[PATCH v2 00/12] proc/stat: Maintain monotonicity of "intr" and "softirq"

Alexei Lozovsky
2021-09-15T17:59:04Z

Here's a patch set that makes /proc/stat report total interrupt counts
as monotonically increasing values, just like individual counters for
interrupt types and CPUs are.

The total counters are not actually maintained but computed from
individual counters. These counters must all have the same width
for their sum to wrap around in correct and expected manner.

This patch set unifies all counters that are displayed by /proc/stat
and /proc/interrupts to use "unsigned long" (instead of "unsigned int"
values that get summed into u64, which causes problems for userspace
monitoring tools when the individual counters wrap around).

v1 -> v2:

- Added READ_ONCE to all reads of per-CPU counters
- Widened and unified all counters to "unsigned long"
instead of clamping them all to "unsigned int"
- Fixed typos in documentation

Since the scope of changes has expanded, I think these patches will
need to be seen and vetted by more people (get-maintainer.pl agrees)
but for now I'll keep the same CC list as for v1.

Unresolved questions:

- Does READ_ONCE addition has any merit without WRITE_ONCE?

- Some counters turned out to be unaccounted for in the total sum.
Should they be included there? Or are they omitted on purpose?

- I haven't tested this on PowerPC since I don't have the hardware

Alexei Lozovsky (12):
genirq: Use READ_ONCE for IRQ counter reads
genirq: Use unsigned long for IRQ counters
powerpc/irq: Use READ_ONCE for IRQ counter reads
powerpc/irq: Use unsigned long for IRQ counters
powerpc/irq: Use unsigned long for IRQ counter sum
x86/irq: Use READ_ONCE for IRQ counter reads
x86/irq: Use unsigned long for IRQ counters
x86/irq: Use unsigned long for IRQ counters more
x86/irq: Use unsigned long for IRQ counter sum
proc/stat: Use unsigned long for "intr" sum
proc/stat: Use unsigned long for "softirq" sum
docs: proc.rst: stat: Note the interrupt counter wrap-around

Documentation/filesystems/proc.rst | 8 +++
arch/powerpc/include/asm/hardirq.h | 20 ++++----
arch/powerpc/include/asm/paca.h | 2 +-
arch/powerpc/kernel/irq.c | 42 +++++++--------
arch/x86/include/asm/hardirq.h | 26 +++++-----
arch/x86/include/asm/hw_irq.h | 4 +-
arch/x86/include/asm/mce.h | 4 +-
arch/x86/kernel/apic/apic.c | 2 +-
arch/x86/kernel/apic/io_apic.c | 4 +-
arch/x86/kernel/cpu/mce/core.c | 4 +-
arch/x86/kernel/i8259.c | 2 +-
arch/x86/kernel/irq.c | 82 +++++++++++++++---------------
fs/proc/softirqs.c | 2 +-
fs/proc/stat.c | 12 ++---
include/linux/kernel_stat.h | 10 ++--
kernel/rcu/tree.h | 2 +-
kernel/rcu/tree_stall.h | 4 +-
17 files changed, 119 insertions(+), 111 deletions(-)

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2.25.1