psi_account_irqtime() reads irq_time_read() into a per-rq cumulative counter and only bails out when the delta vs. the previously accounted amount is negative. A delta of exactly zero is treated as "do the work": psi_write_begin() is taken, cpu_clock(cpu) is read (which on x86 ends up in native_sched_clock() / rdtsc) and the cgroup ancestor chain is walked to add zero to every group's PSI_IRQ_FULL bucket. The zero-delta case is common in practice -- it fires every time a context switch crosses a PSI group boundary on a CPU that hasn't serviced an interrupt between the two switches. Measured on a 176-thread AMD EPYC 9D64 server running a compute intensive production workload, instrumented with bpftrace over a 30s window (irq_time_read() read directly from the per-CPU cpu_irqtime so that delta == 0 and delta < 0 could be separated): @total 17,229,311 (100.0%) @ret_curr_swapper 7,864,195 ( 45.6%) curr->pid == 0 @ret_samegrp 323,299 ( 1.9%) same cgroup as prev @reached_delta 9,041,817 ( 52.5%) @delta_positive 6,358,192 ( 36.9%) real work @delta_zero 2,683,625 ( 15.6%) work wasted (this patch) @delta_negative (0) ( 0.0%) monotonic clock So 15.6 % of all psi_account_irqtime() calls - and 29.7 % of the calls that get past the early returns - hit the delta == 0 case; delta < 0 did not occur once in the 30 s window. Under the current code each of those ~89 k calls per second performs the full seqcount write + cpu_clock() read + cgroup-chain walk just to add 0 to every group's PSI_IRQ_FULL counter. Extend the early-return to also cover delta == 0. rq->psi_irq_time does not need updating in that case (it would store the same value back) and no PSI bucket would change. The existing behaviour for delta > 0 is untouched. Signed-off-by: Usama Arif --- kernel/sched/psi.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c index d9c9d9480a45..848955f8893d 100644 --- a/kernel/sched/psi.c +++ b/kernel/sched/psi.c @@ -1023,7 +1023,7 @@ void psi_account_irqtime(struct rq *rq, struct task_struct *curr, struct task_st irq = irq_time_read(cpu); delta = (s64)(irq - rq->psi_irq_time); - if (delta < 0) + if (delta <= 0) return; rq->psi_irq_time = irq; -- 2.53.0-Meta