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

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Message-ID: <alpine.DEB.2.10.1603101445490.2648@vshiva-Udesk>
Date:	Thu, 10 Mar 2016 14:46:36 -0800 (PST)
From:	Vikas Shivappa <vikas.shivappa@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	"Luck, Tony" <tony.luck@...el.com>,
	Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
	"Shivappa, Vikas" <vikas.shivappa@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"Shankar, Ravi V" <ravi.v.shankar@...el.com>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	"Anvin, H Peter" <h.peter.anvin@...el.com>
Subject: Re: [PATCH 4/6] x86/mbm: Memory bandwidth monitoring event
 management



On Tue, 8 Mar 2016, Peter Zijlstra wrote:

> On Mon, Mar 07, 2016 at 11:27:26PM +0000, Luck, Tony wrote:
>>>> +		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?
>>
>> That's been the topic of a few philosophical debates ... what exactly are
>> we trying to say when we report that a process has a "memory bandwidth"
>> of, say, 1523 MBytes/s?  We need to know both the amount of data moved
>> and to pick an interval to measure and divide by. Does it make a difference
>> whether the process voluntarily gave up the cpu for some part of the interval
>> (by blocking on I/O)? Or did the scheduler time-slice it out to run other jobs?
>>
>> The above code gives the average bandwidth across the last interval
>> (with a minimum interval size of 100ms to avoid craziness with rounding
>> errors on exceptionally tiny intervals). Some folks apparently want to get
>> a "rate" directly from perf.  I think many folks will find the "bytes" counters
>> more helpful (where they control the sample interval with '-I" flag to perf
>> utility).
>
> So why didn't any of that make it into the Changelog? This is very much
> different from any other perf driver, at the very least this debate
> should have been mentioned and the choice defended.
>
> Also, why are we doing the time tracking and divisions at all? Can't we
> simply report the number of bytes transferred and let userspace sort out
> the rest?
>
> Userspace is provided the time the event was enabled, the time the event
> was running and it can fairly trivially obtain walltime if it so desires
> and then it can compute whatever definition of bandwidth it wants to
> use.


We had discussions on removing the bw event. Discussed this with Tony and will 
update the patch by removing the bw events.. So this code will be removed.

thanks,
Vikas

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