Messages in this thread | | | Subject | Re: sched: Move SCHED_DEBUG sysctl to debugfs | From | Christian Borntraeger <> | Date | Wed, 28 Apr 2021 10:58:24 +0200 |
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On 28.04.21 10:54, Christian Borntraeger wrote: > > > On 28.04.21 10:46, Peter Zijlstra wrote: >> On Tue, Apr 27, 2021 at 04:59:25PM +0200, Christian Borntraeger wrote: >>> Peter, >>> >>> I just realized that we moved away sysctl tunabled to debugfs in next. >>> We have seen several cases where it was benefitial to set >>> sched_migration_cost_ns to a lower value. For example with KVM I can >>> easily get 50% more transactions with 50000 instead of 500000. >>> Until now it was possible to use tuned or /etc/sysctl.conf to set >>> these things permanently. >>> >>> Given that some people do not want to have debugfs mounted all the time >>> I would consider this a regression. The sysctl tunable was always >>> available. >>> >>> I am ok with the "informational" things being in debugfs, but not >>> the tunables. So how do we proceed here? >> >> It's all SCHED_DEBUG; IOW you're relying on DEBUG infrastructure for >> production performance, and that's your fail. > > No its not. sched_migration_cost_ns was NEVER protected by CONFIG_SCHED_DEBUG. > It was available on all kernels with CONFIG_SMP.
Have to correct myself, it was SCHED_DEBUG in 3.0. > >> >> I very explicitly do not care to support people that poke random values >> into those 'tunables'. If people wants to do that, they get to keep any >> and all pieces. >> >> The right thing to do here is to analyze the situation and determine why >> migration_cost needs changing; is that an architectural thing, does s390 >> benefit from less sticky tasks due to its cache setup (the book caches >> could be absorbing some of the penalties here for example). Or is it >> something that's workload related, does KVM intrinsically not care about >> migrating so much, or is it something else. >> >> Basically, you get to figure out what the actual performance issue is, >> and then we can look at what to do about it so that everyone benefits, >> and not grow some random tweaks on the interweb that might or might not >> actually work for someone else. > > Yes, I agree. We have seen the effect of this value recently and we want > look into that. Still that does not change the fact that you are removing > an interface that was there for ages.
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