On Thu, Sep 05, 2019 at 12:46:37 +0100, Valentin Schneider wrote...
> On 05/09/2019 12:18, Patrick Bellasi wrote: >>> There's a few things wrong there; I really feel that if we call it nice, >>> it should be like nice. Otherwise we should call it latency-bias and not >>> have the association with nice to confuse people. >>> >>> Secondly; the default should be in the middle of the range. Naturally >>> this would be a signed range like nice [-(x+1),x] for some x. but if you >>> want [0,1024], then the default really should be 512, but personally I >>> like 0 better as a default, in which case we need negative numbers. >>> >>> This is important because we want to be able to bias towards less >>> importance to (tail) latency as well as more importantance to (tail) >>> latency. >>> >>> Specifically, Oracle wants to sacrifice (some) latency for throughput. >>> Facebook OTOH seems to want to sacrifice (some) throughput for latency. >> >> Right, we have this dualism to deal with and current mainline behaviour >> is somehow in the middle. >> >> BTW, the FB requirement is the same we have in Android. >> We want some CFS tasks to have very small latency and a low chance >> to be preempted by the wake-up of less-important "background" tasks. >> >> I'm not totally against the usage of a signed range, but I'm thinking >> that since we are introducing a new (non POSIX) concept we can get the >> chance to make it more human friendly. >> >> Give the two extremes above, would not be much simpler and intuitive to >> have 0 implementing the FB/Android (no latency) case and 1024 the >> (max latency) Oracle case? >> > > For something like latency-<whatever>, I don't see the point of having > such a wide range. The nice range is probably more than enough - and before > even bothering about the range, we should probably agree on what the range > should represent. > > If it's niceness, I read it as: positive latency-nice value means we're > nice to latency, means we reduce it. So the further up you go, the more you > restrict your wakeup scan. I think it's quite easy to map that into the > code: current behaviour at 0, with a decreasing scan mask size as we go > towards +19. I don't think anyone needs 512 steps to tune this. > > I don't know what logic we'd follow for negative values though. Maybe > latency-nice -20 means always going through the slowpath, but what of the > intermediate values? Yep, I think so fare we are all converging towards the idea to use the a signed range. Regarding the range itself, yes: 1024 looks very oversized, but +-20 is still something which leave room for a bit of flexibility and it also better matches the idea that we don't want to "enumerate behaviours" but just expose a knob. To map certain "bias" we could benefit from a slightly larger range. > AFAICT this RFC only looks at wakeups, but I guess latency-nice can be For the wakeup path there is also the TurboSched proposal by Parth: Message-ID: <20190725070857.6639-1-pa...@linux.ibm.com> https://lore.kernel.org/lkml/20190725070857.6639-1-pa...@linux.ibm.com/ we should keep in mind. > applied elsewhere (e.g. load-balance, something like task_hot() and its > use of sysctl_sched_migration_cost). For LB can you come up with some better description of what usages you see could benefit from a "per task" or "per task-group" latency niceness? Best, Patrick -- #include <best/regards.h> Patrick Bellasi
