Messages in this thread | | | Date | Wed, 11 Oct 2023 13:39:12 +0300 | From | Andy Shevchenko <> | Subject | Re: [PATCH RFC] cpumask: Randomly distribute the tasks within affinity mask |
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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 > > Analysis: > There are percpu bounded timer interrupt/rcu threads activities > going on every few microseconds on housekeeping cpus, exercising > find_lowest_rq() -> cpumask_any_and_distribute()/cpumask_any_distribute() > So, per cpu variable distribute_cpu_mask_prev for housekeeping cpus > keep on getting set to housekeeping cpus. Bash/docker processes > are sharing same per cpu variable as they run on housekeeping cpus. > Thus intersection of clobbered distribute_cpu_mask_prev and > new mask(isolcpus) return always first cpu within the new mask(isolcpus) > in accordance to the logic mentioned in commits above. > > Fix the issue by using random cores out of the applicable CPU set > instead of relying on distribute_cpu_mask_prev.
> Fixes: 46a87b3851f0 ("sched/core: Distribute tasks within affinity masks") > Fixes: 14e292f8d453 ("sched,rt: Use cpumask_any*_distribute()")
>
Blank lines are not allowed in the tag block.
> Signed-off-by: Ankit Jain <ankitja@vmware.com>
...
> +/** > + * Returns an arbitrary cpu within srcp. > + * > + * Iterated calls using the same srcp will be randomly distributed > + */
This is invalid. Always run
scripts/kernel-doc -v -none -Wall ...
against the file of interest and fix all warnings and errors reported.
-- With Best Regards, Andy Shevchenko
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