Re: [PATCH v2] sched: Warn on long periods of pending need_resched

Peter Zijlstra
2021-03-26T08:59:48Z

On Thu, Mar 25, 2021 at 02:58:52PM -0700, Josh Don wrote:
> > On Wed, Mar 24, 2021 at 01:39:16PM +0000, Mel Gorman wrote:
> > I'm not going to NAK because I do not have hard data that shows they must
> > exist. However, I won't ACK either because I bet a lot of tasty beverages
> > the next time we meet that the following parameters will generate reports
> > if removed.
> >
> > kernel.sched_latency_ns
> > kernel.sched_migration_cost_ns
> > kernel.sched_min_granularity_ns
> > kernel.sched_wakeup_granularity_ns
> >
> > I know they are altered by tuned for different profiles and some people do
> > go the effort to create custom profiles for specific applications. They
> > also show up in "Official Benchmarking" such as SPEC CPU 2017 and
> > some vendors put a *lot* of effort into SPEC CPU results for bragging
> > rights. They show up in technical books and best practice guids for
> > applications. Finally they show up in Google when searching for "tuning
> > sched_foo". I'm not saying that any of these are even accurate or a good
> > idea, just that they show up near the top of the results and they are
> > sufficiently popular that they might as well be an ABI.
>
> +1, these seem like sufficiently well-known scheduler tunables, and
> not really SCHED_DEBUG.

So we've never made any guarantees on their behaviour, nor am I willing
to make any.

In fact, I propose we merge the below along with the debugfs move. Just
to make absolutely sure any 'tuning' is broken.

---
Subject: sched,fair: Alternative sched_slice()
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu Mar 25 13:44:46 CET 2021

The current sched_slice() seems to have issues; there's two possible
things that could be improved:

- the 'nr_running' used for __sched_period() is daft when cgroups are
considered. Using the RQ wide h_nr_running seems like a much more
consistent number.

- (esp) cgroups can slice it real fine (pun intendend), which makes for
easy over-scheduling, ensure min_gran is what the name says.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/sched/fair.c | 15 ++++++++++++++-
kernel/sched/features.h | 3 +++
2 files changed, 17 insertions(+), 1 deletion(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -680,7 +680,16 @@ static u64 __sched_period(unsigned long
*/
static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
- u64 slice = __sched_period(cfs_rq->nr_running + !se->on_rq);
+ unsigned int nr_running = cfs_rq->nr_running;
+ u64 slice;
+
+ if (sched_feat(ALT_PERIOD))
+ nr_running = rq_of(cfs_rq)->cfs.h_nr_running;
+
+ slice = __sched_period(nr_running + !se->on_rq);
+
+ if (sched_feat(BASE_SLICE))
+ slice -= sysctl_sched_min_granularity;

for_each_sched_entity(se) {
struct load_weight *load;
@@ -697,6 +706,10 @@ static u64 sched_slice(struct cfs_rq *cf
}
slice = __calc_delta(slice, se->load.weight, load);
}
+
+ if (sched_feat(BASE_SLICE))
+ slice += sysctl_sched_min_granularity;
+
return slice;
}

--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -90,3 +90,6 @@ SCHED_FEAT(WA_BIAS, true)
*/
SCHED_FEAT(UTIL_EST, true)
SCHED_FEAT(UTIL_EST_FASTUP, true)
+
+SCHED_FEAT(ALT_PERIOD, true)
+SCHED_FEAT(BASE_SLICE, true)