On 18/09/2019 15:18, Patrick Bellasi wrote: >> 1. Name: What should be the name for such attr for all the possible usecases? >> ============= >> Latency nice is the proposed name as of now where the lower value indicates >> that the task doesn't care much for the latency > > If by "lower value" you mean -19 (in the proposed [-20,19] range), then > I think the meaning should be the opposite. > > A -19 latency-nice task is a task which is not willing to give up > latency. For those tasks for example we want to reduce the wake-up > latency at maximum. > > This will keep its semantic aligned to that of process niceness values > which range from -20 (most favourable to the process) to 19 (least > favourable to the process). >
I don't want to start a bikeshedding session here, but I agree with Parth on the interpretation of the values. I've always read niceness values as -20 (least nice to the system / other processes) +19 (most nice to the system / other processes) So following this trend I'd see for latency-nice: -20 (least nice to latency, i.e. sacrifice latency for throughput) +19 (most nice to latency, i.e. sacrifice throughput for latency) However... >> But there seems to be a bit of confusion on whether we want biasing as well >> (latency-biased) or something similar, in which case "latency-nice" may >> confuse the end-user. > > AFAIU PeterZ point was "just" that if we call it "-nice" it has to > behave as "nice values" to avoid confusions to users. But, if we come up > with a different naming maybe we will have more freedom. > ...just getting rid of the "-nice" would leave us free not to have to interpret the values as "nice to / not nice to" :) > Personally, I like both "latency-nice" or "latency-tolerant", where: > > - latency-nice: > should have a better understanding based on pre-existing concepts > > - latency-tolerant: > decouples a bit its meaning from the niceness thus giving maybe a bit > more freedom in its complete definition and perhaps avoid any > possible interpretation confusion like the one I commented above. > > Fun fact: there was also the latency-nasty proposal from PaulMK :) > [...] > > $> Wakeup path tunings > ========================== > > Some additional possible use-cases was already discussed in [3]: > > - dynamically tune the policy of a task among SCHED_{OTHER,BATCH,IDLE} > depending on crossing certain pre-configured threshold of latency > niceness. > > - dynamically bias the vruntime updates we do in place_entity() > depending on the actual latency niceness of a task. > > PeterZ thinks this is dangerous but that we can "(carefully) fumble a > bit there." > > - bias the decisions we take in check_preempt_tick() still depending > on a relative comparison of the current and wakeup task latency > niceness values. Aren't we missing the point about tweaking the sched domain scans (which AFAIR was the original point for latency-nice)? Something like default value is current behaviour and - Being less latency-sensitive means increasing the scans (e.g. trending towards only going through the slow wakeup-path at the extreme setting) - Being more latency-sensitive means reducing the scans (e.g. trending towards a fraction of the domain scanned in the fast-path at the extreme setting). > $> Load balance tuning ====================== Already mentioned these in [4]: - Increase (reduce) nr_balance_failed threshold when trying to active balance a latency-sensitive (non-latency-sensitive) task. - Increase (decrease) sched_migration_cost factor in task_hot() for latency-sensitive (non-latency-sensitive) tasks. >> References: >> =========== >> [1]. https://lkml.org/lkml/2019/8/30/829 >> [2]. https://lkml.org/lkml/2019/7/25/296 > > [3]. Message-ID: <20190905114709.gm2...@hirez.programming.kicks-ass.net> > > https://lore.kernel.org/lkml/20190905114709.gm2...@hirez.programming.kicks-ass.net/ > [4]: https://lkml.kernel.org/r/3d3306e4-3a78-5322-df69-7665cf01c...@arm.com > > Best, > Patrick >