rq)
>

I tried to identify the start-line precisely, and made the implemention
little more complicated. Using calc_load_update as start-line will cause
it not that accurate, but may work, and keep simple. I will test this on
my environments, and try to port the next patch on this.