Messages in this thread | | | Subject | Re: [PATCH v2 4/4] sched/fair: Prevent active LB from preempting higher sched classes | From | Valentin Schneider <> | Date | Fri, 30 Aug 2019 16:44:05 +0100 |
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On 29/08/2019 15:19, Vincent Guittot wrote: [...] >> Right, if we end up kicking the cpu_stopper this can still happen (since >> we drop the lock). Thing is, you can't detect it on the cpu_stopper side, >> since the currently running is obviously not going to be CFS (and it's >> too late anyway, we already preempted whatever was running there). Though >> I should probably change the name of the patch to reflect that it's not a >> 100% cure. >> >> I tweaked the nr_running check of the cpu_stop callback in patch 3/4 to try >> to bail out early, but AFAICT that's the best we can do without big changes >> elsewhere. >> >> If we wanted to prevent those preemptions at all cost, I suppose we'd want > > I'm not sure that it's worth the effort and the complexity >
My point exactly :)
[...] >> I had this initially but convinced myself out of it: since we hold no >> lock in need_active_balance(), the information we get on the current task >> (and, arguably, on the h_nr_running) is too volatile to be of any use. > > But since the lock is released anyway, everything will always be too > volatile in this case.
We do release the lock if we go kick the cpu_stopper, but can nevertheless make a decision with the most up to date information. I'd say it's for similar reasons that we check busiest->curr->cpus_ptr right before kicking the cpu_stopper rather than in need_active_balance().
The majority of the checks in need_active_balance() (all but one) depend on env/sd stats which aren't volatile.
>> >> I do believe those checks have their place in active_load_balance()'s >> critical section, as that's the most accurate we're going to get. On the >> plus side, if we *do* detect the remote rq's current task isn't CFS, we >> can run detach_one_task() locally, which is an improvement IMO. > > This add complexity in the code by adding another path to detach attach task(s).
Note that it's not a new detach/attach per se, rather it's about doing it in active_load_balance() rather than active_load_balance_cpu_stop() in some cases.
> We could simply bail out and wait the next load balance (which is > already the case sometime) or if you really want to detach a task jump > back to more_balance >
A simple bail-out is what I had in v1, but following Qais' comments I figured I could add the detach_one_tasks().
Jumping back to more_balance is quite different than doing a detach_one_task().
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