Re: [PATCH 1/1] alpha/perf: Account for time spent handling PMU interrupts

[Matt Turner]

On Sat, Nov 7, 2026 at 4:19 AM Magnus Lindholm <linmag7@xxxxxxxxx> wrote:
> Alpha does not report PMU interrupt duration to perf, preventing its
> adaptive throttling from reducing an excessively costly sampling rate.

Nice find, and thanks for the thorough testing write-up. The logic looks
right to me: time_init() runs before early initcalls, and since it zeroes
est_cycle_freq when the HWRPB value is within tolerance, the
"est_cycle_freq ? : hwrpb->cycle_freq" idiom picks the same frequency it
did. A few small things below.

It would be worth adding a sentence on the symptom from the cover letter
(short sampling periods leave the machine unresponsive with RCU
grace-period kthread starvation reports) so the commit stands on its own.

> +#include <linux/math64.h>

On 64-bit, div64_u64() is just a plain division, so this include only
serves that one call. See below for an alternative that drops it.

> +extern unsigned long est_cycle_freq;

This is already declared in arch/alpha/kernel/proto.h. Please
#include "proto.h" instead, like the rest of arch/alpha/kernel does.

> +static unsigned long pmu_cycle_freq __read_mostly;

Since this is only written from an early_initcall, __ro_after_init seems
a better fit.

> +     u32 start = get_cycles();
> +     u32 cycles;
> +
> +     alpha_perf_event_irq_inner(la_ptr, regs);
> +     /* Unsigned subtraction handles a wrap of the low 32-bit counter. */
> +     cycles = (u32)get_cycles() - start;

cycles_t is already a 32-bit unsigned int on Alpha, so the cast isn't
needed. Declaring these as cycles_t would read a bit more naturally.

> +     if (likely(pmu_cycle_freq))
> +             perf_sample_event_took(div64_u64((u64)cycles * NSEC_PER_SEC,
> +                                              pmu_cycle_freq));

Alpha has no integer divide instruction, so this compiles to a __divqu
millicode call on every PMU interrupt. It's small next to the handler
cost you measured, but it's easy to avoid: compute a mult/shift pair
once at init with clocks_calc_mult_shift() and convert with
mul_u64_u32_shr(cycles, mult, shift) here.

> +     /* Match the calibrated frequency selected by time_init(). */
> +     pmu_cycle_freq = est_cycle_freq ? est_cycle_freq : hwrpb->cycle_freq;

Not for this patch, but this is now the second copy of that selection
(show_cpuinfo() has the other). Having time_init() store the frequency
it chose in a global would let both just read it. Fine as a follow-up.

With the proto.h and cast nits fixed (the rest at your discretion),

Reviewed-by <mattst88@xxxxxxxxx>

Thanks,
Matt