[PATCH 0/1] alpha/perf: Enable duration-based sampling throttling

[Magnus Lindholm <linmag7@gmail.com>]

Hi,

While testing Matt's v7 ProfileMe series [1] on my EV67 AlphaServer ES40,
I found that short-period sampling could make the machine unresponsive.
Once serial logging was enabled, the console showed RCU grace-period
kthread starvation reports.

Count-based throttling was working: at HZ=1024 and the default limit of
100000 samples/s, the observed cadence was approximately 98 interrupts
per throttle cycle. However, Alpha never called perf_sample_event_took(),
so perf could not reduce that limit based on interrupt handling cost.
Setting perf_cpu_time_max_percent to 25 did not provide that feedback.

Keeping period 17 but lowering the limit to 1024 or 4096 samples/s made
the tests pass while still exercising repeated stop/park/restart cycles.
Adding duration feedback also made the original high-rate test pass,
without changing those counter-state operations. The minimal fix measured
average handler costs of 24-26 us against a 2.5 us budget and reduced the
limit to 7168-8192 samples/s.

Reproducer
==========

The reproducer requires Matt's ProfileMe kernel series and matching perf
tools. On the ES40 I used the following one-second workload, built
natively as "busy":

  #include <stdint.h>
  #include <time.h>

  static double now(void)
  {
          struct timespec t;

          clock_gettime(CLOCK_MONOTONIC, &t);
          return t.tv_sec + t.tv_nsec * 1e-9;
  }

  int main(void)
  {
          uint64_t x = 1;
          double end = now() + 1;

          do {
                  for (int i = 0; i < 1000; i++)
                          asm volatile("mulq %0,3,%0" : "+r" (x));
          } while (now() < end);
          return 0;
  }

  cc -O2 -o busy busy.c

As root, with serial capture running:

  dmesg -n 7
  sysctl -w kernel.perf_event_max_sample_rate=100000
  sysctl -w kernel.perf_cpu_time_max_percent=25

The period-10007 control completed successfully:

  taskset -c 0 perf record -B -R \
      -e cpu/instructions,profileme,randomize/ -c 10007 \
      -o control.data -- ./busy

Changing only the period reproduced the failure:

  taskset -c 0 perf record -B -R \
      -e cpu/instructions,profileme,randomize/ -c 17 \
      -o period17.data -- ./busy

The unpatched runs required a manual reset; a userspace timeout did not
reliably recover the system. The commands above are the essential
recording commands from the controlled tests, which also used a timeout
supervisor and saved logs on another machine.

Validation and submission scope
===============================

The minimal fix, without diagnostic instrumentation, passed three
repeats of the high-rate reproducer and the two low-rate controls.
Broader testing covered 21 cases on all three CPUs: randomized ProfileMe
instruction sampling at periods 16, 17 and 31, ProfileMe and Aggregate
cycle sampling, and concurrent ProfileMe and mixed-mode workloads.
All completed, with 1352488 samples, no recorded losses and no new RCU
stalls or kernel faults. This was bounded stress testing, not a long soak.

The missing feedback predates ProfileMe. This patch is therefore based
on mainline and can be reviewed and applied independently of Matt's
series. I cross-built vmlinux, the Alpha boot image and modules on
mainline d48723711bae with the ES40 configuration and GCC 15.0.1. The
only build warning was the existing saved_config COMMON-symbol warning.

The hardware results above used Matt's v7 series with this fix applied;
the mainline-only build has not been boot-tested. RPCC supplies a short,
same-CPU interval because Alpha's tick-based sched_clock() is too coarse
for this measurement. Counter programming and event handling are unchanged.

[1] https://lore.kernel.org/linux-alpha/20261105-alpha-profileme-v7-0-c474e892abb0@gmail.com/

Thanks,
Magnus

Magnus Lindholm (1):
  alpha/perf: Account for time spent handling PMU interrupts

 arch/alpha/kernel/perf_event.c | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)


base-commit: d48723711bae9b8cdc56719c448ba42fef28cbee