linux-kernel - Re: [PATCH] perf/x86/intel: Drop kernel samples even though :u is specified

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Date:   Fri, 19 May 2017 10:42:48 +0100
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jin Yao <yao.jin@...ux.intel.com>, acme@...nel.org,
        jolsa@...nel.org, mingo@...hat.com,
        alexander.shishkin@...ux.intel.com, Linux-kernel@...r.kernel.org,
        ak@...ux.intel.com, kan.liang@...el.com, yao.jin@...el.com,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] perf/x86/intel: Drop kernel samples even though :u is
 specified

On Fri, May 19, 2017 at 11:29:05AM +0200, Peter Zijlstra wrote:
> On Fri, May 19, 2017 at 06:19:12PM +0800, Jin Yao wrote:
> > +bool skid_kernel_samples(struct perf_event *event, struct pt_regs *regs)
> > +{
> > +	u64 ip;
> > +
> > +	/*
> > +	 * Without PEBS, we may get kernel samples even though
> > +	 * exclude_kernel is specified due to skid in sampling.
> > +	 */
> > +	if ((event->attr.exclude_kernel) &&
> > +	    (event->attr.sample_type & PERF_SAMPLE_IP)) {
> > +		ip = perf_instruction_pointer(regs);
> > +		if (kernel_ip(ip))
> > +			return true;
> > +	}
> > +
> > +	return false;
> > +}
> > +
> >  EVENT_ATTR(cpu-cycles,			CPU_CYCLES		);
> >  EVENT_ATTR(instructions,		INSTRUCTIONS		);
> >  EVENT_ATTR(cache-references,		CACHE_REFERENCES	);
> 
> 
> I would much rather see this in generic code, somewhere around
> __perf_event_overflow() I suppose. That would retain proper accounting
> for the interrupt rate etc..
> 
> Also it would work for all architectures. Because I'm thinking more than
> just x86 will suffer from skid.

Yes, I think this will affect arm/arm64 too (and probably others that rely
on irqs for sampling the regs).

Will