On Mon, Oct 19, 2015 at 03:16:18PM +0200, Peter Zijlstra wrote: > On Wed, Oct 14, 2015 at 06:47:35PM +0900, byungchul.p...@lge.com wrote: > > > > cpu_load(n) = (1 - 1/s) * cpu_load(n-1) + (1/s) * L > > So I've been taught to use subscripts, not arguments, for progressive > values of the same thing. However I can see the recursive nature of you > definition so I can well imagine people having different views on it. > > > , where n = the current tick - 1, s = scale > > > > = A * cpu_load(n-1) + B > > , where A = 1 - 1/s, B = (1/s) * L > > > > = A * (A * cpu_load(n-2) + B) + B > > > > = A * (A * (A * cpu_load(n-3) + B) + B) + B > > > > = A^3 * cpu_load(n-3) + A^2 * B + A * B + B > > > > = A^i * cpu_load(n-i) + (A^(i-1) + A^(i-2) + ... + 1) * B > > , where i = pending_updates - 1 > > You missed an opportunity here, if you take i==n you avoid the need for > i entirely.
i don't think so. as i said, _n_ is the current tick -1 and _i_ is pending_updates - 1. we cannot take i == n, but should keep (n-i). > > > = A^i * cpu_load(n-i) + B * (A^i - 1) / (A - 1) > > , by geometric series formula for sum > > That's wrong; the limited geometric series expands to: NO, that's not wrong. it doesn't matter at all. a * (1 - r^n) / (1 - r) = a * (-1)(r^n - 1) / (-1)(r - 1) = a * (r^n - 1) / (r - 1) i mean these two are exactly same. > > a * (1 - r^n) / (1 - r) but i think this is also good one. > > Which would give: A^i * cpu_load(n-i) + B * (1 - A^i) / (1 - A) > > > = (1 - 1/s)^i * (cpu_load(n-i) - L) + L > > , by extending A and B > > This appears to be correct however, I think your above mistake must have > been one copying. > > I've rewritten the things a little; does this look good to you? however, your expressions and descriptions below look better than me, except some logical errors. could you keep my logical flow unchagned? thanks anyway, byungchul > > --- > Subject: sched: make __update_cpu_load() handle active tickless case > From: Byungchul Park <byungchul.p...@lge.com> > Date: Wed, 14 Oct 2015 18:47:35 +0900 > > XXX write new changelog... > > Cc: mi...@kernel.org > Signed-off-by: Byungchul Park <byungchul.p...@lge.com> > Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> > Link: > http://lkml.kernel.org/r/1444816056-11886-2-git-send-email-byungchul.p...@lge.com > --- > kernel/sched/fair.c | 49 +++++++++++++++++++++++++++++++++++++++++-------- > 1 file changed, 41 insertions(+), 8 deletions(-) > > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -4298,14 +4298,46 @@ decay_load_missed(unsigned long load, un > return load; > } > > -/* > +/** > + * __update_cpu_load - update the rq->cpu_load[] statistics > + * @this_rq: The rq to update statistics for > + * @this_load: The current load > + * @pending_updates: The number of missed updates > + * @active: !0 for NOHZ_FULL > + * > * Update rq->cpu_load[] statistics. This function is usually called every > - * scheduler tick (TICK_NSEC). With tickless idle this will not be called > - * every tick. We fix it up based on jiffies. > + * scheduler tick (TICK_NSEC). > + * > + * This function computes a decaying average: > + * > + * load[i]' = (1 - 1/2^i) * load[i] + (1/2^i) * load > + * > + * Because of NOHZ it might not get called on every tick which gives need for > + * the @pending_updates argument. > + * > + * load[i]_n = (1 - 1/2^i) * load[i]_n-1 + (1/2^i) * load_n-1 > + * = A * load[i]_n-1 + B ; A := (1 - 1/2^i), B := (1/2^i) * load > + * = A * (A * load[i]_n-2 + B) + B > + * = A * (A * (A * load[i]_n-3 + B) + B) + B > + * = A^3 * load[i]_n-3 + (A^2 + A + 1) * B > + * = A^n * load[i]_0 + (A^(n-1) + A^(n-2) + ... + 1) * B > + * = A^n * load[i]_0 + ((1 - A^n) / (1 - A)) * B > + * = (1 - 1/2^i)^n * (load[i]_0 - load) + load > + * > + * In the above we've assumed load_n := load, which is true for NOHZ_FULL as > + * any change in load would have resulted in the tick being turned back on. > + * > + * For regular NOHZ, this reduces to: > + * > + * load[i]_n = (1 - 1/2^i)^n * load[i]_0 > + * > + * see decay_load_misses(). For NOHZ_FULL we get to subtract and add the > extra > + * term. See the @active paramter. > */ > static void __update_cpu_load(struct rq *this_rq, unsigned long this_load, > - unsigned long pending_updates) > + unsigned long pending_updates, int active) > { > + unsigned long tickless_load = active ? this_rq->cpu_load[0] : 0; > int i, scale; > > this_rq->nr_load_updates++; > @@ -4317,8 +4349,9 @@ static void __update_cpu_load(struct rq > > /* scale is effectively 1 << i now, and >> i divides by scale */ > > - old_load = this_rq->cpu_load[i]; > + old_load = this_rq->cpu_load[i] - tickless_load; > old_load = decay_load_missed(old_load, pending_updates - 1, i); > + old_load += tickless_load; > new_load = this_load; > /* > * Round up the averaging division if load is increasing. This > @@ -4373,7 +4406,7 @@ static void update_idle_cpu_load(struct > pending_updates = curr_jiffies - this_rq->last_load_update_tick; > this_rq->last_load_update_tick = curr_jiffies; > > - __update_cpu_load(this_rq, load, pending_updates); > + __update_cpu_load(this_rq, load, pending_updates, 0); > } > > /* > @@ -4396,7 +4429,7 @@ void update_cpu_load_nohz(void) > * We were idle, this means load 0, the current load might be > * !0 due to remote wakeups and the sort. > */ > - __update_cpu_load(this_rq, 0, pending_updates); > + __update_cpu_load(this_rq, 0, pending_updates, 0); > } > raw_spin_unlock(&this_rq->lock); > } > @@ -4412,7 +4445,7 @@ void update_cpu_load_active(struct rq *t > * See the mess around update_idle_cpu_load() / update_cpu_load_nohz(). > */ > this_rq->last_load_update_tick = jiffies; > - __update_cpu_load(this_rq, load, 1); > + __update_cpu_load(this_rq, load, 1, 1); > } > > /* > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
