Re: [PATCH 1/1] sched/psi: skip irqtime accounting when no new irq time has elapsed

[Shakeel Butt]

On Wed, Jun 17, 2026 at 10:50:06AM -0700, Usama Arif wrote:
> 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 <usama.arif@xxxxxxxxx>

Reviewed-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>