linux-kernel - Re: [RFC][PATCH 4/8] perf/x86: Remove PERF_X86_EVENT

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Message-ID: <20190319210018.GP5996@hirez.programming.kicks-ass.net>
Date:   Tue, 19 Mar 2019 22:00:18 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Stephane Eranian <eranian@...gle.com>
Cc:     Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, tonyj@...e.com,
        nelson.dsouza@...el.com
Subject: Re: [RFC][PATCH 4/8] perf/x86: Remove PERF_X86_EVENT_COMMITTED

On Tue, Mar 19, 2019 at 01:48:18PM -0700, Stephane Eranian wrote:
> On Thu, Mar 14, 2019 at 6:11 AM Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > For x86_pmu_commit_txn(), cpuc->n_events is updated, but we can
> > trivially compute the desired value with cpuc->n_txn -- the number of
> > events added in this transaction.
> >
> I suggest you put this explanation in the code so that it is easier to
> understand.

Right, I actually attempted writing that comment a few times but every
time it became a mess. I'll try again, because you're quite right, this
is a bit magical.

> > -               for (i = 0; i < n; i++) {
> > +               i = cpuc->n_events;
> > +               if (cpuc->txn_flags & PERF_PMU_TXN_ADD)
> > +                       i -= cpuc->n_txn;
> > +
> > +               for (; i < n; i++) {