Messages in this thread | | | From | Benjamin Segall <> | Subject | Re: [RFC PATCH] sched/fair: Interleave cfs bandwidth timers for improved single thread performance at low utilization | Date | Tue, 21 Feb 2023 13:43:27 -0800 |
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shrikanth hegde <sshegde@linux.vnet.ibm.com> writes:
> On 2/20/23 11:08 PM, Peter Zijlstra wrote: >> On Tue, Feb 14, 2023 at 08:54:09PM +0530, shrikanth hegde wrote: >> >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >>> index ff4dbbae3b10..7b69c329e05d 100644 >>> --- a/kernel/sched/fair.c >>> +++ b/kernel/sched/fair.c >>> @@ -5939,14 +5939,25 @@ static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq) >>> >>> void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b) >>> { >>> - lockdep_assert_held(&cfs_b->lock); >>> + struct hrtimer *period_timer = &cfs_b->period_timer; >>> + s64 incr = ktime_to_ns(cfs_b->period) / 10; >>> + ktime_t delta; >>> + u64 orun = 1; >>> >>> + lockdep_assert_held(&cfs_b->lock); >>> if (cfs_b->period_active) >>> return; >>> >>> cfs_b->period_active = 1; >>> - hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period); >>> - hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED); >>> + delta = ktime_sub(period_timer->base->get_time(), >>> + hrtimer_get_expires(period_timer)); >>> + if (unlikely(delta >= cfs_b->period)) { >>> + orun = ktime_divns(delta, incr); >>> + hrtimer_add_expires_ns(period_timer, incr * orun); >>> + } >>> + >>> + hrtimer_forward_now(period_timer, cfs_b->period); >>> + hrtimer_start_expires(period_timer, HRTIMER_MODE_ABS_PINNED); >>> } >> >> What kind of mad hackery is this? Why can't you do the sane thing and >> initialize the timer at !0 in init_cfs_bandwidth(), then any of the >> forwards will stay in period -- as they should. >> >> Please, go re-read Thomas's email. > > Thank you Peter for taking a look and review. > we can scrap this implementation and switch to the one you suggested. > All we need is to initialize the offset. > > Only reason was the way i had implemented. This implementation couldn't be > fit into init_cfs_bandwidth as timers would align if the cgroups are > created together and continue to align forever. > >> >> *completely* untested... >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index 7c46485d65d7..4d6ea76096dc 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -5915,6 +5915,7 @@ void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b) >> >> INIT_LIST_HEAD(&cfs_b->throttled_cfs_rq); >> hrtimer_init(&cfs_b->period_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINNED); >> + cfs_b->period_timer.node.expires = get_random_u32_below(cfs_b->period); > > This approach/implementation is better as the random function provides uniform > distribution. Had to modify this a bit to make it work. Along with setting > setting node.expires, we need to set _softexpires as well. Which is what > hrtimer_set_expires does. > > Here are the similar numbers again. > 8 Core system with SMT=8. Total of 64 CPU > Workload: stress-ng --cpu=32 --cpu-ops=50000 > > 6.2-rc6 | with patch > 8Core 1CG power 2CG power | 1CG power 2CG power > 27.5 80.6 40 90 | 27.3 82 32.3 104 > 27.5 81 40.2 91 | 27.5 81 38.7 96 > 27.7 80 40.1 89 | 27.6 80 29.7 115 > 27.7 80.1 40.3 94 | 27.6 80 31.5 105 > > Will collect some more benchmarks numbers w.r.t to performance. > > >> cfs_b->period_timer.function = sched_cfs_period_timer; >> hrtimer_init(&cfs_b->slack_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL); >> cfs_b->slack_timer.function = sched_cfs_slack_timer; > > This below patch worked. > Does the below patch look okay? shall i send the [PATCH V1] with this > change?
Yeah, this design makes way more sense.
> > Question. > Should we skip this adding the offset for root_task_group?
The value should never come up, so it's just a question of if it's fine to call get_random_* in early contexts, which I don't know offhand.
> > > --- > kernel/sched/fair.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index ff4dbbae3b10..6448533178af 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -5923,6 +5923,9 @@ void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b) > INIT_LIST_HEAD(&cfs_b->throttled_cfs_rq); > hrtimer_init(&cfs_b->period_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINNED); > cfs_b->period_timer.function = sched_cfs_period_timer; > + /* Add a random offset so that timers interleave */ > + hrtimer_set_expires(&cfs_b->period_timer, get_random_u32_below(cfs_b->period)); > + > hrtimer_init(&cfs_b->slack_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL); > cfs_b->slack_timer.function = sched_cfs_slack_timer; > cfs_b->slack_started = false;
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