linux-kernel - Re: [PATCH 4/6] x86/mbm: Memory bandwidth monitoring event management

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Date:	Tue, 8 Mar 2016 00:03:29 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vikas Shivappa <vikas.shivappa@...ux.intel.com>
Cc:	vikas.shivappa@...el.com, linux-kernel@...r.kernel.org,
	x86@...nel.org, hpa@...or.com, tglx@...utronix.de,
	mingo@...nel.org, ravi.v.shankar@...el.com, tony.luck@...el.com,
	fenghua.yu@...el.com, h.peter.anvin@...el.com
Subject: Re: [PATCH 4/6] x86/mbm: Memory bandwidth monitoring event management

On Tue, Mar 01, 2016 at 03:48:26PM -0800, Vikas Shivappa wrote:

> Lot of the scheduling code was taken out from Tony's patch and a 3-4
> lines of change were added in the intel_cqm_event_read. Since the timer
> is no more added on every context switch this change was made.

It this here to confuse people or is there some actual information in
it?

> +/*
> + * MBM Counter is 24bits wide. MBM_CNTR_MAX defines max counter
> + * value
> + */
> +#define MBM_CNTR_MAX		0xffffff

#define MBM_CNTR_WIDTH	24
#define MBM_CNTR_MAX	((1U << MBM_CNTR_WIDTH) - 1)


>  #define QOS_L3_OCCUP_EVENT_ID	(1 << 0)
> +/*
> + * MBM Event IDs as defined in SDM section 17.15.5
> + * Event IDs are used to program EVTSEL MSRs before reading mbm event counters
> + */
> +enum mbm_evt_type {
> +	QOS_MBM_TOTAL_EVENT_ID = 0x02,
> +	QOS_MBM_LOCAL_EVENT_ID,
> +	QOS_MBM_TOTAL_BW_EVENT_ID,
> +	QOS_MBM_LOCAL_BW_EVENT_ID,
> +};

QOS_L3_*_EVENT_ID is a define, these are an enum. Rather inconsistent.

>  struct rmid_read {
>  	u32 rmid;

Hole, you could've filled with the enum (which ends up being an int I
think).

>  	atomic64_t value;
> +	enum mbm_evt_type evt_type;
>  };

> +static bool is_mbm_event(int e)

You had an enum type, you might as well use it.

> +{
> +	return (e >= QOS_MBM_TOTAL_EVENT_ID && e <= QOS_MBM_LOCAL_BW_EVENT_ID);
> +}
>  

> +static struct sample *update_sample(unsigned int rmid,
> +				    enum mbm_evt_type evt_type, int first)
> +{
> +	ktime_t cur_time;
> +	struct sample *mbm_current;
> +	u32 vrmid = rmid_2_index(rmid);
> +	u64 val, bytes, diff_time;
> +	u32 eventid;
> +
> +	if (evt_type & QOS_MBM_LOCAL_EVENT_MASK) {
> +		mbm_current = &mbm_local[vrmid];
> +		eventid     =  QOS_MBM_LOCAL_EVENT_ID;
> +	} else {
> +		mbm_current = &mbm_total[vrmid];
> +		eventid     = QOS_MBM_TOTAL_EVENT_ID;
> +	}
> +
> +	cur_time = ktime_get();
> +	wrmsr(MSR_IA32_QM_EVTSEL, eventid, rmid);
> +	rdmsrl(MSR_IA32_QM_CTR, val);
> +	if (val & (RMID_VAL_ERROR | RMID_VAL_UNAVAIL))
> +		return mbm_current;

> +	val &= MBM_CNTR_MAX;

> +	if (val < mbm_current->prev_msr)
> +		bytes = MBM_CNTR_MAX - mbm_current->prev_msr + val + 1;
> +	else
> +		bytes = val - mbm_current->prev_msr;

Would not something like:

	shift = 64 - MBM_CNTR_WIDTH;

	bytes = (val << shift) - (prev << shift);
	bytes >>= shift;

be less obtuse? (and consistent with how every other perf update
function does it).

What guarantee is there we didn't wrap multiple times? Doesn't that
deserve a comment?

> +	bytes *= cqm_l3_scale;
> +
> +	mbm_current->total_bytes += bytes;
> +	mbm_current->interval_bytes += bytes;
> +	mbm_current->prev_msr = val;
> +	diff_time = ktime_ms_delta(cur_time, mbm_current->interval_start);

Here we do a / 1e6

> +
> +	/*
> +	 * The b/w measured is really the most recent/current b/w.
> +	 * We wait till enough time has passed to avoid
> +	 * arthmetic rounding problems.Having it at >=100ms,
> +	 * such errors would be <=1%.
> +	 */
> +	if (diff_time > 100) {

This could well be > 100e6 instead, avoiding the above division most of
the time.

> +		bytes = mbm_current->interval_bytes * MSEC_PER_SEC;
> +		do_div(bytes, diff_time);
> +		mbm_current->bandwidth = bytes;
> +		mbm_current->interval_bytes = 0;
> +		mbm_current->interval_start = cur_time;
> +	}
> +
> +	return mbm_current;
> +}

How does the above time tracking deal with the event not actually having
been scheduled the whole time?


> +static void init_mbm_sample(u32 rmid, enum mbm_evt_type evt_type)
> +{
> +	struct rmid_read rr = {
> +		.value = ATOMIC64_INIT(0),
> +	};
> +
> +	rr.rmid = rmid;
> +	rr.evt_type = evt_type;

That's just sad.. put those two in the struct init as well.

> +	/* on each socket, init sample */
> +	on_each_cpu_mask(&cqm_cpumask, __intel_mbm_event_init, &rr, 1);
> +}