linux-kernel - Re: [PATCH 2/4] perf/x86/mbm: Store bytes counted for mbm during recycle

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Date:	Tue, 3 May 2016 09:46:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vikas Shivappa <vikas.shivappa@...el.com>
Cc:	Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
	tony.luck@...el.com, ravi.v.shankar@...el.com,
	fenghua.yu@...el.com, x86@...nel.org, linux-kernel@...r.kernel.org,
	hpa@...or.com, tglx@...utronix.de, mingo@...nel.org,
	h.peter.anvin@...el.com
Subject: Re: [PATCH 2/4] perf/x86/mbm: Store bytes counted for mbm during
 recycle

On Mon, Apr 25, 2016 at 02:12:09PM -0700, Vikas Shivappa wrote:


> >start:
> >      prev_count = read_hw_counter();
> 
> I am assuming this means we keep the prev_count when event is initialized.
> This is done in the mbm_init which calls update_sample with first parameter
> set to true..

No, when pmu::start() is called.

> >read:
> >      do {
> >              prev = prev_count;
> >              cur_val = read_hw_counter();
> >              delta = cur_val - prev;
> >      } while (local_cmpxchg(&prev_count, prev, cur_val) != prev);
> >      count += delta;

And this you do on pmu::{stop,read}()

> the update_sample does the work to compute the delta and add the delta to
> total_bytes..  it has all the code except for the while loop.

No, no, no, you add rc_count and st_count and generally make a huge mess
of things. The above needs none of that.

Because the above only cares about deltas against the hw counter (as per
prev_count). Therefore count can be an absolute value that carries all
your history as per rc_count, and you don't need st_count because per
prev_count you don't care about the absolute value of the hw counter.