Messages in this thread | | | Date | Wed, 11 Oct 2023 13:46:42 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH RFC] cpumask: Randomly distribute the tasks within affinity mask |
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On Wed, Oct 11, 2023 at 12:53:29PM +0200, Peter Zijlstra wrote: > On Wed, Oct 11, 2023 at 12:49:25PM +0530, Ankit Jain wrote: > > commit 46a87b3851f0 ("sched/core: Distribute tasks within affinity masks") > > and commit 14e292f8d453 ("sched,rt: Use cpumask_any*_distribute()") > > introduced the logic to distribute the tasks at initial wakeup on cpus > > where load balancing works poorly or disabled at all (isolated cpus). > > > > There are cases in which the distribution of tasks > > that are spawned on isolcpus does not happen properly. > > In production deployment, initial wakeup of tasks spawn from > > housekeeping cpus to isolcpus[nohz_full cpu] happens on first cpu > > within isolcpus range instead of distributed across isolcpus. > > > > Usage of distribute_cpu_mask_prev from one processes group, > > will clobber previous value of another or other groups and vice-versa. > > > > When housekeeping cpus spawn multiple child tasks to wakeup on > > isolcpus[nohz_full cpu], using cpusets.cpus/sched_setaffinity(), > > distribution is currently performed based on per-cpu > > distribute_cpu_mask_prev counter. > > At the same time, on housekeeping cpus there are percpu > > bounded timers interrupt/rcu threads and other system/user tasks > > would be running with affinity as housekeeping cpus. In a real-life > > environment, housekeeping cpus are much fewer and are too much loaded. > > So, distribute_cpu_mask_prev value from these tasks impacts > > the offset value for the tasks spawning to wakeup on isolcpus and > > thus most of the tasks end up waking up on first cpu within the > > isolcpus set. > > > > Steps to reproduce: > > Kernel cmdline parameters: > > isolcpus=2-5 skew_tick=1 nohz=on nohz_full=2-5 > > rcu_nocbs=2-5 rcu_nocb_poll idle=poll irqaffinity=0-1 > > > > * pid=$(echo $$) > > * taskset -pc 0 $pid > > * cat loop-normal.c > > int main(void) > > { > > while (1) > > ; > > return 0; > > } > > * gcc -o loop-normal loop-normal.c > > * for i in {1..50}; do ./loop-normal & done > > * pids=$(ps -a | grep loop-normal | cut -d' ' -f5) > > * for i in $pids; do taskset -pc 2-5 $i ; done > > > > Expected output: > > * All 50 “loop-normal” tasks should wake up on cpu2-5 > > equally distributed. > > * ps -eLo cpuid,pid,tid,ppid,cls,psr,cls,cmd | grep "^ [2345]" > > > > Actual output: > > * All 50 “loop-normal” tasks got woken up on cpu2 only > > Your expectation is wrong. Things work as advertised.
That is, isolcpus results in single CPU balance domains and as such we must not distribute -- there is no load balancing.
Ideally we'd reject setting cpumasks with multi bits set on domains like that, but alas, that would break historical behaviour :/
Now, looking at the code, I don't think the current code actually behaves correct in this case :-(, somewhere along the line we should truncate cpu_valid_mask to a single bit. Let me see where the sane place is to do that.
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