linux-kernel - Re: [PATCH] perf_counter: Prevent oopses from per-cpu software counters

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Date:	Fri, 6 Feb 2009 16:31:38 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf_counter: Prevent oopses from per-cpu software
	counters


* Paul Mackerras <paulus@...ba.org> wrote:

> Ingo Molnar writes:
> 
> > So how about fixing these sw counts to properly work as percpu counters too? 
> 
> OK, so for page faults it looks like I want to look at
> 
> 	get_cpu_var(vm_event_states).event[PGFAULT]
> 
> to get the per-cpu page fault count, as long as
> CONFIG_VM_EVENT_COUNTERS is set.

Yeah - i'd suggest that. Note hat VM_EVENT_COUNTERS is default enabled on 
99.99% of kernels:

 config VM_EVENT_COUNTERS
        default y
        bool "Enable VM event counters for /proc/vmstat" if EMBEDDED

> It looks like the scheduler doesn't keep per-cpu counts of context 
> switches or task migrations, or if it does I couldn't find them.  We could 
> do stuff in perf_counter_task_sched_in/out to implement per-cpu context 
> switch and migration counters by adding up the delta values for each task 
> that gets scheduled onto the cpu.  Or we could add explicit per-cpu 
> counters for these things in the scheduler.
> 
> What do you think?

For per-cpu counts of context switches we already have rq->nr_switches. 

We dont have per-cpu counts of migrations - but could add them.

We should do it this way becaue it would be nice to make per-cpu counters 
work just fine even if they are never switched in and out during context 
switches. That turns per-cpu counters into even lower-overhead ways of 
monitoring those values.

	Ingo
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