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Date: Tue, 3 May 2016 09:46:50 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Vikas Shivappa <vikas.shivappa@intel.com>
Cc: Vikas Shivappa <vikas.shivappa@linux.intel.com>, tony.luck@intel.com,
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        mingo@kernel.org, h.peter.anvin@intel.com
Subject: Re: [PATCH 2/4] perf/x86/mbm: Store bytes counted for mbm during
 recycle
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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.