Soft IRQ statistics under /proc/stat

From: Elad Lahav
Date: Thu Sep 11 2008 - 21:39:05 EST

I've been observing some oddities in the statistics produced by mpstat with respect to soft IRQs (for example, considerable soft IRQ time on processors sending UDP packets on dummy NICs). While looking at the kernel code, I noticed that ticks are attributed to soft IRQs when softirq_count() is greater than 0. This happens in __local_bh_disable(), which is called from __do_softirq(), but also from local_bh_disable(). Thus, the number of ticks reported in /proc/stat refers to any execution path that runs with soft IRQs disabled, not just code called from __do_softirq().
I hacked the kernel to differentiate between the two cases, and indeed the anomalies I saw can be explained as code executing under local_bh_disable().

Is this behaviour by design? References to /proc/stat on the web refer to this number simply as "soft IRQ time" (e.g., I would have expected that to include only execution paths starting from __do_softirq().

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