Re: [PATCH V2 5/6] x86/intel_rdt: Use perf infrastructure for measurements

[Peter Zijlstra]

On Thu, Aug 16, 2018 at 01:16:08PM -0700, Reinette Chatre wrote:
> +	l2_miss_event = perf_event_create_kernel_counter(&perf_miss_attr,
> +							 plr->cpu,
> +							 NULL, NULL, NULL);
> +	if (IS_ERR(l2_miss_event))
> +		goto out;
> +
> +	l2_hit_event = perf_event_create_kernel_counter(&perf_hit_attr,
> +							plr->cpu,
> +							NULL, NULL, NULL);
> +	if (IS_ERR(l2_hit_event))
> +		goto out_l2_miss;
> +
> +	local_irq_disable();
> +	/*
> +	 * Check any possible error state of events used by performing
> +	 * one local read.
> +	 */
> +	if (perf_event_read_local(l2_miss_event, &tmp, NULL, NULL)) {
> +		local_irq_enable();
> +		goto out_l2_hit;
> +	}
> +	if (perf_event_read_local(l2_hit_event, &tmp, NULL, NULL)) {
> +		local_irq_enable();
> +		goto out_l2_hit;
> +	}
> +
> +	/*
> +	 * Disable hardware prefetchers.
>  	 *
> +	 * Call wrmsr direcly to avoid the local register variables from
> +	 * being overwritten due to reordering of their assignment with
> +	 * the wrmsr calls.
> +	 */
> +	__wrmsr(MSR_MISC_FEATURE_CONTROL, prefetch_disable_bits, 0x0);

So what about virt?

> +
> +	/* Initialize rest of local variables */
> +	/*
> +	 * Performance event has been validated right before this with
> +	 * interrupts disabled - it is thus safe to read the counter index.
> +	 */
> +	l2_miss_pmcnum = x86_perf_rdpmc_ctr_get(l2_miss_event);
> +	l2_hit_pmcnum = x86_perf_rdpmc_ctr_get(l2_hit_event);
> +	line_size = plr->line_size;
> +	mem_r = plr->kmem;
> +	size = plr->size;

You probably want READ_ONCE() on that, the volatile cast in there
disallows the compiler from re-loading the values later.

> +
> +	/*
> +	 * Read counter variables twice - first to load the instructions
> +	 * used in L1 cache, second to capture accurate value that does not
> +	 * include cache misses incurred because of instruction loads.
> +	 */
> +	rdpmcl(l2_hit_pmcnum, l2_hits_before);

And this again does do virt.

> +	rdpmcl(l2_miss_pmcnum, l2_miss_before);
> +	/*
> +	 * From SDM: Performing back-to-back fast reads are not guaranteed
> +	 * to be monotonic. To guarantee monotonicity on back-toback reads,
> +	 * a serializing instruction must be placed between the two
> +	 * RDPMC instructions
> +	 */
> +	rmb();

You're copying the horrid horrid (did I say truly horrid?) use of
'serializing' from the SDM. Please don't do that.

LFENCE is not a serializing instruction. But given the (new) definition
LFENCE does ensure all prior instructions are retired before it
proceeds.

> +	rdpmcl(l2_hit_pmcnum, l2_hits_before);
> +	rdpmcl(l2_miss_pmcnum, l2_miss_before);
> +	/*
> +	 * rdpmc is not a serializing instruction. Add barrier to prevent
> +	 * instructions that follow to begin executing before reading the
> +	 * counter value.
> +	 */
> +	rmb();
> +	for (i = 0; i < size; i += line_size) {
> +		/*
> +		 * Add a barrier to prevent speculative execution of this
> +		 * loop reading beyond the end of the buffer.
> +		 */
> +		rmb();
> +		asm volatile("mov (%0,%1,1), %%eax\n\t"
> +			     :
> +			     : "r" (mem_r), "r" (i)
> +			     : "%eax", "memory");

Why does that need to be asm?

> +	}

I think you want another LFENCE here, to ensure the RDPMCs don't overlap
with the last LOAD in the loop above.

> +	rdpmcl(l2_hit_pmcnum, l2_hits_after);
> +	rdpmcl(l2_miss_pmcnum, l2_miss_after);
> +	/*
> +	 * rdpmc is not a serializing instruction. Add barrier to ensure
> +	 * events measured have completed and prevent instructions that
> +	 * follow to begin executing before reading the counter value.
> +	 */
> +	rmb();
> +	/* Re-enable hardware prefetchers */
> +	wrmsr(MSR_MISC_FEATURE_CONTROL, 0x0, 0x0);


So what I do in userspace is:

	mmap_read_pinned(ctx); /* prime */

	for (many-times) {

		cnt = mmap_read_pinned(evt);
		barrier();
		cnt = mmap_read_pinned(evt) - cnt;
		update_stats(&empty, cnt);

		cnt = mmap_read_pinned(evt);
		barrier();
		/* the thing */
		barrier();
		cnt = mmap_read_pinned(evt) - cnt;
		update_stats(&stat, cnt);

	}

	sub_stats(&stat, &empty);

Maybe I should've used asm("lfence" ::: "memory") instead of barrier(),
but the results were good enough.