linux-kernel - [PATCH v2 12/12] docs: proc.rst: stat: Note the interrupt counter wrap-around

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Date:   Thu, 16 Sep 2021 02:58:48 +0900
From:   Alexei Lozovsky <me@...mmy.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        Christoph Lameter <cl@...ux.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org
Subject: [PATCH v2 12/12] docs: proc.rst: stat: Note the interrupt counter wrap-around

Let's make wrap-around documented behavior so that userspace has
no excuses for not handling it properly if they want accurate values.

On 32-bit platforms "intr" and "softirq" counters can and will
wrap-around, given enough time since boot. This can be days or hours,
depending on the load.

On 64-bit platforms these counters use 64-bit values and these are
very unlikely to oveflow before the heat death of the universe,
but it's still technically possible.

Many other counters can wrap-arond too but I'm not going to enumerate
all of them here. The interrupt counters are most likely to overflow.

Signed-off-by: Alexei Lozovsky <me@...mmy.net>
---
 Documentation/filesystems/proc.rst | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 042c418f4090..a33af0074838 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -1513,6 +1513,14 @@ interrupts serviced  including  unnumbered  architecture specific  interrupts;
 each  subsequent column is the  total for that particular numbered interrupt.
 Unnumbered interrupts are not shown, only summed into the total.
 
+.. note::
+
+   On 32-bit platforms interrupt counters are 32-bit, including the total
+   count of all interrupts. Depending on the system load, these values will
+   sooner or later wrap around. If you want accurate accounting of the rate
+   and *actual* number of interrupts serviced, you should monitor the value
+   closely and handle wrap-arounds.
+
 The "ctxt" line gives the total number of context switches across all CPUs.
 
 The "btime" line gives  the time at which the  system booted, in seconds since
-- 
2.25.1