linux-kernel - Re: Getting interrupt every million cache misses

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Date:   Thu, 27 Oct 2016 11:33:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Pavel Machek <pavel@....cz>
Cc:     acme@...hat.com, kernel list <linux-kernel@...r.kernel.org>,
        mingo@...hat.com, alexander.shishkin@...ux.intel.com
Subject: Re: Getting interrupt every million cache misses

On Thu, Oct 27, 2016 at 11:11:04AM +0200, Pavel Machek wrote:
> How to work around rowhammer, break my system _and_ make kernel perf
> maintainers scream at the same time: (:-) )
> 
> I think I got the place now. Let me try...

Lol ;-)

> 
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index d31735f..ce83f5e 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -1495,6 +1495,11 @@ perf_event_nmi_handler(unsigned int cmd, struct pt_regs *regs)
>  
>  	perf_sample_event_took(finish_clock - start_clock);
>  
> +	/* Here */
> +	{
> +		udelay(58000);
> +	}
> +
>  	return ret;
>  }
>  NOKPROBE_SYMBOL(perf_event_nmi_handler);

Like you guess, not quite ;-)


I think you want to register a custom overflow handler with your event.

So you get something like:


struct perf_event_attr rh_attr = {
	.type	= PERF_TYPE_HARDWARE,
	.config = PERF_COUNT_HW_CACHE_MISSES,
	.size	= sizeof(struct perf_event_attr),
	.pinned	= 1,
	.sample_period = 1000000,
};

static DEFINE_PER_CPU(struct perf_event *, rh_event);
static DEFINE_PER_CPU(u64, rh_timestamp);

static void rh_overflow(struct perf_event *event, struct perf_sample_data *data, struct pt_regs *regs)
{
	u64 *ts = this_cpu_ptr(&rh_timestamp); /* this is NMI context */
	u64 now = ktime_get_mono_fast_ns();
	s64 delta = now - *ts;

	*ts = now;

	if (delta > 64 * NSEC_PER_USEC)
		udelay(58000);
}

__init int my_module_init()
{
	int cpu;

	/* XXX borken vs hotplug */

	for_each_online_cpu(cpu) {
		struct perf_event *event = per_cpu(event, cpu);

		event = perf_event_create_kernel_counter(&rh_attr, cpu, NULL, rh_overflow, NULL);
		if (!event)
			/* meh */
			;

	}
}

__exit void my_module_exit()
{
	int cpu;

	for_each_online_cpu(cpu) {
		struct perf_event *event = per_cpu(event, cpu);

		if (event)
			perf_event_release_kernel(event);
	}
}