Re: [PATCH v2] powerpc/entry: Fix double accounting of user time on interrupt entry

Aboorva Devarajan [off-list ref] writes:

Since the switch to generic entry, an interrupt from user mode
accounts user time twice: once in arch_interrupt_enter_prepare()
and again in arch_enter_from_user_mode(), which irqentry_enter()
invokes for the same interrupt:

	arch_interrupt_enter_prepare()
	  account_cpu_user_entry()		/* first */
	irqentry_enter()
	  arch_enter_from_user_mode()
	    account_cpu_user_entry()		/* second */

The second call charges the same interval again, because
account_cpu_user_entry() accumulates the time spent in user mode
since the last return to user space.

The two calls come from the GENERIC_ENTRY preparation series,
where each step was a no-op on its own. Commit 09a9d3a8499d
("powerpc: introduce arch_enter_from_user_mode") added the hook
with the user-time accounting in it, but nothing called it yet.
Commit 893082ac769b ("powerpc: Prepare for IRQ entry exit")
copied interrupt_enter_prepare() verbatim into entry-common.h as
arch_interrupt_enter_prepare(); that copy was equally unused, as
handlers still called interrupt_enter_prepare().

Commit bee25f97ad24 ("powerpc: Enable GENERIC_ENTRY feature")
made both live. On the syscall side it did the full conversion:
system_call_exception() now accounts once through the hook via
syscall_enter_from_user_mode(), rather than calling
account_cpu_user_entry() directly. On the interrupt side it
switched the handler macros to arch_interrupt_enter_prepare()
followed by irqentry_enter(), which also runs the hook, but the
accounting in arch_interrupt_enter_prepare() was not removed to
match. The double accounting starts with that commit.

With CONFIG_VIRT_CPU_ACCOUNTING_NATIVE=y this roughly doubles the
reported user time of any workload that takes interrupts. The
other accounting modes compile account_cpu_user_entry() to an
empty stub, so they are not affected.

Remove the accounting from arch_interrupt_enter_prepare() and rely
on arch_enter_from_user_mode(), which already runs for both
syscalls and interrupts. The duplicate account_stolen_time() call
is removed the same way.

On a pseries LPAR a busy loop reports 6s user time in 3s elapsed
(~210% CPU) before the fix, and 3s (~105% CPU) after it:

  $ python3 -c 'while True: pass' &
  $ sleep 3; ps -p $! -o etime,time,pcpu

            ELAPSED     TIME  %CPU
  Before      00:03 00:00:06   210
  After       00:03 00:00:03   105

A 50% load reports ~70% usr / 30% idle before the fix, and
~49% usr / 51% idle after it:

  $ taskset -c 6 stress-ng --cpu 1 --cpu-load 50 &
  $ mpstat -P 6 1

            CPU    %usr   %idle
  Before      6   69.74   30.26
  After       6   48.51   50.50

Fixes: bee25f97ad24 ("powerpc: Enable GENERIC_ENTRY feature")
Reviewed-by: Mukesh Kumar Chaurasiya (IBM) <redacted>
Signed-off-by: Aboorva Devarajan <redacted>
---

Looks good. Thanks for the descritive commit msg!

Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>