linux-kernel - Re: [PATCH] scripts/checksyscalls.sh: only whine perf_counter

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Date:	Sat, 13 Jun 2009 14:37:16 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Mike Frysinger <vapier@...too.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scripts/checksyscalls.sh: only whine perf_counter_open
	when supported

Ingo Molnar writes:

> PMU support is not required to get perfcounters support: if an 
> architecture hooks up the syscall it will get generic software 
> counters and the tools will work as well.

It looks to me that to work properly, each architecture has to provide
set_perf_counter_pending() and some way to arrange to call
perf_counter_do_pending() once interrupts get re-enabled.  We need
this because some software counters (the context-switch counter at
least, as well as all the tracepoint counters) use nmi = 1 because
their events happen at times when we can't do a wakeup.

Actually, as far as the tracepoints are concerned, I can't see any
reason to be sure that interrupts will be disabled when a tracepoint
event occurs.  On powerpc, calling set_perf_counter_pending() when
interrupts are enabled won't result in perf_counter_do_pending()
getting called right away; the perf_counter_do_pending() call won't
happen until something disables and re-enables interrupts.

Paul.
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