linux-kernel - Re: [RFC PATCH 5/6] bpf: Implement function bpf_read_pmu() that get the selected hardware PMU conuter

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Message-ID: <20150717132657.GM19282@twins.programming.kicks-ass.net>
Date:	Fri, 17 Jul 2015 15:26:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	pi3orama <pi3orama@....com>
Cc:	"Wangnan (F)" <wangnan0@...wei.com>,
	kaixu xia <xiakaixu@...wei.com>,
	"ast@...mgrid.com" <ast@...mgrid.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"acme@...nel.org" <acme@...nel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"masami.hiramatsu.pt@...achi.com" <masami.hiramatsu.pt@...achi.com>,
	"jolsa@...nel.org" <jolsa@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hekuang@...wei.com" <hekuang@...wei.com>
Subject: Re: [RFC PATCH 5/6] bpf: Implement function bpf_read_pmu() that get
 the selected hardware PMU conuter

On Fri, Jul 17, 2015 at 08:57:00PM +0800, pi3orama wrote:
> >> But what if we really want cross-cpu PMU accessing? Impossible?
> > 
> > Under the assumption that the eBPF program is called from tracing, and
> > therefore from any context (task, softirq, irq and nmi), yes impossible.
> > 
> > You cannot do (synchronous) IPIs from either IRQ context or with IRQs
> > disabled. And both are valid trace contexts.
> 
> What about software perf event? For example, tracepoints?

Some of them, tracepoints would work. So you could exempt
TYPE_TRACEPOINT, but I would suggest starting as constrained as possible
and relaxing when we really need/want.


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